Ionosphere-thermosphere space weather issues

Journal ofAtmospheric

and Terrestrial

Pergamon PLI: SOO21-9169(%)00029-3

Ionosphere-thermosphere

Physics, Vol. 58, No. 14, pp. 1527-1574, 1996 Copyright 0 19% Elsevier Science Ltd Printed in Great Britain. All rights reserved OoZl-9169/96 $15.00+0.00

space weather issues

R. W. Schunk and J. J. Sojka Center for Atmospheric and Space Sciences, Utah State University, UT 843224405, Logan, U.S.A. (Received 21 November

1995; accepted 1 March 1996)

Abstract-Weather disturbances in the ionosphere-thermosphere system can have a detrimental effect on both ground-based and space-based systems. Because of this impact and because our field has matured, it is now appropriate to develop specification and forecast models, with the aim of eventually predicting the occurrence, duration, and intensity of weather effects. As part of the new National Space Weather Program, the CEDAR community will focus on science issues concerning space weather, and this tutorial/review is an expanded version of a tutorial presentation given at the recent CEDAR annual meeting. The tutorial/review provides a brief discussion of weather disturbances and features, the causes of weather, and the status of weather modeling. The features and disturbances discussed include plasma patches, boundary and aurora1 blobs, sun-aligned polar cap arcs, the effects of traveling convection vortices and SAID events, the lifetime of density structures, sporadic E and intermediate layers, spread F and equatorial plasma bubbles, geomagnetic storms and substorms, traveling ionospheric disturbances (TID’s), and the effects of tides and gravity waves propagating from the lower atmosphere. The tutorial/review is only intended to provide an overview of some of the important .scientific issues concerning ionospheric-thermospheric weather, with the emphasis on the ionosphere. Tutorials on thermospheric and magnetospheric weather issues are given in companion papers. Copyright 0 1996 Elsevier Science Ltd

1. INTRODUCHON

The ionosphere-thermosphere system has been studied extensively during the last four decades. Experimentally, balloons, rockets, satellites, coherent and incoherent scatter radars, magnetometers, and a wide range of ground-based optical instruments have been

used to study the system. On the theoretical side, studies have been conducted with l-, 2-, and 3-dimensional numerical models. Initially, the modeling was restricted to specific regions (high-, mid-, and low-latitudes) and the ionosphere and thermosphere were modeled separately. But in recent years, fully coupled global models been ionosphere-thermosphere have developed. As a result of this extensive research effort, it is now well known that the ionospherethermosphere system varies markedly with altitude, latitude, longitude, universal time, season, solar cycle, and geomagnetic activity. This variation results from the couplings, time delays, and feedback mechanisms that are inherent in the system as well as from the effects of solar, interplanetary, magnetospheric and mesospheric processes. The primary driving mechanism for the ionosphere-thermosphere system is solar EUV and UV radiation, but magnetospheric electric fields, particle precipitation, and heat flows have a significant effect on the system. The strength

and form of the magnetospheric effect is determined, to a large degree, by the solar wind dynamic pressure and the orientation of the interplanetary magnetic field (IMF), i.e. by the state of the interplanetary medium. Also, gravity waves and tides propagating up from the lower atmosphere can appreciably affect the ionosphere-thermosphere system. The various driving processes act in concert to determine the density, composition, and temperature morphologies of the ionized and neutral constituents in the ionosphere-thermosphere system. Because these driving processes display characteristic trends, the ionosphere-thermosphere system also displays ‘characteristic’ or ‘average’ features, which correspond to the climatology of the system. For example, at mid-latitudes, the average electron density distribution tends to be uniform, with a gradual transition from dayside high densities to nightside low densities across the terminator. The equatorial electron densities also tend to be uniform on the dayside, but at night Appleton ionization peaks appear on both sides of the magnetic equator. At high latitudes, additional large-scale electron density features are evident when the IMF is southward, including a tongue of ionization, a polar hole, a main trough, and an overall enhancement in the aurora1 oval. With regard to the neutral gas, the general circulation is away

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R. W. Schunk and J. J. Sojka

from the solar-generated high-pressure region in the afternoon, around the globe to the low-pressure region in the early morning sector, with the flow over the polar caps being moderated by magnetospheric processes. Currently, the chemical and physical processes responsible for the ‘average’ or ‘background’ ionospheric-thermospheric features are known, and how these large-scale features vary with universal time, season, solar cycle, and geomagnetic activity is also understood. In other words, the climatology of the system is basically understood. However, the ionosphere-thermosphere system can vary appreciably from hour to hour or from day to day, and it can display a considerable amount of structure. These weather features occur because the external driving mechanisms can be localized, spatially structured, and unsteady, and because there are time delays associated with some of the ionosphere-thermosphere coupling processes. The structure in the system varies from less than a meter to more than 1000 km, and it can appear in the form of propagating plasma patches, boundary blobs, aurora1 blobs, propagating density holes, as well as density irregularities caused by plasma instabilities. Also, rapid temporal variations can occur during geomagnetic storms and substorms and when the IMF changes direction. The net effect is that the weather disturbances in the ionosphere-thermosphere system are significant, and they can adversely affect numerous civilian and military systems and/or operations. Weather disturbances can affect over-the-horizon (OTH) radars, HF communications, surveying and navigation systems that use Global Positioning System (GPS) satellites, surveillance (optical and radar), satellite tracking, satellite lifetimes, power grids, and pipelines (cf. Kleusberg, 1992; Shapka, 1992; Cliffswallow and Human, 1992; and references therein). For example, changes in the IMF can lead to the occurrence of either propagating plasma patches or sun-aligned arcs in the polar cap. Enhanced electron density fluctuations are associated with these ionospheric structures. These fluctuations also occur in the equatorial ionosphere in association with spread F when substorm electric fields penetrate to low latitudes. The small fluctuations in the electron density then cause signal fluctuations (scintillations), which affect HF communications. Scintillations and ionospheric structures also affect OTH radar systems, which are currently being used in counter-drug operations. The advantage of an OTH system is that large ocean areas can be monitored at a relatively low cost. However, ionospheric structures and scintillations can cause clutter and it then becomes difficult to identify a slow moving target (boat or plane) embedded in the

radar clutter. The ionosphere also affects the satellite radar altimetry program (TOPEX/Poseidon and Geosat satellites), which focuses on measuring features on the ocean’s surface as well as surface heights. A radar altimeter is a nadir-pointing ranging instrument that emits a short, high frequency, microwave pulse toward the ocean surface. By measuring the two-way time delay for the echo to return to the satellite and knowing the satellite altitude, one can calculate the height of features on the ocean surface. However, the ionosphere modifies the radar pulse and, hence, the time delay, which then affects the accuracy of the ocean height calculation. Geomagnetic storms and substorms can be particularly damaging to both satellite and ground-based systems. The increased ionospheric currents that flow during geomagnetic storms can induce voltages on the ground that are large enough to cause transformer failures and blackouts. During the severe geomagnetic storm of March 1989, the entire Hydro-Quebec power system collapsed leaving millions of people without electricity for several hours. The increased particle and Joule heating that occurs during geomagnetic storms also leads to significantly enhanced neutral densities. The consequent increase in atmospheric drag on satellites can be sufficient to cause a large difference between the predicted and actual orbits, and when this happens a considerable effort has to be expended to find several thousand ‘lost’ space objects. After more than 40 yr of extensive research, our field has reached the point where specification and forecasting are possible. Because forecasting will help mitigate the detrimental effects of space weather, a National Space Weather Program has been created, with all of the relevant government agencies taking part (Air Force, Navy, Defense Nuclear Agency, NASA, NOAA, and NSF). The NSF effort will primarily be carried out via the CEDAR program, and the emphasis will be on scientific issues rather than on products. At the first CEDAR meeting devoted to space weather, we were asked to present a tutorial on ionosphere/thermosphere space weather issues, and this article is an expanded version of our CEDAR tutorial. In Section 2, we show examples of weather events. In Section 3, we discuss the causes of weather, and in Section 4, we describe the current state of weather modeling. Finally, in Section 5, we provide a list of some of the unresolved scientific issues that are impeding progress with regard to specifications and forecasting. 2. WEATHER

DISTURBANCES

AND FEATURES

Weather disturbances in the ionosphere-thermosphere system occur at all altitudes throughout the

Ionosphere-thermosphere space weather issues E-region, F-region, and topside, and at all latitudes. During geomagnetic: storms and substorms, and when the IMF and solar wind dynamic pressure change, the temporal variation of the ion and neutral densities can be large and density structures can be created. These features are manifestations of space weather effects. In the following subsections, some of the more prominent, mainly ionospheric, weather features are briefly discussed. The goal is to introduce the feature, its associated nomenclature, and describe its characteristics. Also, the basic characteristics of geomagnetic storms and substorms are briefly described because they are instrumental in creating weather features. In all cases, the emphasis is on ‘measured’ characteristics, and the aim of this section is to provide a ‘quick look’ at the whole weather picture. The topics discussed were selected because they are at the forefront of research at this time and because they directly relate to the ‘modeling’ efforts that are discussed in Section 4. 2.1. Propagating plasma patches Plasma patches are regions of enhanced plasma density and 630-nm emission that occur at polar latitudes. They have been observed for more than 15 yr via optical, digisonde, and in situ satellite measurements (Buchau et a!., 1983, 1985; Weber et al., 1984, 1986; Buchau and Fleinisch, 1991; Fukui et al., 1994; 29

23 :310 UT

2 3*36

23832

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Valladares et al., 1994b; Rodger et al., 1994). Patches typically appear when the IMF turns southward. They have been observed in summer and winter at both solar maximum and minimum. They seem to be created either in the dayside cusp or just equatorward of the cusp. Once formed, they convect in an antisunward direction across the polar cap at the prevailing convection speed, which typically varies from 300 m/s to 1 km/s. Patch densities are a factor of 310 greater than background densities and their horizontal dimensions vary from 200 to 1000 km. As they convect across the polar cap, the electron temperature is low, which indicates an absence of particle precipitation. However, intermediate-scale irregularities (l-10 km) and scintillations are usually associated with propagating plasma patches (Basu et al., 1995). Figure 1 shows an example of plasma patches observed at Qaanaaq, Greenland, on 29 October 1989 (Fukui et al., 1994). The figure corresponds to a digitization of a sequence of all-sky photographs taken at 2-min intervals. The direction of the sun is indicated by an arrow on the first and last photographs. At 23.30 UT, a patch that is extended in the dawn-dusk direction is observed and it subsequently moves in an antisunward direction. Six minutes later, another patch appears in the all-sky camera’s field-of-view and it also moves in an antisunward direction. The velocity of the patches is about 730 m/s.

OCT

1969

23134

23142

23136

23’44

Fig. 1. Propa.gating plasma patches observed at Qaanaaq on 29 October 1989. The dials represent a digitization of all-sky images taken at 2-min intervals. The solid and shaded areas show two plasma patches moving in an antisunward direction. From Fukui et al. (1994).

R. W. &hunk and J. J. Sojka

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-7

4

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4

-3

GEOMAGNETICNORM

-2

-1

0

1

2

3

OISTANCE FROM CHATA,,IKA

s

4 -

(I

x ld

7 km

Fig. 2. Contours of electron density measured on November 11, 1981, by the Chatanika incoherent scatter radar. The contours are plotted as a function of altitude and geomagnetic north distance from the radar (in 100 km units).

From Rino et al. (1983).

2.2. Boundary and aurora1 blobs Boundary and aurora1 blobs are regions of enhanced plasma density that are located either inside or on the equatorward edge of the aurora1 oval. Figure 2 shows examples of such features. The figure shows contours of the electron density measured on November 11, 1981 by the Chatanika incoherent scatter radar (Rino et al., 1983; Tsunoda, 1988). The contours are plotted as a function of altitude and geomagnetic north distance from the radar (in 100 km units). Two 15-min radar scans are shown that are close to each other in time. The aurora1 blob is seen in the first scan and is located about 500 km north of the radar. The structure extends from about 180 to 300 km in altitude and is about 200 km wide. The structure is no longer evident in the second scan. The boundary blob is seen in both radar scans and is situated just equatorward of the aurora1 E layer and poleward of the mid-latitude trough. The aurora1 E layer is evident in the second scan as enhanced densities north of the radar at about 130 km altitude, while the mid-latitude trough is located south of the radar and is the narrow latitudinal region of low plasma densities. At still lower latitudes, a classical F-region is clearly evident. Although not shown in Fig. 2, boundary blobs can persist for many hours and can extend over large longitudinal distances. 2.3. Sun-alignedpolar cap arcs Sun-aligned polar cap arcs are discrete 6300 A emission structures in the polar cap (Weber et al., 1989; Niciejewski et al., 1989; Valladares and Carlson, 1991; Valladares et al., 1994a). The arcs appear when the

IMF is near zero or northward and are a result of electron precipitation, with the characteristic energy varying from 300 eV to 5 keV and the energy flux varying from 0.1 to a few erg/cm2/s’. They are relatively narrow (< 300 km), but are extended along the noon-midnight direction (1000-3000 km). Under conditions of large (> 10 nT) northward IMF, a single arc can form that extends all the way from the dayside to the nightside aurora1 oval, with the associated optical emission forming the Greek letter theta when viewed from space (Frank et al., 1986). Typically, however, the arcs do not completely extend across the polar cap, and frequently, multiple arcs are observed. Once formed, the arcs tend to drift toward either the dawn or dusk of the polar cap at speeds of a few hundred meters per second. Figure 3 shows the temporal evolution of multiple polar cap arcs observed at Qaanaaq, Greenland, on February 19, 1987. The arcs are reconstructions of 6300 A images displayed in a corrected geomagnetic coordinate system (Valladares et al., 1994a). Initially, three arcs were visible; but at 22.57 UT a fourth arc appeared which then drifted toward the other arcs. After carefully studying the motion of 150 polar cap arcs, the authors found that the arc direction of motion depended on both the IMF BYcomponent and the arc location in the polar cap (Fig. 4). For a given value of BY, two well-defined regions (or cells) exist. Within each cell, the arcs move in the same direction toward the boundary between the cells. The arcs located in the duskside cell move toward the dawn, while those in the dawnside cell move toward the dusk. The relative size of the dawn and dusk cells is determined by the magnitude of BY. Recently coordinated campaigns involving incoherent scatter radars, ground-based optical instruments, satellites, and rockets have elucidated the electrodynamics, precipitation features, electron density morphology, and thermal structure of polar cap arcs. The data collected from a large number of polar cap arcs were then used by Valladares and Carlson (1991) to construct a schematic of a ‘typical’ polar cap arc (Fig. 5). The typical arc is characterized by an asymmetric plasma density enhancement and distinct regions of elevated electron and ion temperatures. The convection in the arc is primarily along the arc, but it is sheared with the flow reversing direction across the arc. Usually, there is also a slow drift of the plasma across the arc (not shown in Fig. 5). Although the schematic diagram is valuable for defining a typical polar cap arc, some arcs can be distinctly different from that shown in Fig. 5. For example, Weber et al. (1989) observed a moderate density decrease in a polar cap arc even in the presence of precipitating electrons.

Ionosphere-thermosphere

space weather issues

(a) 15

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Cc) 15

17

19

I

21

23

23:Ol ’ Fig. 3. Multiple polar cap arcs observed at Qaanaaq, Greenland, on February 19, 1989. The arcs are displayed in a corrected geomagnetic coordinate system. From Valladares et al. (1994a).

2.4. Density variability at high latitudes The electron density at high latitudes is not only affected by solar EUV radiation and neutral winds,

but by plasma convection and particle precipitation as well. Because precipitation and convection are timedependent, spatially nonuniform, and vary with both K, and the IMF, a large hour-by-hour density variation should be expected when the ionosphere is viewed from a singbe station. This is particularly true when the IMF changes direction and plasma patches or sun-aligned arcs can be present. The variation observed at Qaanaaq, Greenland, over a 3-day winter period (January 17.-19, 1989) is shown in Fig. 6. In

this figure, the critical frequency, f,,F,, is plotted every 5 min over a 24-h period, but data from all three days are plotted on the same 24-h time axis. Although a diurnal trend is evident, the hour-by-hour and dayby-day variability is large in winter. In summer, the bulk of the polar cap is sunlit and the variability is smaller (not shown). 2.5. Sporadic E and intermediate layers Sporadic E layers are ionization enhancements in the E-region at altitudes between 9C120 km (Whitehead, 1989; Biggin et al., 1986; Mathews et al., 1993). The layers tend to occur sporadically and can be seen

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R. W. Schunk

and J. J. Sojka

DUSK-DAWN MOTION OF POLAR CAP ARCS

12 YLT

12 MLT

12 MLT

24 MLT

24 MLT

24 YLT

By<<0

By=0

By>+0

b)

cl

3

Fig. 4. Schematic diagram of the relative location of polar cap arcs and their corresponding dusk-dawn motion for different values off?,: (a) By i < 0, (b) By = 0, and (c) B, > r 0. From Valladares et al. (1994a).

at all latitudes. Relative to background values, the layer densities can be an order of magnitude greater. A distinct feature of sporadic E layers is that they are

very narrow (0.6-2 km), and the primary ions in the layers are metallic (Fe+, Mg+), which are produced during meteor ablation. Multiple layers can occur sim-

k~

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enhanced Ti

Altitude

(km) 300

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DAWN 200

0

-100

8

??

000~

100

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Plasma

drift

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Fig. 5. Schematic diagram of a cross-section of a representative sun-aligned polar cap arc. The contours show the region of enhanced electron densities, while the cross-hatched areas show the regions of enhanced electron and ion temperatures. The plasma drift direction is shown at the bottom. From Valladares and Carlson (1991).

Ionosphere-themrosphere NASSPLOT -l-t-+-+-.

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space weather issues

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Fig. 6. Measurements off,F, at Qaanaaq, Greenland, plotted every 5 min over a 24-h period. Three days of data are plotted on the same time axis.

ultaneously, separated by 610 km, and after formation the layers ten-d to descend at a slow speed (0.6 4 m/s). Sometimes the sporadic E layers are flat and uniform in the hon.zontal direction, while at other times they are like clouds (2-100 km in size) that move horizontally at speeds of between 2CL130 m/s (cf. Whitehead, 1989). Figure 7 shows an example of a sporadic E layer, as measured by the incoherent scatter radar at Arecibo on May 7,1983 (Riggin et al., 1986). The figure shows electron density profiles as a function of altitude at different times. During the early evening, from 17.10 to 19.10 Atlantic Standard Time (AST), a sporadic E layer was present at 116 km with a peak electron density of about 5 x 10S/cm3. After sunset (18.10 AST), the densities below the F-region decayed rapidly and a deep valley formed. However, the sporadic E layer persisted, but it descended to 114 km and its peak density decrea.sed to about 1 x 104/cm3. After 19.10 AST, the layer continued to descend and it remained weak until 21.48 AST, at which time it reached 105 km. Subsequently, the layer density started to increase. In contrast to sporadic E layers, intermediate layers are broad (l&20 km wide), are composed of molecular ions (NO+, Oz), and occur in the altitude range from 120 to 180 km (Shen et al., 1976; Miller et al., 1993; Mathews et a/., 1993). They frequently appear at night in the valley between the E- and F-regions, which is a low density region, but they can also occur

during the day. They tend to form on the bottomside of the F-region and then slowly descend throughout the night toward the E-region. As with sporadic E layers, intermediate layers can occur at all latitudes, can have a large horizontal extent, and can have an order of magnitude density enhancement relative to background densities. Figure 7 shows an example of the formation and subsequent downward descent of an intermediate layer, from 160 to 120 km, which appeared at about 20.30 AST on May 7,1983 (Riggin et al., 1986). 2.6. Spread F and equatorial bubbles Plasma irregularities or inhomogeneities in the Fregion caused by plasma instabilities manifest as spread F echoes on ionograms (Tsunoda, 1985; Hysell et al., 1990; Abdu et al., 1992; Mendillo et al., 1992; Jayachandran et al., 1993; Aggson et al., 1992). The scale sizes of the irregularities range from a few centimeters to a few hundred kilometers, and the irregularities can appear at all latitudes. However, spread F in the equatorial region can be particularly severe. At night, fully developed equatorial spread F is characterized by plasma bubbles, which are vertically elongated wedges of depleted plasma that drift upward from beneath the bottomside F layer to altitudes greater than 1000 km. The entire north-south extent of a bubble flux tube is typically depleted. The east-west extent of a disturbed region can be several thousand kilometers, with the horizontal distance between indi-

R. W. Schunk and J. J. Sojka

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HEIGHT (kms) Fig. 7. Electron density profiles vs altitude at different times showing both a sporadic E layer and a descending intermediate layer. The measurements were made with the Arecibo incoherent scatter radar on May 7, 1983. From Riggin et al. (1986).

vidual depleted regions being tens to hundreds of kilometers. When bubbles form, they drift upward with

a speed that varies from 100 to 1000 m/s. But after formation, their upward drift ceases (fossil bubbles). The spread F disturbance drifts toward the east with the background plasma, but the high-altitude bubbles tend to lag behind. Figure 8 shows a schematic diagram of the evolution of equatorial spread F and bubbles that are consistent with simultaneous HF radar, rocket, and Jicamarca VHF radar measurements on March 1415, 1983 (Argo and Kelley, 1986). Near the dusk terminator, the equatorial F layer rises due to the action of dynamo electric fields and subsequently it descends. On the day the measurements were made, the layer

BOTTOMSIDE SPREAD-F

0

SUN

Fig. 8. Schematic diagram showing the evolution of equatorial spread F and plasma bubbles that is consistent with the measurements of Argo and Kelley (1986).

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space weather issues

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Fig. 9. Plasma density variation along 15 orbits of the DMSP satellite F-10 for day 74 of 1991. The numbers in the plots correspond to the longitude at the magnetic equatorial crossing. The solar zenith angle at the magnetic equator varies from 113” to 116”, corresponding to a local solar time of about 19.40. From Hanson and Urquhart (1994).

was in the process of moving downward when spread F occurred. Plasma bubbles formed on the bottomside of the F layer and drifted to higher altitudes as the entire disturbed region convected toward midnight. Past midnight, the spread F disturbance ceased but the bubbles (detached plumes) persisted. When a satellite traverses bubbles, the ambient plasma density can decrease by more than an order of magnitude (Fig. 9). In this figure, the electron density variation along the polar orbiting DMSP F-10 satellite track is shown for 15 orbits on day ‘74of 199 1 (Hanson and Urquhart, 1994). The satellite altitude varied from 745 to 855 km and data were taken every 2 s. Note that on some orbits there were large density depletions and the density in the deplssted region was irregular. These depleted regions are equatorial bubbles. 2.7. Geomagnetic storms Geomagnetic storms occur when there is a large sudden increase in the solar wind speed (cf. Fairfield,

1992). They can be particularly strong when the increased solar wind speed is accompanied by a large southward IMF component. Following a sudden storm commencement (SSC), there are growth, main, and recovery phases, respectively. During the growth phase, the magnetospheric electric field and particle precipitation patterns expand, the electric fields become stronger, and precipitation becomes more intense. During this phase, the Joule and particle heating rates and the electrojet currents increase. The energy input to the upper atmosphere maximizes during the main phase, while during the recovery phase the geomagnetic activity and energy input decrease. For large storms, the density, composition, and circulation of the ionosphere-thermosphere system can be significantly modified on a global scale and the modifications can persist for several days after the geomagnetic activity ceases. If the electron density increases as a result of storm dynamics, it is called a ‘positive ionospheric storm’, while a decrease in elec-

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R. W. Schunk

tron density is called a ‘negative ionospheric storm.’ During a sudden storm commencement, gravity waves can be excited at high latitudes and their subsequent propagation toward lower latitudes leads to a ‘traveling ionospheric disturbance.’ Unfortunately, the response of the ionosphere-thermosphere system to different geomagnetic storms can be significantly different, and even for a given storm the system’s response can be very different in different latitudinal and longitudinal regions (Rishbeth, 1991; Priilss et al., 1991; Rich and Denig, 1992; Buonsanto et al., 1992; Yeh et al., 1992, 1994; Ma et al., 1995; PrBlss, 1995). A large magnetic storm that has been extensively studied is the October 1989 storm (Allen et al., 1989; Yeh et al., 1994; Ma et al., 1995). This magnetic storm was triggered by a great solar flare which appeared at 12.29 UT on October 19, 1989. Associated with this flare was an enhanced solar wind speed of about 2000 km/s, and the IMF turned southward two times (between 12.5fL13.40 UT and 16.5&19.00 UT). As a consequence, a SSC occurred at 09.17 UT on October 20, and after an initial phase the storm displayed two periods of enhanced activity during the following 48 h. The magnetic activity index Ap reached 162 and aurora1 glows were seen at about 29” N geomagnetic latitude in America during the height of the storm. In response to the storm, there were long-lasting electron

and J. J. Sojka

density depletions at high latitudes, as measured by a world-wide network of ionosondes (Fig. 10). In the equatorial region, both negative and positive storm effects occurred at different times. In addition, largescale traveling ionospheric disturbances (TID’s) were observed on two nights, with equatorward propagation velocities in the range of 33G-680 m/s (not shown). 2.8. Substorms Substorms correspond to electromagnetic disturbances that suddenly appear in the aurora1 region near midnight magnetic local time (cf. Akasofu, 1964; McPherron et al., 1973). Following onset, there are expansion, maximum, and recovery phases, with the expansion phase typically lasting about 30 min and the entire substorm about 2-3 h. When viewed via the associated optical emission, the substorm first appears as a localized region of bright emission on the poleward edge of the aurora1 oval near local magnetic midnight. This so-called bulge is part of a westward traveling surge that occurs during the expansion phase of a substorm Associated with substorms are locally enhanced electric fields, particle precipitation, and both field-aligned and electrojet currents. Also, intense discrete aurora1 arcs typically appear near the poleward and westward fronts of the bulge. Eventu-

-10 -15 -20 -25 -30 -35 -40 --45 UT(hr.)

Fig. 10. Contours ofQ’* deviations vs latitude and time for the Asian/Pacific sector during 2&22 October 1989. The diurnal variation offJr on October 19 was used as a quiet time reference and it was subtracted from they& variations on October 20-22 to yield thef& deviations. The solid curves correspond to positive deviations and the dashed curves to negative deviations. The interval between adjacent contours is 1 MHz. From Ma et al. (1995).

Ionosphere-thermosphere space weather issues

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Generic Aurora

Middle Surge (Sector 4)

yetc2~

I

Fig. 11. Schematic diagram of a generic amoral bulge deduced from DE 1 images of 35 substorms. region is broken up into six sectors, as labeled in the diagram. From Fujii et al. (1994).

ally, the substorm associated disturbances encompass the entire high latitude region. Recently, Fujii Int al. (1994) examined the electrodynamic parameters in the nighttime sector during 35 aurora1 substorms in an effort to determine characteristic features. To aid in the comparison of the different substorms, a generic aurora was introduced and the region was divided into six sectors based on their characteristic features (Fig. 11). The six sectors were the region west of the bulge, the surge horn (which extends from the surge toward dusk along the oval), the surge (which is an intense aurora in the western and poleward part of the bulge), the middle surge (which is a bright aurora just east of the surge), the eastern bulge, iand the region east of the bulge. Data from the DE :I and 2 satellites were studied with

The

respect to this generic aurora for 35 isolated substorm events, and in this way it was possible to identify specific electrodynamic features during substorm expansion. Figure 12 shows the synthesis of the results, including the field-aligned currents, electric fields, and auroras. Note that at the poleward boundary of the bulge, a pair of upward and downward fieldaligned currents occurs in association with a narrow eastward and/or antisunward plasma how. During the recovery phase of a substorm, a subaurora1 ion drift (SAID) event can occur. SAID events correspond to relatively narrow regions of rapid westward ion drifts located in the evening sector just equatorward of the aurora1 oval @middy et al., 1977; Anderson et al., 1993). In SAID events, the ion drifts can reach 4 km/s. The latitudinal width of the region

Dawn

Dusk

Midnight Fig. 12. Schematic associated

diagram showing the distributions, field-aligned currents, plasma flows, and auroras with a generic aurora during a bulge-type substorm. From Fujii et al. (1994).

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R. W. Schunk and J. J. Sojka

varies from 0.1 to 2”, and the lifetime of the event ranges from less than 30 min to up to 3 h. The SAID events are extended in longitude, but usually are confined to the 18.O(r24.00 local time sector. These events are commonly thought to occur because of a separation between ion and electron drift paths in the plasma sheet that develops during the recovery phase of substorms.

3. CAUSES

Sunward

Antisunward

Sunward

OF WEATHER

The underlying causes of weather in the ionosphere-thermosphere system can be traced to the effects of magnetospheric convection and precipit.ation and to the effects of upward propagating tides and gravity waves from the lower atmosphere. The spatial and temporal variability of these processes, coupled with time delays and feedback mechanisms in the ionosphere-thermosphere system, lead to the weather features and disturbances. Therefore, it is useful to provide a brief overview of the current state of knowledge of the important system drivers. Note, however, that this section is not intended to provide a comprehensive review of the subject, but merely to introduce the novice to the current status. It is now well-known that the convection electric field is strongly correlated with Kp and that it depends on the solar wind dynamic pressure and the direction of the interplanetary magnetic field (B,, By, BJ. When the IMF is southward (Bz < 0), the convection pattern takes a standard form, and hence, it is not surprising that this case is better understood than the northward case. However, even for southward IMF, the bulk of the work conducted to date was directed towards obtaining ‘empirical’ or ‘statistical’ patterns of plasma convection. Typically, these empirical models are constructed from data collected over many months or years from many ground-based sites or satellite orbits. The data are synthesized, binned, and then fitted with simple analytical expressions. As a consequence, the empirical convection models represent average magnetosphere conditions, not instantaneous patterns. Also, the convection and precipitation boundaries that exist in these models are smooth and they overlap only in a general sense. Currently, numerous ‘empirical’ convection electric field models exist, including models based on satellite data (Heppner, 1977; Volland, 1978; Heelis et al., 1982; Heelis, 1984; Sojka et al., 1986; Heppner and Maynard, 1987; Rich and Hairston, 1994; Weimer, 1995), incoherent scatter radar data (Foster, 1983; Holt et al., 1987), coherent radar data (Greenwald et al., 1995), and mag-

t0

Fig. 13. Schematic diagram showing two-cell convection for southward IMF. The solid lines correspond to streamlines of the plasma flow in the polar region at F-region altitudes.

netometer data (Kamide et al., 1994; Richmond et al., 1988). Likewise, several ‘empirical’ models have been developed to describe aurora1 particle precipitation (Spiro et al., 1982; Hardy et al., 1985; Evans, 1987). 3.1. Southward IMF When the IMF is southward (B, ~0) plasma convection at high latitudes exhibits a 2-cell pattern with antisunward flow over the polar cap and return flow equatorward of the aurora1 oval, as shown schematically in Fig. 13. The potential drop across the polar cap, which determines the convection speed, varies with the solar wind dynamic pressure. Also, the potential drop can be distributed uniformly or asymmetrically between the two cells depending on the IMF By component. For B,zO, the convection cells are symmetric, while for By >O, the convection in the morning cell is enhanced and for By ~0 the convection in the evening cell is enhanced in the northem hemisphere. Associated with each convection pattern is an aurora1 precipitation pattern, which is shown schematically in Fig. 14. Although the precipitation pattern can have both continuous and discrete features for southward IMF, the precipitation is generally confined to the ‘classical’ aurora1 oval. 3.2. Northward IMF When the IMF is northward (B, >O), the plasma convection, particle precipitation and Birkeland (or field-aligned) current patterns are more complex than

Ionosphere-thermosphere space weather issues

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Cusp region

Diffuse aurora1

Discreteaurora1 Diffuse aurora Fig. 14. Schematic

diagram

iUCS

showing the different particle precipitation southward IMF. From Akasofu (1976).

for a southward IMF configuration. In this case, even the ‘average’ or ‘statistical’ patterns have not been clearly established. However, measurements have shown that when l.he IMF is northward, the convection in the polar cap can be sunward and aurora1 precipitation can occur in the polar cap in addition to the classical aurora1 oval. They have also shown that an additional field-aligned current system (NBZ currents) exists in the polar cap. However, the interpretation of these features is still controversial. Maezawa (1976) was the first to recognize that plasma convection does not assume the standard twocell configuration during quiet periods or when the IMF is northward. IFrom an analysis of magnetic dis-

06

18

Fig. 15. Schematic diagram of the plasma the southern polar region for a northward (middle dial) and BY ~0 (right dial). The satellite are shown at

06

regions

in the aurora1

oval for

turbances, Maezawa deduced that a characteristic current system forms in the polar cap as the northward component of the IMF increases. He also deduced that sunward plasma convection should occur in the polar cap in association with this new current system. Subsequently, satellite electric field measurements confirmed the presence of sunward convection in the polar cap for northward IMF. The sunward convection was interpreted by Burke et al. (1979) to be a signature of a four-cell convection pattern. However, Burke et al. (1979) found that four-cell convection patterns are clearly seen only on the sunlit side of the polar region. On the nightside, the electric field was found to be irregular. Later, Potemra et al. (1984)

18

08

18

convection patterns and NBZ Birkeland current directions in IMF. The patterns are shown for BY >O (left dial), BY = 0 traces of AB/electric field observed by a dawn-dusk orbiting the bottom. From Potemra et al. (1984).

1540

R. W. Schunk and J. J. Sojka B, > 0

Northward IMF

BJ
Oh Oh Fig. 16. Distorted two-cell convection patterns for a strongly northward IMF and for BY> 0 (left dial) and BY ~0 (right dial) in the northern hemisphere. From Heppner and Maynard (1987).

suggested that three-cell convection patterns can occur for northward B,depending on the direction of the IMF Bycomponent. Figure 15 shows the proposed convection patterns and NBZ current directions in the southern polar region for Bz >O and three By cases. On the other hand, Heppner and Maynard (1987) interpreted the sunward convection in the polar cap in terms of a ‘severely distorted’ two-cell convection pattern (Fig. 16). This interpretation was based on electric field measurements from the Dynamics Explorer 2 satellite, but the data were used only when Bz was clearly northward. Subsequent studies that were based on measured field-aligned current patterns, magnetometer data, satellite electric field data, and/or radar data produced arguments favoring one or the other type of pattern, with no clear resolution of the controversy (Rasmussen and Schunk, 1988; Zanetti et al., 1990; Reiff and Heelis, 1994; Greenwald et al., 1995). For example, Reiff and Heelis (1994) presented strong evidence, based on Atmosphere Explorer C satellite measurements, that fourcell convection exists, and they suggested that the distorted two-cell pattern may be a transitional pattern associated with IMF state changes. But, Greenwald et al. (1995) presented evidence for a distorted two-cell pattern that was stable. Recently, in an attempt to resolve this issue, two independent studies have been conducted. Rich and Hairston (1994) performed a comprehensive analysis of DMSP F8 and F9 satellite data and concluded that, “the development of more than two convection cells

for northward IMF is either uncommon or nonexistent. A distorted 2-cell pattern is observed.” On the other hand, Weimer (1995) performed a comprehensive study using DE 2 satellite data and concluded that, “for northward IMF, evidently there are four convection cells rather than a distortion of the 2cell pattern.” Since the two independent and comprehensive studies led to opposite conclusions, it is obvious that we still don’t know what the convection patterns look like when the IMF is northward. Perhaps, as suggested by Greenwald et al. (1995), both patterns can exist. In this case, it is important to determine the conditions under which the various northward Bzconvection patterns occur. With regard to particle precipitation for northward IMF, measurements have clearly shown that precipitation occurs in the polar cap (Buchau et al., 1983; Weber et al., 1984; Mende et al., 1988). The precipitation can be uniform or appear in the form of sun-aligned arcs (Fig. 3). There can be single or multiple sun-aligned arcs, and once they appear, they drift toward either the dawn or dusk at speeds of a few hundred m/s. For B,strongly northward, a single arc can occur that extends from the dayside to nightside aurora1 oval (e-aurora). Presumably, sun-aligned arcs are associated with the flow reversal boundaries in the convection pattern (3-cell, 4-cell, or distorted 2-cell), which implies that the convection pattern is continually evolving in time since the arcs continually drift. However, measurements indicate that plasma flows across the arc in addition to that parallel to

Ionosphere-thermosphere space weather issues the arc (Carlson et al., 1988), which complicates the interpretation of the arc electrodynamic features. 3.3. Structured convection andprecipitation The ‘empirical’ or ‘statistical’ patterns of plasma convection and particle precipitation do not display spatial structure owing to the averaging processes used in their construction. However, even for a southward IMF configuration, the patterns can exhibit an appreciable amount of structure. For example, Moses et al. (1988) developed a two-dimensional ionospheric convection model, and then compared the model predictions with plasma drift velocities measured along DE 2 satellite orbits. From the model-data comparisons, the authors found that roughly 35% of the DE 2 passes that crossed the dayside between 08.00 and 14.00 MLT could not be modeled with a ‘single’ narrow flow entry region. The authors noted that the electric field can be concentrated along portions of the polar cap entrance and the entry region may contain multiple ‘throats.’ For northward IMF, the structure in the plasma convection pattern can be even more pronounced, as shown in Fig. 17. The ion drift velocities shown in this3figure were measured by the DE 2 satellite during a, crossing of the northern polar region when the IMF was northward. Although this case corresponds to an extreme case of electric field

ION

DRIFT

UNIVERSITY

METER,

DE-2

OF TEXAS

AT DALLAS

OCTOBER Ii’, 1981 1634 - 1646 UT

I KM/SEC Fig. 17. Plasma convection velocities in the high latitude Fregion in a magnetic latitude-MLT reference frame. The data were obtained with the ion drift meter on the Dynamics Explorer-2 satellite. From Frank et al. (1986).

1541

structure, it does indicate what is missing from the statistical convection models. Also, from a single satellite pass it is not possible to separate the temporal and spatial changes. That is, in general, it is not possible to determine if the structure seen in Fig. 17 is spatial structure measured by the satellite as it crossed a fixed convection pattern or if the pattern varied in time as the satellite crossed the polar region. 3.4. Time-varying convection and precipitation. Another limitation of the ‘statistical’ or ‘average’ convection and precipitation patterns is that they are static, i.e. they do not display the strong temporal variability known to exist at high latitudes. During the growth phase of geomagnetic storms, the convection and precipitation patterns expand, convection speeds increase, and precipitation becomes more intense throughout the polar region, while the reverse occurs during the declining phase of geomagnetic storms. During substorms, the convection and precipitation features can vary markedly in a localized region, with a time-constant as short as a few minutes. The well known westward traveling surge that appears near midnight at the start of a substorm is an example of a rapidly varying phenomenon (cf. Kan et al., 1985; Kamide et al., 1986, 1994). In addition to the dramatic changes that occur during storms and substorms, the precipitation and convection patterns change continuously in response to temporal variations in both the solar wind dynamic pressure and the IMF direction. Figure 18 shows the components of the IMF (B,, BJ for a representative 12-h time period. During this period, BY started out positive and then changed direction eight times. Likewise, B, started out positive and then changed direction four times. However, the ‘statistical’ convection patterns indicate that convection electric fields vary markedly with the IMF BY and B, directions. This implies that the plasma convection pattern changed its configuration several times during the 12-h period shown in Fig. 18. For example, when B, was northward, the convection pattern could have been a threecell, four-cell, severely distorted two-cell or turbulent pattern, while for B, southward it was probably a twocell pattern. Variations in BY probably modified the pattern and variations in the strength of B, and BY imply that the speed of the convection varied with time even if the IMF did not change direction. This raises the following questions: What does the ‘instantaneous’ convection pattern look like for different IMF directions? When the IMF changes direction, how long does it take to establish a new convection pattern?

1542

R. W. &hunk and J. J. Sojka

-20 : 6

8

10

12

14

16

18

20

Universal time Fig. 18. Variation of the interplanetary magnetic field (B,, B,) vs universal time for a representative 12-h period. From Roble et al. (1987).

The response time of the high-latitude ionosphere to ‘sudden’ changes in the solar wind/IMF configuration has been studied for several years (Akasofu et al., 1982; Bargatze et al., 1985; Etemadi et al., 1988; Knipp et al., 1991; Hairston and Heelis, 1995). The more recent studies indicate that the plasma convection velocities on the dayside respond fairly rapidly, within S-10 min, to sudden changes in the north-south component of the IMF, while the entire pattern is established in about 20-30 min. This result is consistent with the time constant for the formation of sun-aligned arcs. When the IMF turns northward, it typically takes about 2&30 min for the sun-aligned polar cap arcs to fully develop, which implies that a new convection pattern forms in the polar cap during this time period. However, more work is needed to establish the time constant for different seasonal and solar cycle conditions. Evidence that the convection pattern can change significantly during the time it takes a satellite to cross the polar region can be given by integrating the electric field measured along the satellite track to obtain the associated potential change. If the convection pattern does not vary with time, then the positive and negative potentials should cancel if one integrates along a dawn-dusk orbit from mid-latitudes (- 45”) on one side, across the polar region, to mid-latitudes (_ 45”) on the other side. Therefore, if you assume a zero potential at one side, you should get back to zero on the other side. A potential residual indicates that the convection pattern changed during the satellite traversal. To show that the convection pattern can change

appreciably during a satellite traversal of the polar region, we selected a 6-day interval (days 7&75) during the March 1989 storm period and obtained the relevant passes for the DMSP F8 satellite, which was in a dawn-dusk orbit. A total of 156 polar passes were available and for each pass the electric field was integrated along the orbit, as described above. The F8 satellite traversed the polar region from 45” on one side to 45” on the other side in about 25 min. The difference in the two mid-latitude potentials is called the ‘offset’ or ‘residual,’ and it is shown in Fig. 19 for the 156 polar passes. A zero offset means the convection pattern did not change, while a large offset implies large changes in the convection pattern during the satellite traversal. Only 35% of the polar passes displayed a small offset (offset potential less than 20% of the polar cap potential). For some passes, the offset potential was comparable to the polar cap potential, which implies major convection pattern changes in 25 min. 3.5. Attempts precipitation

to model time-varying

convection

and

There have been relatively few attempts to obtain ‘snapshots’ of the plasma convection pattern and these have required the use of multi-instrument data sets (Heelis et al., 1983; Rasmussen et al., 1986; Marklund et al., 1988; Richmond et al., 1988; Kamide et al., 1994). In the study by Heelis et nl. (1983) data from three radars (Chatanika, Millstone Hill and the STARE facility) and four satellites (NOAA 6 and 7, ISEE 2 and DE 2) were used to obtain the plasma convection pattern during the time interval from 02.30

Ionosphere-thermosphere

x

5 ;‘=J

5

x x

x

x

x

X

x

XX

(typically two in the Heelis study) helped identify the ‘instantaneous’ plasma convection pattern. The above study by Heelis et al. (1983) was successful in obtaining instantaneous convection patterns only at specific times, corresponding to the simultaneous crossing of the polar region by at least two satellites. However, what is needed are plasma convection and particle precipitation patterns as a function of time. In this situation, the so-called AMIE technique corresponds to the state-of-the-art (Richmond et al., 1988; Knipp et al., 1995; Kamide et al., 1994). With this technique, a current-conductivity (electrodynamic) model of the polar region is solved to obtain the convection electric field, given ionospheric current and conductivity distributions. Initially, ionospheric current and conductivity distributions are assumed and then they are adjusted until the calculated electric fields agree with those measured along available satellite tracks and at localized regions probed by ground-based radars. The magnetic perturbations associated with the adopted current distribution must also agree with those measured by ground-based magnetometers. In the latest AMIE simulations, 100 magnetometer sites are used, which can provide snapshots of the convection pattern every 5 min (Knipp et al., 1995). Because of the spatial and temporal resolution of the magnetometers, the AMIE technique can provide time varying convection patterns. However, because

Fig. 19. Offset potential vs polar cap potential for 156 polar passes of the DMSP I:8 satellite. The offset potential is calculated from 45” at mid-latitudes on one side of a polar pass to 45” on the other side. The polar cap potential is calculated from the dusk to dawn polar cap boundaries.

to 14.00 UT on October 25, 1981. Data from ISEE 2 indicated that the IMF was southward (& < 0), which corresponds to the relatively simple case of 2-cell convection. Nevertheless, this study showed that ‘simultaneous’ data from multiple satellite crossings

*Ml-

r-. __-N

0

1543

!

x

xx

space weather issues

-

-

-\

\ \.

\

a” -*al -

DE-2

AL-I- (km)

WIT (deg) MLT

945

80.6 17.0

919

66.1 19.3

Fig. 20. Comparison of magnetic perturbations observed by DE 2 along the satellite track with that produced by the AMIE technique. The A& and AB, are, respectively, (dipole) south and east components of the magnetic disturbance field in nT. From Kamide et al. (1994).

1544

R. W. Schunk and J. J. Sojka

relatively smooth ionospheric conductivity distributions are adopted in the simulations, the resulting convection patterns tend to resemble the statistical convection patterns at a given instant of time, with localized distortions in the pattern coinciding with ingested ground-based radar or satellite data. Also, regardless of what data are ingested, the current/convection boundaries tend to be smooth owing to the adopted conductivity distributions. This is clearly shown in Fig. 20 where the AMIE technique, based on multi-site magnetometer data, was compared with DE 2 satellite measurements for a substorm period (Kamide et al., 1994). The figure shows a comparison of the magnetic perturbations observed by DE 2 with those produced by the AMIE modeling along a DE 2 track. Note that the DE 2 data were not ingested into the model and that the AMIE results are due to ground-based magnetometer and radar data. It is clear that without radar or satellite measurements the AMIE technique sannot properly describe the convection boundaries. 3.6. Conjugacy issues An important issue concerning ionosphere-magnetosphere coupling is whether or not the convection and precipitation patterns are the same (or similar) in both hemispheres. In a general sense it is known that for southward IMF if you have enhanced convection in the morning cell in the northern hemisphere then the evening cell has enhanced convection in the southern hemisphere (i.e. the BYdependence is reversed). Likewise, for northward IMF, sun-aligned polar cap arcs were shown to be conjugate by Mizera et al. (1987) using the NOAA-7 and DMSP-F6 satellites and by Obara et al. (1988) using data from the EXOSC and Viking satellites. However, at solstice, the convection pattern in the sunlit hemisphere is frequently observed to be strong and well established, while in the dark hemisphere it is weak and disorganized. Also, the sun-aligned arc studies mentioned above were conducted at the times when the arcs were bright and stable. At other times the electric fields appear to be turbulent and both arcs and diffuse glows exist simultaneously. Therefore, additional detailed studies are needed to properly address the issue of conjugacy. 3.7. Convection andprecipitation

boundaries

When convection and precipitation patterns are used as inputs to ionospheric and thermospheric models, an important issue is how to overlay the patterns so that the respective boundaries match up. One of the problems relates to the identification of boundaries in the aurora1 precipitation pattern. Specifically,

Geographic latitude Fig. 21. Counts in selected channels of the SSJ/3 detector showing the variation in aurora1 zone boundary location with electron energy. From Gussenhoven et al. (1981).

the precipitation boundaries are a function of electron energy. Gussenhoven et al. (1981) generalized this by integrating over the entire downward flux and used J,,, > 10’ /(cm2 sr s) as their definition of both equatorward and poleward boundaries of the aurora1 regions. However, this single definition of a boundary is only an average. For the energy range used in the Gussenhoven et al. (198 1) study, this boundary ranged over a 2.5” latitude range for energies extending from 50 to 5500 eV. Figure 21 shows this extended range of boundaries as a function of electron energy from the aurora1 dusk sector as observed by the DMSP/FZ satellite. At each energy, the equatorward boundary of the aurora1 region can readily be identified by a factor of 10 increase in the electron number flux (counts) over a range of 0.1 to 1.O” in latitude. At 50 eV, the electron number flux across the boundary

Ionosphere-thermosphere AE-E MAR-APR

space weather

1978-79

issues

1545

Kp < 3

MAY-AUG

SEP-OCT

c ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ r ’ I ‘I

DO

04

08

12

16

20

24

0IO SOLAR

LOCAL

04

06

12

16

20

24

TIME

Fig. 22. Empirical model of vertical plasma drifts in four longitude sectors and for three seasons. The results arc fcr low magnetic activity and moderate to high solar activity. Also shown arc the seasonal Jicamarca drift patterns for similar solar flux and geomagnetic conditions. From Fejer et al. (1995).

cm* sr s), ranges from N lo7 to lo9 electrons/(keV while at 5500 eV, the electron number flux ranges from 3 x lo4 to 3 x lo6 electrons/(keV cm’ sr s). The peak fluxes at both energies can be as much as a factor of 100 greater than the larger of these fluxes. 3.8. Dynamo andpenetration electric$elds At low latitudes, dynamo electric fields generated by thermospheric winds have a pronounced effect on the equatorial ionosphere. During the day, the dynamo electric field is eastward, which induces an upward plasma drift, while the reverse is true at night. a comRecently, Fejer er’ al. (1995) conducted prehensive study of the vertical plasma drifts (zonal electric fields) measured by the Atmosphere Explorer E satellite at low 1,atitudes and then constructed an empirical model which describes the observed variations. The model includes diurnal, seasonal, solar cycle, and longitudinal dependencies and is a very useful input for numerical modeling of the ionosphere and thermosphere. Figure 22 shows the vertical plasma drifts obtained from this equatorial electric field model in four longitude sectors and for three seasonal periods. As noted above, the vertical plasma drifts are upward during the day and downward at night, with typical magnitudes in the range of from

l&30 m/s. A feature that is evident in most longitude sectors and seasons is the prereversal enhancement in the upward plasma drift near dusk (N 18 local time). This feature is linked to equatorial spread F. When magnetic activity changes rapidly, such as occurs during storms and substorms, disturbance electric fields appear in the equatorial region (Blanc and Richmond, 1980; Spiro et al., 1988; Fejer and Scherliess, 1995). The disturbance electric fields result from the prompt penetration of magnetospheric electric fields from high- to low-latitudes and from the dynamo action of storm-generated neutral winds. The direct penetration electric fields have a lifetime of about 1h. The disturbance dynamo (wind-driven) electric fields have a longer lifetime and amplitudes proportional to the energy input into the ionospherethermosphere system at high latitudes.

4. WEATHER

MODELING

In order to model the ionosphere-thermosphere system with a physical model, it is necessary to prescribe global distributions of the magnetospheric particle precipitation and electric fields. These inputs must be specified as a function of time during changing geomagnetic activity. Since such extensive data sets

1546

R. W. Schunk and J. J. Sojka

are rarely available, it is not surprising that by far the bulk of the numerical modeling efforts have been based on ‘empirical’ or ‘statistical’ model inputs. Typically, the empirical models are based on data collected over an extended period of time from many groundbased sites, rocket flights, or satellites. The data are synthesized, binned with the aid of appropriate indices, and then fitted with simple analytical expressions. The empirical models, therefore, represent ‘average’ not instantaneous magnetospheric conditions, pictures, and the ionosphere-thermosphere simulations based on them describe the climatology of the system. With regard to southward IMF, literally hundreds of ionospheric, thermospheric, and coupled ionospheric-thermospheric simulations have been conducted using the various empirical models of plasma convection and particle precipitation that are available (cf. Schunk, 1988a, b; Sojka, 1989; Sojka and Schunk, 1995; Roble et al., 1988; Fuller-Rowe11 et al., 1984; and references therein). Simulations have been conducted for different solar cycle, seasonal, magnetic activity, and IMF conditions. However, when ‘empirical’ or ‘statistical’ models are used to describe the plasma convection and particle precipitation patterns, only large-scale ionospheric and thermospheric features can be modeled (i.e. the climatology). Nevertheless, these simulations have been very successful in describing all of the observed large-scale ionospheric features, such as the enhanced dayside densities, the enhanced aurora1 densities, the tongue of ionization, the polar hole, the main electron density trough, and the Appleton anomaly. In comparison to the southward IMF case, relatively little attention has been given to simulating the ionospheric response to northward IMF convection and precipitation patterns. Sojka and Schunk (1987) calculated the ionospheric response to 2-, 3-, and 4cell convection patterns, and Maynard et al. (1990) calculated the response to distorted 2-cell convection patterns. Although only a limited number of geophysical cases were considered, the simulations clearly showed that the ionosphere is sensitive to the different convection patterns and that unique ionospheric features are associated with each pattern. Therefore, since the northward IMF convection and precipitation patterns have not been clearly established, it is currently not possible to determine the climatology of the ionosphere-thermosphere system for this direction of the IMF. The rest of this section will focus on the status of weather modeling. This involves the modeling pertaining to both mesoscale and smaller scale density

structures modeling.

as well as geomagnetic

storm and substorm

4.1. Propagating plasma patches Several mechanisms have been proposed to explain the appearance of plasma patches. Kelley et al. (1982) suggested that ionization from soft electron precipitation can create patches. Anderson et al. (1988) proposed that the sudden expansion and then contraction of the plasma convection pattern can create patches. When the convection pattern expands, high density plasma from the sunlit ionosphere is transported through the cusp and into the polar cap. When the convection pattern contracts, high densities no longer flow into the polar cap and those already there become isolated, forming a plasma patch. This basic idea was extended by Lockwood and Carlson (1992), who proposed that flux transfer events in the cusp cause short-lived enhanced flows that transport patches of plasma into the polar cap. Time-dependent changes in the IMF BY-component have also been proposed as a mechanism for creating plasma patches (Sojka et al., 1993, 1994a; Decker et al., 1994). With this mechanism, the tongue of ionization that normally extends from latitudes equatorward of the cusp and into the polar cap is broken into patches as the convection throat moves in response to Bv changes. Rodger et al. (1994) provide measurements that By changes are important for creating indicate patches, but they propose that electron precipitation and a short-lived flow channel event (FCE) are also necessary for patch formation. Specifically, they suggest that electron precipitation in the cusp causes enhanced electron densities throughout the cusp region (Fig. 23). At some instant, a short-lived, latitudinally narrow, convection channel appears that is extended in the east-west direction and that contains large drift speeds. The enhanced drifts cause a rapid depletion of the plasma in the channel (cf. Schunk et al., 1975). Subsequently, By changes direction, and both the low density region associated with the fossil FCE and the high density region just poleward of it convect into the polar cap, forming a plasma patch. A flow channel event, or fast plasma jet, was also invoked by Valladares et al. (1994b) to explain patch formation. Their measurements indicated that a tongue of ionization extended through the cusp and into the polar cap. Then, a fast plasma jet appeared that was similar to that shown in Fig. 23. The plasma jet was latitudinally narrow (300 km), extended in the east-west direction (2000 km), and contained eastward velocities in excess of 2 km/s. The plasma jet was

Ionosphere-thermosphere space weather issues

t = 0 By negative

1547

\

t=bmikBvnO

b F‘\

t = 3 mins -&& -‘!;$

/ t=9minsBvzO

By negative

Cusp precipitation hence high &

%

Region of low Ne

\

??

~~~~~~~nNe’

Fig. 23. Schematic diagram showing how a plasma patch can be formed via a flow channel event (plasma jet) in the cusp in combination with a IMF BYchange. From Rodger et al. (1994).

located just poleward of the cusp and perpendicular to the tongue of ionization (Fig. 24). The jet caused a rapid density depletion and acted to break the tongue of ionization into patches. The appearance of the plasma jet coincided with a change in the BY component of the IMF. With regard to modeling, only a couple of the proposed patch formation mechanisms have been simulated. Anderson et al. (1988) showed that a sudden expansion and contraction of the convection pattern can indeed create plasma patches, and model studies were conducted to show the effect of a time-dependent variation of BY(Sollka et al., 1993, 1994a; Decker et al., 1994). In the study by Sojka et al. (1993), the USU ionospheric model was used to determine whether or not time-varying electric fields can create plasma patches. This work required a very fine spatial grid (37 x 39 points) in the polar cap in order to resolve patch features, as shown in Fig. 25. In the simulation, the IMF was assumed to be southward, a Heppner and Maynard (1987) 2-cell convection pattern for B,, negative was adopted, and then ionospheric densities were calculated for diurnally reproducible conditions in winter. Subsequently, the convection pattern was changed and the efixt on the densities was observed. The ‘A’convection pattern was used initially to obtain the diurnally reproducible densities, and then at 21 .OO UT the pattern was changed to the ‘DE’ pattern and one-half hour later it was changed back to the ‘A’ pattern. Both of these convection patterns are for

BYnegative, but for the ‘A’ pattern the antisunward convection is fairly uniform at about 500 m/s (Fig. 25). In contrast, the ‘DE’ pattern displays a sharp asymmetry in the polar cap with enhanced convection (1 km/s) in the dusk sector and reduced convection (< 200 m/s) in the dawn sector. When the diurnally reproducible densities were obtained with the ‘A’pattern, a fairly uniform tongue of ionization extended across the polar cap from the dayside to the nightside. However, after the convection pattern changes to the ‘DE’ pattern at 21.00 UT (Fig. 26, panel l), a segment of the tongue breaks off, forming a plasma patch, which then convects across the dark polar cap toward midnight. At 21.30 UT (panel 5), the convection pattern reverts back to the ‘A’pattern and a new uniform tongue of ionization starts to form near noon. However, the detached plasma patch from the original tongue continues to convect across the dark polar cap. Simulations have also been conducted of the effect that propagating plasma patches have on the thermosphere (Ma and Schunk, 1995). In the simulations, 2-cell plasma convection and particle precipitation patterns were initially adopted that were appropriate for quiet geomagnetic activity and then diurnally reproducible neutral densities, temperatures, and winds were calculated that were consistent with these magnetospheric drivers. Subsequently, at a selected time (t = 0), a plasma patch was created in the cusp vicinity and simultaneously the aurora1 precipitation

R. W. Schunk and J. J. Sojka

1548 DISTANCE (km)

SOUTH iz

I

200

400

NORTH 600

SOUTH

DISTANCE (km)

NORTH

*

5

Fig. 24. Schematic diagram showing how a plasma patch can be formed by the sudden appearance of a plasma jet just poleward of the cusp that breaks the tongue of ionization into a patch. From Valladares et al. (1994b).

and plasma convection speeds were increased to simulate a southward turning of the IMF. The plasma

patch then convected antisunward across the dark (southern hemisphere, winter) polar cap. The simulation results in Fig. 27 correspond to the thermospheric response at t = 1 h. The model predicts that the propagating patch acts as a collisional snowplough, creating a build-up of neutrals at the front of the patch and a decreased neutral density both in and behind the plasma patch. The neutral disturbance that is induced by, and moves along with, the plasma patch is also characterized by a temperature enhancement, an increased wind speed, neutral gas upwelling, and O/N2 composition changes. At later times, the patchinduced disturbance spreads out over a region that is much larger than the patch dimensions (not shown). 4.2. Boundary and aurora1 blobs Aurora1 blobs are thought to be produced by nonuniform particle precipitation in the oval. Indeed, the measurements in Fig. 2 show that a significant ionization enhancement in both the E- and F-regions can occur within 10 min after precipitation commences. After the precipitation ceases, the Eregion ionization decays rapidly, leaving an aurora1 blob in the F-region. This picture of aurora1 blob creation is also consistent with ionospheric simulations (Sojka and Schunk, 1986). Boundary blobs appear to be polar cap patches that have convected through the nightside aurora1 oval and around toward dusk. That this is the most likely explanation was first suggested by Robinson et al. (1985). They performed a trajectory calculation using a 2-cell convection pattern obtained from the Heelis et al. (1982) model. Specifically, they defined a circular region in the dusk convection cell and then followed the distortion of the circle as the elements inside the

circle moved along the convection trajectories. As shown in Fig. 28, after 3 h the circle becomes an extremely elongated structure in longitude and very narrow in latitude, and its location coincides with the equatorward edge of the aurora1 oval. Note that Robinson et al. (1985) did not perform density calculations, but simply showed how a region can deform in response to plasma convection. However, Anderson et al. (1996) have recently performed ionospheric simulations and showed how plasma patches in the polar cap evolve into boundary blobs. Specifically, the authors generated patches by varying the B_vcomponent of the IMF, as shown in Fig. 26, and then followed their subsequent temporal evolution. As suggested by Robinson et al. (1985), the patches became elongated structures on the equatorward edge of the aurora1 oval. A latitudinal cross section of these structures yields the characteristic signature of boundary blobs, as shown in Fig. 29. 4.3. Sun-aligned polar cap arcs The modeling concerning sun-aligned polar cap arcs has been focused on two important aspects: the arc electrodynamics and the effect of arcs on the ionosphere. The electrodynamic studies were based on the model developed by Zhu et al. (1993). This model is a 2-dimensional time-dependent model in which the electrodynamics of the arc is treated self-consistently in the coupled magnetosphere-ionosphere system. The physical processes that occur in a given model run are as follows. A magnetospheric shear flow is assumed and the information, carried by an AlfvCn wave, propagates toward the ionosphere, which is characterized by simple background conductivity and convection patterns. The downward propagating AlfvCn wave can be partially reflected from the ionosphere and can bounce back and forth between the

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issues

Solar Max Medium Kp Northern Hemisphere

oooo

Fig. 25. A segment of the spatial grid used in the high-resolution plasma patch simulation flow directions in the grid segment for the Heppner-Maynard ‘A’ and ‘DE’ convection side). From Sojka et al. (1993).

5.0

5.1

5.2

5.3

5.5 Log,0

Fig. 26. N,,,F, distributions spatial grid of Fig. 25. The separated by 7.5 min. Prior existed, and

5.6

5.7

(left dial). The patterns (right

5.8

N,Fz

at selected times during the plasma patch simulation. N,_Fz is shown only in the panels labeled l-8 correspond to successive snapshots starting at 21 .OOUT and to panel 1, the ‘A’ convection pattern existed, for panels 14 the ‘DE’ pattern for panels 5-8 the ‘A’ pattern existed. From Sojka et al. (1993).

Ionospherethermosphere

$=

1~/------\

ATbatch)

$=

Fig. 27. Contours of the neutral mass density (top), the enhancement in neutral temperature (middle), and the increase in the wind speed (bottom) due to the presence of the patch. The circled area in the top panel shows the patch location. The time is I h after the appearance of the patch. From Ma and Schunk (1995).

space weather issues

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ionosphere and magnetosphere. Meanwhile, the upward field-aligned current associated with the AlfvCn wave enhances the conductance in the ionosphere, and the conductance change can launch a secondary Alfvtn wave toward the magnetosphere. The entire process is transient, during which all parameters in the ionosphere change in time self-consistently and the polar cap arc develops. Due to the finite conductivity in the ionosphere, the temporal change of the Alfvtn wave in the coupled M-I system decays with time, and the polar cap arc approaches an asymptotic state. The Zhu et al. (1993) model has been used to study various aspects of arc electrodynamics, including the time constant for arc formation, the occurrence frequency of and the spacing between multiple arcs, the effect of different background convection and conductance patterns on arc dynamics, and the effect of the arc precipitation characteristics on its development (Zhu et al., 1994a, b, 1995; Sojka et al., 1994c). Perhaps the most significant result obtained from these simulations is that multiple polar cap arcs may not be due to multiple shears in the magnetosphere, but instead may result from the active role the ionosphere plays in the electrodynamical coupling between the ionosphere and magnetosphere. It was found that an initial single arc tends to split into multiple arcs owing to conductivity changes in the ionosphere. It was also found that the occurrence of multiple arcs has a strong dependence on the magnitude of the large-scale background convection. Multiple polar cap arcs are more likely to occur when the large-scale convection field in the polar cap is greater than 20 mV/m and the background Hall conductance is in the range of from 0.5 to 2 mho. When the large-scale convection is weak and the ionospheric conductance is high, a single arc is more likely to form in the polar cap. When the multiple arcs occur, the simulations indicate that the spacing between them is mainly determined by the hardness of the arc electron precipitation. A harder precipitation causes a wider spacing between individual arcs. The edge-to-edge spacing between arcs is predicted to vary from about 20 km to about 60 km when the ratio of the Hall-to-Pedersen conductances in the arc varies from 1 to 2. Recently, the theoretical predictions concerning multiple polar cap arcs have been verified (Zhu et al., 1996). The model predictions were compared with multiple arc dynamics observed using the All-Sky Intensified Photometer (ASIP) located near Qaanaaq, Greenland. Figure 30 shows a sequence of all-sky images starting at OS.22 UT on November 12, 1990. The four images, which are shown at a 2-min interval, show the formation and development of a pair of

R. W. Schunk

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and J. J. Sojka 12

12

18

06 18

06

18

0818

08

0

0

Fig. 28. Distortion of a circular blob of ionization as it convects from the polar cap through and around the amoral oval toward dusk. A Heelis et al. (1982) 2-cell convection pattern was used. From Robinson et al. (1985).

Log

o+

L-z

5.2 5.4 5.6 5.8 203OlJT223OMLT

D

Lag

o+

4 4.4 4.8 5.2 5.6

8

:* .E!* P a

i

w-

I/L 6s

70

75

80

CGM Latih&

Fig. 29. Snapshot ofthe O+ density distribution at 21.30 UT. The snapshot corresponds to a latitude-altitude slice through a 3-D density distribution along the 22.30 MLT meridian. The two boundary blobs shown were previously polar cap patches. From Anderson et al. (1996).

sun-aligned polar cap arcs. Also shown in Fig. 30 is the evolution of the field-aligned current, as predicted by the Zhu et al. (1996) model for the conditions relevant to the observations. The all-sky images indicate that polar cap arcs start to appear at about 08.22 UT and that a steady state is reached in about 6 min. The images also indicate that the arcs are structured and have an edge-to-edge spacing of about 40 km. The corresponding model predictions are in good agreement with the measurements. The information gained from the electrodynamic studies about arc convection and precipitation features was the basis for a series of ionospheric model simulations aimed at elucidating the ionospheric perturbations associated with sun-aligned arcs (Crain ef al., 1993, 1994). In the simulations, the key arc parameters were varied, including the width, the electric field structure, and the precipitation energy flux and characteristic energy. Figure 31 shows how the electrostatic potential, electric field, and electron precipitation varied across the sun-aligned arc for the simulations. Note that this variation is consistent with the available measurements (cf. Valladares and Carl-

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1553 0

I

Arc

Potential WV)

2

40 20

El&Jc

0

W/d

_20

-40

-150

Fig. 30. A sequence of images (left column) showing polar cap arcs starting at 08.22 UT on November 12, 1990, and snapshots of the corresponding field-aligned current distribution predicted by an electrodynamic model (right column). The snapshots and images are shown at 2-min intervals. The dashed lines indicate upward field-aligned currents. The ionospheric dimensions of the images are about 1430 km in the noon-midnight (x) direction and about 1165 km in the dawn-dusk I(Y)direction. From Zhu et al. (1996).

son, 1991). Also note that there are oppositely directed electric fields in the arc. On the left side of the arc, the arc electric field induces a sunward drift, while on the right side it induces an antisunward drift. When this arc electric field structure is added to the background convection pattern, the resulting plasma motion is shown in panel d of Fig. 3 1. The main conclusions of the arc studies were that the ionospheric response to sun-aligned arcs is nonlinear, with the largest ,modifications occurring for intermediate arc widths and electric field strengths, and that the E- and F-region responses are very different. Additionally, it was found that as the ionospheric plasma drifts into, across, and then out of a sunaligned arc, it is modified in a nonuniform manner in response to the production and heating in the arc (Fig. 32). The ionospheric: modification is characterized by enhanced E-region densities within the precipitation

.

0

. ,;o

Dusk-Dawn

Distance

(km)

Test Trajectory through Arc (not

to scale)

Fig. 3 1. Dawn-dusk variation of a model sun-aligned polar cap arc. The horizontal axis represents spatial size in km. The panels show potential (a), electric field (b), precipitation energy flux (c), and plasma motion through the arc (d). From Crain et al. (1993).

200 loo

0

1

2

3

4

Elapsed time (hr)

Fig. 32. Electron density distribution across a sun-aligned arc. The plasma drifts across the arc from left to right. The calculations were for winter, solar minimum, an arc width of 200 km, an arc electric field of 40 mV/m, and precipitation energy flux and characteristic energy of 1 erg/c&/s and 0.5 keV, respectively. From Crain et al. (1993).

1554

R. W. Schunk and J. J. Sojka

region, enhanced F-region densities due to production from the soft component of precipitation and to upward diffusion from the lower ionosphere, and enhanced topside densities due to increased scale heights associated with the ion and electron heating in the arc. As the flux tube convects out of the arc, the E-region densities decrease rapidly due to the fast recombination of the molecular ions. However, the Fregion density actually increases as the flux tube first leaves the arc due to downward diffusion from the topside ionosphere, which is in response to the decrease in T, and TP Subsequently, the F-region density decays slowly due to the relatively slow Of recombination rate. This produces the distinctive ‘candle flame in the wind’ in the contour plot of Ne, with cross arc convection corresponding to the wind. However, it should be emphasized that the density variation shown in Fig. 32 is that obtained by following a single convecting flux tube of plasma. Outside the arc, the flux tube moves in a direction parallel to the arc as well as perpendicular to it, with the parallel motion greater than the perpendicular motion.

4.4. Ionospheric disturbance due to convection vortices Ground-based magnetometers have observed the signatures of traveling convection twin vortices (Friiset al., 1988; Heikkila et al., 1989; Christensen McHenry et al., 1990; Kivelson and Southward, 1991; Glassmeier and Heppner, 1992). The twin vortices correspond to spatially localized, transient, convection cells embedded in a large-scale convection pattern. The vortices are characterized by enhanced electric fields, particle precipitation, and an upward/downward field-aligned current pair. They are typically observed in the prenoon and postnoon sectors at geomagnetic latitudes between 6G-75” and are aligned predominantly in the east-west direction. The E-W extent of a vortex structure ranges from several 100’s to 1000 km and its N-S extent is about 500 km. Although the field-aligned currents are opposite in a given twin vortex, either polarity is possible. The magnitude of the current is estimated to be about 1 mA/m’ and the characteristic energy of the precipitation in the upward current cell is about 1 keV. Once formed, the twin vortices propagate toward the nightside at speeds of from 3 to 15 km/s, but they weaken as they propagate and only last for about l& 20 min. Although single vortex structures are more common, a continuous series of traveling twin vortices was observed to move in the antisunward direction for a period of several hours with about a 15-min time interval between vortices. Schunk et al. (1994) calculated the ionospheric

response to traveling convection vortices for both summer and winter conditions. In their simulations, the vortex structure initially had a maximum convection electric field of about 100 mV/m (initial circulation speeds of about 2 km/s), but as the twin vortex structure moved toward the nightside at a speed of 3 km/s, its intensity decreased and it disappeared after approximately 15 min. The electron precipitation associated with the vortex structure also decreased, from an initial energy flux of 2.3 ergs/cm*/s, as the vortex structure propagated. The simulation indicated that the traveling convection twin vortex had its greatest effect on the ionosphere at altitudes between 14&300 km, with the maximum effect occurring near 200 km (Fig. 33). The effect was largest during the first 68 min after the appearance of the vortex. In response to the enhanced vortex electric fields, T, initially increased to about 4000 K in both convection cells due to ion-neutral frictional heating. Associated with the increased Tts was a substantial O+ -+ NO+ composition change owing to the energy dependence of this reaction. Initially, n(O+) was decreased by a factor of 10 and n(NO+) was increased by a factor of 2.5. Like Ti, T, was also elevated in the twin vortex location, but only by about 300 K. Unlike T,, however, the T, enhancement was not symmetric. T, was increased due to both coupling to the hot ions and energy transfer from the precipitating electrons. The latter mechanism dominated, and since it only existed in the dayside cell (upward current region), the T, enhancement was asymmetric. The elevated ion and electron temperatures also resulted in plasma upwelling events, which, in turn, led to changes in topside plasma scale heights. The main conclusion from this study was that there is an appreciable ionospheric response to traveling convection vortices and that measurements of the response should help in elucidating ionosphere-magnetosphere coupling processes.

4.5. Lifetime

of density structures

Once an ionospheric density structure is created in the high-latitude ionosphere, it is important to know how long it will last and where it will go. To address this issue, Schunk and Sojka (1987) conducted a series of ionospheric simulations in an effort to study the lifetime and transport characteristics of ‘large’ plasma density structures (factors of l&-100). In this study, a density structure was created at a specific location in the high-latitude F-region and the subsequent evolution was followed for different seasonal and solar cycle conditions as well as for different orientations of the IMF, i.e. different convection patterns. However,

Ionosphere-thermosphere

4.2

4.4

4.6 Loglo

4.n

5.0

space weather issues

3.2

cm.”

Fig. 33. Schematic of a traveling convection vortex on the dawn side of the polar cap (top panel). Snapshots of the O+ and NO+ densities, and the ion and electron temperatures, at 220 km (bottom panel). The snapshots are shown at 1, 3, and 6 min after a traveling convection vortex impacts the polar ionosphere. The simulation results are for moderate solar activity and summer conditions. From Schunk et al. (1994).

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Ionosphere-thermosphere 12

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space weather issues 12

6

24

24

Fig. 34. Plasma drift trajectories in different coordinate systems for a Volland (1978) convection pattern with corotation added. The convection pattern is a symmetric two-cell pattern with a cross polar cap potential of 62 kV. The left panel shows the drift trajectories in a magnetic latitude-MLT reference frame. Also shown in this panel is a Spiro et al. (1982) amoral oval for Kp = 3 and a selected ‘test’ trajectory (solid curve). The right panel shows the path of a plasma flux tube in the geographic inertial frame for 3.5 traversals of the test trajectory. Also shown in the right panel are the terminator locations for summer and winter solstice. From Schunk and Sojka (1987). only two ‘structure’ Locations were considered, one on the dayside and one in the dusk sector. For the simulations involving the dayside structure location, a standard two-cell convection pattern was adopted. The pattern, which is shown in the left panel of Fig. 34, corresponds to a symmetric, two-cell configuration of the Volland (1978) type with corotation added. This pattern is representative of moderate geomagnetic activity with K, = 3 and a total cross polar cap potential of 62 kV. In the polar cap, the electric field is about 17 mV/m and the corresponding antisunward convection speed is about 300 m/s. The solid line is an adopted ‘test’ trajectory, which was used to follow convecting plasma flux tubes in different reference frames. In the right panel, this test trajectory is followed for 3 l/2 traversals in a geographic inertial frame starting on the dayside between 10.00 and 11.00 local time (shown by the dot in the left panel). Three complete traversals of the test trajectory take about 27 h. Although the 3 l/2 trajectory loops overlap in the magnetic frame, they do not overlap in the geographic inertial frame because the convection pattern is fixed in a magnetic frame and the magnetic pole rotates about the geographic pole. If a plasma patch were following this trajectory, it may or may not survive depending on how long it is exposed to ion production due to either aurora1 particle precipitation or solar EUV radiation (the terminators for summer and winter solstice are denoted by dashed lines).

The test trajectory shown in Fig. 34 was followed for 3 l/2 traversals starting on the dayside at the location marked by the dot (285” E longitude). Different seasonal and solar cycle cases were considered as well as different initial density perturbations. A start time of 15.30 UT was selected so that the starting location was sunlit in both summer and winter. As a consequence, the background conditions at the starting location could be obtained from daytime steady state solutions. Figure 35 shows the varifor three ation of N,F, along the test trajectory different density enhancements and for both winter and summer conditions at solar minimum. In each panel, the solid curve corresponds to the natural plasma, while for the dotted and dashed curves the ‘initial’ normal O+ density profile was multiplied by factors of 10 and 100, respectively. The variations of the solar zenith angle and the precipitating aurora1 electron energy flux along the test trajectory are shown at the top and bottom of each panel, respectively. In winter, the flux tube is in darkness most of the time and, therefore, aurora1 precipitation is the only source of plasma along most of the test trajectory. For the natural case, N,,,F, at the starting location is about 5 x 105/cm3, but as the flux tube convects across the polar cap in darkness, N,JZ decreases until it reaches the nocturnal aurora1 oval. Because the test trajectory has a fairly long residence time in the oval, there are distinct N,,,E; enhancements during each of the three traversals of the oval. When the initial O+

R. W. Schunk and J. J. Sojka SOLAR

MINIMUM

WINTER tos x

1

1

0 -1

,”

lifetimes are much shorter in summer than in winter. The patch with a factor of 10 enhancement basically disappears in about 4 h, while for the factor of 100 enhancement, the decay time is about 11 h. The faster patch decay rate in summer is a result of both the change in the atmospheric O/N, density ratio and the greater importance of solar EUV production, which helps to maintain background plasma densities.

I

dE

4.6. Sporadic E and intermediate

layers

0

P 1 1= Od -1 -2 -3 16 24 30 36 42 ‘4

TIME

ALONG

& 5 6

16 24 30 36 42 48

TRAJECTORY

(hr)

Fig. 35. N,,,F2 variation along the test trajectory for three plasma flux tubes in both summer and winter at solar minimum. The flux tubes start on the dayside at the location marked by the dot in Fig. 34. The solid curve corresponds to the background plasma, while for the dotted and dashed curves the initial 0’ density profile was multiplied by factors of 10 and 100, respectively. The solar zenith angle variation along the trajectory is shown at the top, and the log of the aurora1 electron energy flux along the trajectory is shown at the bottom. From Schunk and Sojka (1987).

density profile is multiplied by factors of 10-100 to create density structures (patches), the subsequent plasma decay occurs at the same rate as the background plasma (solid curve) owing to the absence of sunlight. However, this is not true when the patch flux tubes enter the nocturnal aurora1 oval. Even though the patch densities are much greater than the background density when the Ilux tubes first encounter the nocturnal oval, aurora1 precipitation is sufficient to slow the decay of the patches. Therefore, in winter at solar minimum, the lifetime of a patch with a factor of 10 enhancement relative to the background plasma is about 11 h, while for a factor of 100 enhancement the lifetime is about 19 h. In summer, the temporal evolution of the density structures is similar to that in winter, but there are some quantitative differences. First, for the natural case (solid curve) N,F, at the dayside starting location is greater in winter than in summer owing to the wellknown winter anomaly. The difference between summer and winter NmF2 is smaller in the aurora1 oval because the adopted precipitation pattern is the same in summer and winter. With regard to the density structures, the important thing to note is that the

At mid-latitudes, intermediate layers are primarily a result of wind shears connected with the semi-diurnal tide (Whitehead, 1989; Miller et al., 1993; Wilkinson et al., 1992; Osterman et al., 1994, 1995). Between 13&l 80 km, the meridional wind is effective in causing induced upward and downward ion drifts. When the wind blows toward the poles a downward ion drift is induced, whereas when it blows toward the equator an upward ion drift is induced. If the wind changes direction with altitude (a wind shear), the plasma will either diverge and decrease its density or converge and increase its density (layer formation). As a wind shear null moves down in altitude, the ion convergence, and hence intermediate layer, also descend. Although the meridional wind component of tidal motion is the primary mechanism for creating intermediate layers at mid-latitudes, zonal winds, electric fields, and acoustic-gravity waves can affect the dynamics of intermediate layers. Recent modeling of intermediate layers has been conducted by several investigators (Wilkinson et al., 1992; Szuszczewicz, 1995; Szuszczewicz et al., 1995; Osterman et al., 1994, 1995). The modeling has clearly shown that shears in the meridional wind create ion convergence regions where intermediate layers form. This is shown in Fig. 36, where electron density profiles between lOG200 km are shown at selected times throughout the night (Osterman et al., 1994). In the simulations, a meridional wind was applied at 20.00 LT. The wind had a sinusoidal variation with altitude, with an amplitude of 20 m/s, a vertical wavelength of 40 km, and a downward phase velocity of 1.5 m/s. These values were selected to be consistent with the & semidiurnal tide. When the wind was initially imposed, there were convergent wind shear nulls at 183 and 143 km. Subsequently, intermediate layers formed at the convergent nulls and then drifted downwards at a speed consistent with the phase velocity of the neutral wind. Although shears in the meridional wind were shown to be the main mechanism for creating intermediate layers at mid-latitudes, the modeling also showed that electric fields, zonal winds and metallic ions may contribute at times (Wilkinson et

Ionosphere-thermosphere

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1559

*90~ . . ,..... . . .,..J.yhit~

, , .... _

. . ,7

Density (N/CM**3) Fig. 37. Ion density profiles showing the formation of a thin metallic ion layer at 108 km. The profiles are shown 2000 s after the application of a 50 mV/m westward electric field. From Bristow and Watkins (1991).

Totail

Ion

Concentration

(cm

3

Fig. 36. Calculated electron density profiles at selected times after the application Iof a sheared meridional wind. Note the formation of intexmediate layers and their subsequent descent. From Osterman et al. (1994).

al., 1992; Szuszczewicz et al., 1995; Osterman et al., 1995). Sporadic E layers at mid-latitudes are also primarily a result of wind shears, but they can be generated by diurnal and semi-diurnal tides as well as by gravity waves (cf. Whitehead, 1989). At the altitudes where these layers occur (9G120 km), it is the zonal wind that is most effective in inducing vertical ion drifts, which result from a V, x B dynamo action (V, is the zonal wind and B is the geomagnetic field). Therefore, a wind reversal with altitude will yield ion convergence or divergence regions. Ions can be accumulated in the ion convergence regions, but since the molecular ions (NO+, Oz, N:) rapl:dly recombine, it is the long-lived metallic ions that survive and dominate the sporadic E layers. At equatorial latitudes, gradient instabilities play an important role in creating sporadic E layers, while at high latitudes recent modeling has shown that sporadic E layers can be created solely by con-

vection electric fields (Bristow and Watkins, 1991, 1994). An example of a sporadic E layer at high latitudes that was created by electric fields is shown in Fig. 37 (Bristow and Watkins, 1991). In the l-dimensional simulation, a westward electric field of 50 mV/m was applied to the ionosphere and the subsequent evolution of the plasma was modeled. This electric field did not produce a convergent null in the vertical ion velocity, which usually occurs during sporadic E layer formation. However, the ion drift was downward at all altitudes, with a magnitude that decreased with altitude. This situation resulted in an accumulation of ions at low altitudes. As time proceeded, the molecular ions recombined, leaving a relatively thin metallic ion layer at about 108 km. Subsequent 3-dimensional ionospheric simulations have shown that sporadic E layers can form for a wide range of large-scale convection electric field patterns, but the region where they form depends on the specific convection pattern (Bristow and Watkins, 1994). 4.7. Spread F and equatorial bubbles During the last 10 yr, significant progress has been made in elucidating the main mechanisms responsible for equatorial spread F and plasma bubbles (Abdu et al., 1992; Mendillo et al., 1992; Hysell et al., 1990; Tsunoda, 1985). The scenario that has evolved from these and other studies is as follows. During the day, the thermospheric wind produces a dynamo electric

1560

R. W. Schunk

field in the lower ionosphere that is eastward. This Efield is then mapped to F-region altitudes along the geomagnetic field. The eastward E-field in combination with the northward B-field produces an upward E x B drift of the F-region plasma in the equatorial region. As the ionosphere corotates with the Earth toward dusk, the zonal (eastward) component of the neutral wind increases, with the wind blowing predominantly across the terminator from the dayside to the nightside. The increased eastward wind, in combination with the sharp day-night conductivity gradient across the terminator, leads to a prereversal enhancement in the eastward E-field. This, in turn, acts to raise the F layer as the ionosphere rotates into darkness. In the absence of sunlight, the lower ionosphere rapidly decays and a steep vertical density gradient develops on the bottomside of the elevated F layer. This produces the classical configuration for the Rayleigh-Taylor (R-T) instability in which a heavy fluid is situated above a light fluid, being supported by the horizontal geomagnetic field. Depending on the conditions, a density perturbation can trigger the R-T instability on the bottomside F layer. Once triggered, density irregularities develop, and the field-aligned depletions bubble up through the equatorial F layer. The irregular plasma structure associated with these depletions cause spread F radar clutter. However, the F layer height and bottomside density gradient may not be the only conditions necessary for the R-T instability and subsequent spread F. Studies have shown that a meridional wind component, which produces a northsouth density asymmetry along B, can act to stabilize the plasma. On the other hand, upward propagating gravity waves, which induce vertical winds, act to trigger the R-T instability both by providing an initial perturbation and by affecting the instability condition. Also, the R-T instability, and hence spread F, have been shown to display seasonal and longitudinal dependencies, which implies that the integrated conductivity is important. The longitudinal dependence is related to the declination of the B-field (i.e. its deviation from geographic north) and the associated conductivity differences at the two ends of the field line. With regard to modeling, the time-dependent evolution of equatorial spread F has been simulated in a 2-dimensional configuration via a numerical solution of the nonlinear fluid equations. Initial simulations showed that the Rayleigh-Taylor instability does lead to bottomside spread F, which then evolves into plasma bubbles (Scannapieco and Ossakow, 1976). Further simulations showed the dependence of spread F on the F-region peak altitude, the bottomside den-

and J. J. Sojka

sity gradient, zonal winds, and E-region conductivities (Ossakow et al., 1979; Zalesak and Ossakow, 1980; Zalesak et al., 1982). More recently, the effects of vertical winds and electric fields have been simulated (Sekar et al., 1994). Figure 38 shows the results of a typical simulation of the nonlinear evolution of equatorial spread F (Sekar et al., 1994). The simulation domain extends from 250 to 550 km in altitude and from -100 to 100 km in the east-west direction. The magnetic field direction is out of the plane of the diagram. The dotted curves in each plot show the initial electron density profile. The solid curves show the electron density contours at 700 (left panel), 900 (middle panel), and 1100 (right panel) s after the plasma becomes unstable. The top row corresponds to the case when only gravity is included, while for the bottom row both gravity and a downward wind are included. In both cases, a plasma bubble forms on the bottomside of the F layer and then drifts upwards. However, with allowance for a downward wind, the bubble formation is accelerated. 4.8. Geomagnetic storm effects During geomagnetic storms, there is a large energy input into the ionosphere-thermosphere system at high-latitudes. As noted earlier, the plasma convection and precipitation patterns expand, electric field strengths increase, and particle precipitation becomes more intense. In response to these changes, aurora1 Eregion densities increase, dayside high-densities convect into the polar cap at F-region altitudes, polar holes fill in, the main trough moves equatorward, the ion and neutral temperatures increase, ion and neutral composition changes occur, wind speeds increase, and equatorward propagating gravity waves are excited. At mid-latitudes, the equatorward propagating waves drive the F-region ionization toward higher altitudes, which results in an ionization enhancement and positive storm effect, particularly on the dayside. Behind the wave disturbance are enhanced meridional winds. These diverging winds cause upwelling and neutral composition changes, which then lead to a negative storm effect (decreased electron densities). For big storms, the winds and composition changes can penetrate all the way to the magnetic equator, but that is generally not the case. However, the dynamo electric fields generated by the enhanced mid-low latitude winds can affect the equatorial ionosphere. Also, at storm onset penetrating electric fields can directly affect the equatorial plasma. A further, more complete discussion of magnetic storm effects is given by Priilss (1995) and Fuller-Rowe11 et al. (1994).

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~bxl&S’

-60

of

0

60

loo

ZoNAL DISTANCE Mm)

Fig. 38. Numerical simulation of the nonlinear evolution of an equatorial bubble. The dotted curves in each panel correspond to the initial F layer density profile. The solid lines are electron density contours. The left, middle, and right panels correspond to snapshots at 700, 900, and 1100 s, respectively, after instability onset. The top row is the case for gravity only and the bottom row is for both gravity and a downward neutral wind of 20 m/s. From Sekar et al. (1994).

Both regional and global numerical models have been useful tools for elucidating the effects that geomagnetic storms have on the ionosphere-thermosphere system (8ojka and Schunk, 1984; Crowley et al., 1989; Schunk and Sojka, 1989; Burns et al., 1991; Fuller-Rowe11 et al., 1991, 1994; Codrescu et al., 1992; Sojka et al., 1.994b; Richards et al., 1994). In general, the models have produced results that are qualitatively similar to the measured response described in the preceding paragraph. To date, however, the model studies have been limited both in the way the storm wa.s modeled and in the formulation of the model itself. ln most of the studies, empirical convection and precipitation patterns were adopted and these were then driven with an assumed, relatively simple, time-dependent K, (or A& variation that was supposed to represent a geomagnetic storm. However, there have been attempts to use more realistic patterns for both plasma convection and particle precipitation in the global circulation models. Magnetometer and satellite data were trsed with the AMIE technique to obtain time-dependent convection and precipitation

patterns, which were then used in both NCARTGCM (Crowley et al., 1989) and NCAR-TIGCM (Lu et al., 1995) simulations. In some of the studies only the thermospheric response was modeled, while in other studies only the ionospheric response was modeled. Even in the recent fully-coupled global ionospherethermosphere simulations (Fuller-Rowe11 et al., 1994), part of the ionosphere was modeled with an empirical model (the equatorial region), while the rest of the ionosphere was based on a numerical formulation. Despite the limitations, the Fuller-Rowe11 et al. (1994) simulations produced some very impressive results. In the simulations, the storm was characterized by an increase in the magnetospheric energy input at high latitudes for a 12-h period. The elevated neutral temperatures and increased wind speeds at high-latitudes resulted in initial global wind surges from both polar regions that propagated toward the equator with phase speeds of about 600 m/s. Behind the surges were meridional winds that reached 100 m/s at mid-latitudes. The surges were larger on the nightside than on the

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R. W. &hunk and J. J. Sojka

dayside. The divergent wind field produced neutral gas upwelling and composition changes. In response to the increased winds, upwelling, and composition changes, the ionosphere displayed a complicated behavior with both positive and negative storm effects occurring at different locations and times both during and after the storm. Figure 39 shows the increase in the neutral temperature and winds at 24.00 UT and 300 km in the northern polar region that is attributed to the storm. The storm results in a maximum of 400 K increase in T, and a 600 m/s wind increase at this time, which is 12 h after the storm commencement. The temporal response of the ionosphere at 0” longitude is shown in Fig. 40, where the ratio of the stormto-quiet N,,,F2 is plotted vs latitude and UT. Note the large depletions in electron density at high-latitudes in both hemispheres that occur in the recovery phase of the storm. These result from storm-induced neutral composition changes that persist after the storm ceases. Overall, the simulation results are in good agreement with the measurements of storm characteristics.

4.9. Substorm effects To date, there have been very few studies of the effects that substorms have on the ionosphere and thermosphere, primarily because of the lack of electrodynamic measurements with the spatial and temporal resolutions needed for rigorous modeling. However, some simplistic model studies have been conducted that provide clues of what probably happens to the ionosphere and thermosphere during the recovery phase of substorms when SAID events occur. With regard to the ionosphere, the sudden application of a large northward electric field in an extended longitudinal, but narrow latitudinal, domain will result in a channel of rapid westward plasma flow at F-region altitudes. In the channel will be elevated ion temperatures and non-Maxwellian ion velocity distributions due to ion-neutral collisions, rapid O+ -+ NO+ composition changes and ion upflows due to the elevated ion temperatures, and electron density depletions (Schunk et al., 1975, 1976; St.-Maurice and Schunk, 1979; Schunk and Sojka, 1982; Gombosi and Killeen, 1987; Sellek et al., 1991). The effect of a 100 mV/m meridional electric field, with a 15min duration, on the nocturnal ionosphere is shown in Fig. 41. In the simulation, daytime steady state density profiles were initially calculated, and then the photoionization rates were set to zero (at t = 0) for the nocturnal decay part of the simulation. After a 2-h decay period, a 100 mV/m meridional electric field was applied for 15 min. In response to this electric

field, there was a rapid decrease of the O+ density, a buildup of the NO+ density, and a moderate decrease of the electron density. After the application of the electric field, the NO+ density buildup decayed at a fairly rapid rate, nearly returning to its initial value in 15 min (t = 2.30). The thermospheric response to SAID events (or channels of rapidly convecting plasma) was modeled by several authors and these simulations showed that a large thermospheric disturbance can occur both inside and outside the SAID region (St.-Maurice and Schunk, 1981; Fuller-Rowell, 1984; Chang and St.Maurice, 1991). The disturbance moves out of the SAID region via advection, viscous dissipation, and wave propagation. Figure 42 shows the temporal response of the thermospheric wind to the sudden application of a 100 mV/m electric field in a 1” wide convection channel (Fuller-Rowell, 1984). The simulation domain was extended in longitude so that it was actually a 2-dimensional simulation (altitude and latitude). Also, initially the thermosphere was stationary, and the convecting ionosphere had a ‘fixed’ density profile throughout the simulation. Despite these simplifications, the simulations showed that an appreciable acceleration of the thermosphere occurs after only 10 min. It is also apparent that the effects of the plasma convection channel extend well beyond the channel at F-region altitudes. Along with the enhanced winds are elevated neutral temperatures, upwelling, and neutral composition changes (not shown). Simulations of the type shown in Fig. 42 indicate that ionospheric structure at the mesoscale level (lo& 1000 km) can lead to thermospheric structure. Recently, this general conclusion has been verified in careful experimental studies (Eastes et al., 1992; Batten and Rees, 1990; Batten et aI., 1988). In particular, during geomagnetic disturbances and near discrete aurora1 features, the experimentalists found the thermosphere to be highly structured, with spatial scales varying from 50-500 km. They also found that it exhibited fairly rapid temporal variations, with time scales as short as l&30 min. The perturbed wind magnitudes were observed to be larger than predicted and in the range of 300 to 450 m/s. 4.10. Tides and gravity waves Tides and gravity waves play an important role in the dynamics and energetics of the lower thermosphere (Fesen et al., 1991, 1993, 1995; Forbes and Vial, 1989; Forbes et al., 1993, 1995; Fritts and Vincent, 1987; Bristow et al., 1994). These waves can be generated in situ by solar UV and EUV heating as

Ionospherethermosphere

cTIM-DIFF

NEUTRAL TEMPERATURE 12

10

space weather issues

(DEG. K)

1563

~SI

Perim

-

355

lat =

50.0

6

Pressure level 12 al5 Y/S UT 24.0 YlMMUY 08.0.WYUM 4% 1.CoNlwRIlmRvAL 30I 4 Fig. 39. Relative increase in the thermospheric temperatures and winds in the northern polar region at 300 km and 24.00 UT, which is 12 h after storm commencement. The maximum temperature increase exceeds 400 K and winds change by 600 m/s. From Fuller-Rowe11 er al. (1994).

R. W. Schunk

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CTIM-DIFF

and J. J. Sojka

NmF2

grl-s5S

(m-3)

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logarithm

I I, maximum

Values at NmF2 0.0552. contour interval 0.0580

of the storm-to-quiet N,,,F, ratio vs latitude Fuller-Rowe11 et al. (1994).

and UT for 0” longitude.

Ionosphere-thermosphere 6

600

4

space weather

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230 c

500 -400 3 cm 3 a

200 100 0 6

600 500 -400 g 0300 3 a

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Fig. 41. Electron, O+, NO+ and 0: density profiles at selected times during an ionospheric decay. At t = 0, the photoionization rates are set to zero and at f = 2 h the plasma is subjected to a 100 mV/m meridional electric field for 15 min. From Schunk et al. (1976).

well as by temporally varying momentum and energy inputs associated with magnetospheric processes (convection, precipitation, currents). Tides and gravity waves are also generated in the lower atmosphere and then they propagate up to the thermosphere. For example, heating due to the absorption of solar radiation by Hz0 in the troposphere and by O3 in the stratosphere produces upward propagating tides that

can reach the thermosphere. These upward propagating tides and gravity waves have a significant effect on the lower thermosphere, but at present it is difficult to include them in a realistic manner owing to the lack of global measurements of the forcing function. An example of the effect of semidiurnal tides has been presented by Crowley et al. (1989) based on

Neutral wind Grid=3 lat range 68 to 72 re.s=O.2 Height range 70 to 420 km Simulation time=10 mins 68

Neutral wind Grid=3 lat range 68 to 72 11x=0.2 Height range 70 to 420 km Simulation time=60 mins

Height (km)’ 68

70

Fig. 42. Thenaospheric panel) after tlhe sudden 70” magnetic latitude,

velocities in and near a convection channel at 10 min (left panel) and 60 mm (right application of a 100 mV/m electric field in the channel. The channel is centered at is 1” wide (x 100 km), and is extended in longitude. From Fuller-Rowe11 (1984).

R. W. Schunk and J. J. Sojka

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-400

‘300 ‘200 5 E .s 2

-100 -90

-60

-30

0

30

60

90

30

60

90

GROG lat [deg]

‘-90

-60

-30

0 GEOG lat [degl

Fig. 43. Variation of the meridional neutral wind vs altitude and latitude for 70”W at 18 UT on a quiet day. The variation is shown both without (a) and with (b) tidal effects. Solid contours are for winds blowing toward the south and dashed contours correspond to northward winds. The contour interval is 10 m/s. From Crowley et al. (1989).

NCAR thermospheric general circulation model (TGCM) simulations. The simulations were for the September 18-19, 1984 period of the Equinox Transition Study. For the magnetically quiet time period, calculations were performed both with and without the semidiurnal tides. Figure 43 shows the variation of the meridional neutral wind vs altitude and latitude for 70” W at 18 UT. The top panel shows the wind without semidiurnal tides, while the bottom panel shows the wind with tidal effects. It is apparent that semidiurnal tides are very important in the lower thermosphere. There is a complex wind structure below about 300 km, with reversals of the wind direction clearly evident. The semidiurnal tides also have a similar effect on the neutral temperature and densities. The tidal-induced structure in the neutral parameters then affects the ionospheric densities at D- and E-

region altitudes because of the short time constant for chemical reactions. More complete discussions of the sources, propagation characteristics, and effects of tides and gravity waves are given in the above references.

5. SUMMARY Because of the detrimental effects of space weather on both ground-based and space-based systems, a National Space Weather Program has been created. The ultimate goal of the program is to provide a forecasting capability for predicting the duration and intensity of space weather disturbances. To accomplish this goal, it will eventually be necessary to have global models of the various solar-terrestrial domains that are capable of ingesting data from both ground-based and space-based monitoring systems. In the meantime, additional science issues must be addressed in order to fully elucidate the causes and consequences of space weather. With regard to the ionosphere-thermosphere system, the main weather disturbances can be traced to the effects that magnetospheric (convection, precipitation, field-aligned currents) and lower atmospheric (tides, gravity waves) processes have on the system. Therefore, a reliable forecasting capability cannot be achieved without a complete understanding of the spatial and temporal characteristics of these driving processes. In this regard, some of the important unresolved issues are as follows: (1) The shape of the large-scale convection pattern for northward IMF needs to be clearly established. Is it a 3-cell, 4-cell or distorted 2-cell pattern?; (2) the spatial and temporal evolution of the convection and precipitation patterns during storms and substorms needs to be fully elucidated; (3) the relationship between convection, precipitation, and plasma structure boundaries needs to be established, including boundaries associated with the aurora, convection reversals, sun-aligned polar cap arcs, subauroral ion drifts, boundary blobs, and plasma patches; (4) it is important to determine under what conditions features in the northern and southern hemispheres are conjugate; and (5) tides and gravity waves propagating from the lower atmosphere contribute a significant variability to the state of the thermosphere, yet even the climatologies for these processes are not well understood. New ways are required to include these effects in the global models. As far as modeling is concerned, the main challenge will be to self-consistently incorporate both mesoscale and small-scale features in the global models. This will probably require a new generation of models involv-

Ionosphere-thermosphere

space weather

ing nested grid, adaptive grid, and nested model approaches. These models will be needed to further elucidate the creation, transport, and decay of a wide range of ionosphere-thermosphere structures and irregularities. Additional studies are required to determine the effect on the ionosphere-thermosphere system of propagating plasma patches, sun-aligned polar cap arcs, aurora1 arcs, SAID events, traveling convection vortices, flux transfer events in the cusp, sporadic E layers, intermediate layers, spread F, and equatorial plasma bubbles. Finally, it should be noted that this paper was based on a tutorial given at a recent CEDAR meeting. The paper was expanded to include additional review material, but because of a page limitation it was not possible to provide a comprehensive review of all the

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issues. Instead, the paper was written to provide an overview of some of the important scientific issues concerning ionospheric-thermospheric weather, with the emphasis on the ionosphere. A companion tutorial on magnetospheric weather topics is given by Dan Baker, in this issue of the journal. Also, several comprehensive review papers have been written recently that are relevant to space weather, including reviews on thermospheric winds (Titheridge, 1995) ionospheric-thermospheric disturbances during geomagnetic storms (Rees, 1995), ionospheric F-region storms (Prolss, 1995) and geomagnetic storms in general (Gonzalez et al., 1994). Acknowledgements-This research was supported, in part, by NASA grant NAG-5-1484, NSF grant ATM-9308163, and ONR grant N00014-95-1-0652 to Utah State University.

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The effect of interplanetary magnetic field orientation on dayside high-latitude ionospheric convection. J. geophys. Res. 89,2873-2880. A model of the high-latitude ionospheric convection pattern. J. geophys. Res. 87, 6339-6345. Multistation measurements of high-latitude ionosphericconvection. J.geophys. Res. 88,lOll l-10121. A transient aurora1 event on the dayside. J. geophys. Res. 94, 1529 1. Empirical models of high-latitude electric fields. J. geophys. Res. 82, 1115-l 125. Empirical high-latitude electric field models. J. geophys. Res. 92,44674I89. Empirical models for the plasma convection at high latitudes from Millstone Hill observations, J. geophys. Res. 92,203-212. Seeding and layering of equatorial spread F by gravity waves. J. geophys. Res. 95, 17253. HF doppler and ionosonde observations on the onset conditions of equatorial spread F. J. geophys. Res. 98, 13741. Modeling substorm current systems using conductivity distributions inferred from DE aurora1 images. J. geophys. Res. 91, 11235511256. Ground-based studies of ionospheric convection associated with substorm expansion. J. geophys. Res. 99,19451. Simulation of the westward traveling surge and Pi2 pulsations during substorms. J. geophys. Res. 90, 1091 l-10922. On the origin and spatial extent of high latitude F region irregularities. J. geophys. Res. 87,4469. Ionospheric traveling vortex generation by solar wind buffeting of the magnetosphere. J. geophys. Res. 96, 1661-1667. The Global Positioning System and ionospheric conditions. Solar-Terrestrial Predictions IV, 142146. Ionospheric convection response to changing IMF direction. Geophys. Res. Len. 18, 721-724. Interhemispheric comparisons of ionospheric response to the geomagnetic storm of March 20-21 1990. EOS Suppl. 76,261. The production of polar cap electron density patches Geophys. by transient magnetopause reconnection. Res. Left. 19, 1731-1734. Magnetosphere-ionosphere-thermosphere coupling: effect of neutral winds on energy transfer and field-aligned current. J. geophys. Res. 100, 1964319659. A study of ionospheric electron density deviations during two great storms. J. afmos. terr. Phys. 57, 10371043. Effect of polar cap patches on the polar thermosphere, J. Geophys. Res. 100, 19,701-19,713. Magnetospheric convection induced by the positive and negative z component of the interplanetary magnetic field: quantitative analysis using polar cap magnetic records. J. geophys. Res. 81,2289-2303. Snapshots of high-latitude electrodynamics using Viking and DMSP F7 observations. J. geophys. Res. 93,14479-14492. Observations of ion layer motions during the AIDA campaign. J. atmos. terr. Phys. 55,447457.

Ionosphere-thermosphere Maynard N. C., Sojka J. J., Schunk R. W., Heppner J. P. and 13race L. H. McHenry M. A., Clauer C. R. and Friis-Christensen

McPherron

R. L., Russell C. T. and Aubry

space weather 1990

E.

M. P.

1990

1973

Mende S. B., Doolittll: J. H., Robinson R. M., Vondrak R. R. and Rich F. J. Mendillo M., Baumgardner, J., Pi Z., Sultan P. J. and Tsunoda R. Miller N. J., Grebowsky J. M., Hedin A. E. and Spencer N. W.

1988

Mizera P. F., Gorney

1987

D. J. and Evans D. S.

1992 1993

Moses J. J., Siscoe, G. L., Heelis R. A. and Winmngham J. D. Niciejewski R. J., Meriwether J. W. Jr, McCormac F. G., Hecht J. H., Christensen A. B., Sivjee G. G., Stricklland D. J., Swenson G., Mende S. B., Valance-Jones A., Gottinger R. L., Carlson H. C. and Valladares C. E. Obara T., Kitayama M., Mukai T., Kaya N., Murphree J. S. and Cogger L. L.

1988

Ossakow S. L., Zalesak Chatarvedi P. K.

1979

S. T., McDonald

B. E. and

1989

1988

Osterman

G. B., Heelis R. A. and Bailey G. J.

1994

Osterman

G. B., Heelis R. A. and Bailey G. J.

1995

Potemra T. A., Zanetti L. J., Bythrow Lui A. T. Y. and Lijima T.

P. F.,

1984

Prolss G. W

1995

Priilss G. W., Brace L H., Mayr H. G., Carignon G. R., Killeen T. L. and Klabucher Rasmussen C. E. and :&hunk R. W.

1991 J. A. 1988

Rasmussen C. E., Schunk R. W., Sojka J. J., Wickwar V. B.. de la Beauiardiere 0.. Foster J.. Holt J., Evans D. S. and Nielson E. Rees D.

1986

Reiff P. H. and Heelis R. A

1994

Rich F. J. and Denig W. F.

1992

Rich F. J. and Hairston

1994

M.

Richmond A. D., Kamide H., Ahn B.-H., Akasofu S.-I., Alcayde D., Blanc M., de la Beaujardiere O., Evans D. S., Foster J. C., Friis-Christenson E., Fuller-Rowe11 T. J., Holt. J. M., Knipp I)., Kroehl H. W., Lepping R. P., Pellinen R. J., Senior C. and Zaitzev A. P.

1995

1988

issues

1571

A test of convection models for IMF B, north. Planet. Space Sci. 38, 1077. Relationship of solar wind parameters to continuous dayside, high latitude traveling ionospheric convection vortices. J. geophys. Res. 95, 15007. Satellite studies of magnetospheric substorms on August 15,196s. 9. Phenomenalogical model for substorms. J. geophys. Res. 78, 3131-3149. Plasma drifts associated with a system of sun-aligned arcs in the polar cap. J. geophys. Res. 93,256264. Onset conditions for equatorial spread F. J. geophys. Res. 97, 13865. Equatorial ion concentration, 14&200 km, based on Atmosphere Explorer E data. J. geophys. Res. 98, 15685515692. On the conjugacy of the aurora, high and low latitude. Geophys. Res. Left. 14, 19G-193. A model for multiple throat structures in the polar cap flow entry region. J. geophys. Rex 93,9785-9790. Coordinated satellite and ground-based measurements of the energy characteristics of a sun-aligned arc over Sondre Stromtjord. J. geophys. Res. 94,17201-17213.

Simultaneous observations of sun-aligned polar cap arcs in both hemispheres by EXOS-C and Viking. Geophys. Res. Lett. 15, 713-716. Dependence of altitude of F peak and bottomside background electron density gradient scale length. J. geophys. Res. 84, 17. Modeling the formation of intermediate layers at Arecibo latitudes. J. geophys. Res. 99, 11357-l 1365. Effects of zonal winds and metallic ions on the behavior of intermediate layers. J. geophys. Res. 100, 78297838. By-dependent convection patterns during northward interplanetary magnetic field. J. geophys. Res. 89, 9753-9760. Ionospheric F-region storms. In Handbook of Atmospheric Elertrodynamics, Vol. 2, (Edited by Volland H.), pp. 195-248. CRC Press, Boca Raton, FL. Ionospheric storm effects at subauroral latitudes: a case study. J. geophys. Res. 96, 127551288. Ionospheric convection inferred from interplanetary magnetic field-dependent Birkeland currents. J. geophys. Res. 93, 1909-1921. Comparison of simultaneous Chatanika and Millstone Hill observations with ionospheric model predictions. J. geophys. Res. 91, 69866998. Observations and modeling of ionospheric and thermospheric disturbances during major magnetic storms: a review. J. atmos. terr. Phys. 57, 1433-1457. Four cells or two? Are four cells really necessary? J. geophys. Res. 99,3955-3959. The major magnetic storm of March 13-14, 1989 and associated ionospheric effects. Can. J. Phys. 70,51& 525. Large-scale convection patterns observed by DMSP. J. geophys. Res. 99,3827. Mapping electrodynamic features of the high-latitude ionosphere from localized observations: combined incoherent-scatter radar and magnetometer measurements for January 18-19, 1984. J. geophys. Res. 93, 5760-5776.

R. W. Schunk

1512 Richards P. G., Torr D. G., Bounsanto Sipler D. P. Riggin D., Swartz W. E., Providakes Farley D. T.

M. J. and

J. and

R. T., Vickrey

Roble R. G., Forbe J. M. and Marcos

1994

1986

Rino C. L., Livingston R. C., Tsunoda R. T., Robinson R. M., Vickrey J. F., Senior C., Cousins M. D., Owen J. and Klobuchar J. A. Rishbeth H. Robinson R. M., Tsunoda Gverin L.

and J. J. Sojka

J. F. and

F. A.

1983

1991 1985

1987

Roble R. G., Ridley E. C., Richmond A. D. and Dickinson R. E. Rodger A. S., Pinnock M., Dudeney J. R., Baker K. B. and Greenwald R. A. Scannapieco A. J. and Ossakow S. L. Schunk R. W.

1976 1988a

Schunk

R. W.

1988b

Schunk

R. W. and Sojka J. J.

1982

Schunk

R. W. and Sojka J. J.

1987

Schunk

R. W. and Sojka J. J.

1989

Schunk

R. W., Raitt W. J. and Banks P. M.

1975

Schunk

R. W., Banks P. M. and Raitt W. J.

1916

Schunk

R. W., Zhu L. and Sojka J. J.

1994

Sekar R., Susasini

R. and Raghavaro

Sellek R., Bailey G. J., Moffett Anderson P. C. Shapka R.

R.

R. J., Heelis R. A. and

1988 1994

1994

1991 1992

Shen J. S., Swartz W. E., Farley D. T. and Harper R. M. Smiddy M., Kelley M. C., Burke W., Rich F., Sagalyn R., Shuman B., Hays R. and Lai S.

1976

Sojka J. J.

1989

1977

Sojka J. J. and Schunk

R. W.

1984

Sojka J. J. and Schunk

R. W.

1986

Sojka J. J. and Schunk

R. W.

1987

Sojka J. J. and Schunk

R. W.

1995

Ionospheric effects of the March 1990 magnetic storm: comparison of theory and measurement. J. geophys. Res. 99,23359-23365. Radar studies of long-wavelength waves associated with mid-latitude sporadic E layers. J. geophys. Res. 91,801 l-8024. Recent studies of the structure and morphology of auroral zone F region irregularities. Radio Sci. 18, 1167. F-region storms and thermospheric dynamics. J. geomagn. Geoelectr. 43, 513-524. Sources of F region ionization enhancements in the nighttime aurora1 zone. J. geophys. Res. 90, 75337546. Thermospheric dynamics during the March 22, 1979, magnetic storm. 1. Model simulations. J. geophys. Rex 92,6045-6068. A coupled thermosphere/ionosphere general circulation model. Geophys. Res. Left. 15, 1325-1328. A new mechanism for polar patch formation. J. geophys. Res. !99,642556436. Nonlinear equatorial spread F. Geophys. Res. Lett. 3,451. A mathematical model of the middle and high latitude ionosphere. Pure appl. Geophys. 127,255-303. Magnetosphere-ionosphere-thermosphere coupling processes. In Solar-Terrestrial Energy Program (STEP): Major Scientific Problems, pp. 52-l 10. Ion temperature variations in the daytime high-latitude F region. J. geophys. Res. 87, 5169-5183. A theoretical study of the lifetime and transport of large ionospheric density structures. J. geophys. Res. 92, 12343-12351. A three-dimensional time-dependent model of the polar wind. J. geophys. Res. 94,8973-8991. Effect of electric fields on the daytime high-latitude E and F regions. J. geophys. Res. 80,3121-3130. Effects of electric fields and other processes upon the nighttime high-latitude F layer. J. geophys. Res. 81, 3271-3282. Ionospheric response to traveling convection twin vortices Geophys. Res. Lett. 21, 1759-1762. Effects of vertical winds and electric fields in the nonlinear evolution of equatorial spread F. J. geophys. Res. 99,2205-2213. Effects of large zonal plasma drifts on the subauroral ionosphere. J. atmos. terr. Phys. 53, 557-565. Geomagnetic effects on modem pipeline systems. SolarTerrestrial Predictions IV, 163-170. Ionization layers in the nighttime E-region valley above Arecibo. J. geophys. Res. 81, 5517-5526. Intense poleward directed electric fields near the ionospheric projection of the plasmapause. Geophys. Res. Lett. 4, 543. Global scale, physical models of the F region ionosphere. Rev. Geophys. 27,371403. A theoretical F-region study of ion compositional and temperature variations in response- to magnetospheric storm inputs. J. ueophvs. Res. !98,2348-2358. A theoretical study of the production and decay of localized electron density enhancements in the polar ionosphere. J. geophys. Res. 91,3245. Theoretical study of the high-latitude ionosphere’s response to multicell convection patterns. J. geophys. Res. 92,8733-8744. Simulations of high latitude ionospheric climatology. J. atmos. terr. Phys., in press.

Ionosphere-thermosphere Sojka J. J., Rasmussen

C. E. and Schunk

space weather

R. W.

1986

Sojka J. J., Bowline M. D., Schunk R. W., Decker D. T., Valla’dares C. E., Sheehand R., Anderson D. N. and Heelis R. A. Sojka J. J., Bowline M. D. and Schunk R. W.

1993

Sojka J. J., Schunk

1994b

R. W. and Denig W. F.

Sojka J. J., Zhu L., Crain D. J. and Schunk

Spiro R. W., Reiff P. II. and Maher

R. W.

1994a

1994c

L. J.

1982

Spiro R. W., Wolf R. A. and Fejer B. G.

1988

St-Maurice

J.-P. and Schunk

R. W.

1979

St-Maurice

J.-P. and Schunk

R. W.

1981

Szuszczewicz

E. P.

1995

Szuszczewicz

E. P., et al.

1995

Titheridge

J. E.

1995

Tsunoda

R. T.

1985

Tsunoda

R. T.

1988

Valladares

C. E. and Carlson

Valladares

C. E., Carl:ion H. C. and Fukui F.

1994a

Valladares

C. E., Carllson H. C. and Fukui F.

1994b

Volland

H. C. Jr

1991

1978

H.

Weber E. J., Buchau J., Moore J. G., Sharber J. R., Livingston R. C., Winningham J. D. and Reinisch B. W. Weber E. J., Klobuchar J. A., Buchau J., Carlson H. C. Jr., Livingston R. C., de la Beaujardiere G., McCready M., Moore J. G. and Bishop G. J. Weber E. J., Kelley M. C., Ballenthin J. O., Basu S., Carlson H. C., Fleischman J. R., Hardy D. A., Maynard N. C., Pfaff R. F., Rodriguez P., Sheehan R. E. and Smiddy M. Weimer D. R.

1984

Whitehead

1989

J. D.

issues

1573

An interplanetary magnetic field dependent model of the ionospheric convection electric field. J. geophys. 1290. Res. 91,11281-l Modeling polar cap F-region patches using time varying convection. Geophys. Res. Left. 20, 17831786. Patches in the polar ionosphere: UT and seasonal dependence. J. geophys. Res. 99, 14959-14970. The ionospheric response to the sustained high geomagnetic activity during the March 1989 great storm. J. neophys. Res. 99, 21341-21352. Effect of high latitude ionospheric convection on sunaligned polar cap arcs. J. geophys. Res. 99, 88518863. Precipitating electron energy flux and aurora1 zone conductance: an empirical model. J. geophys. Res. 87, 8215-8227. Penetration of high-latitude electric field effects to low latitudes during SUNDIAL 1984. Ann. Geophys. 6, 39-50. Ion velocity distribution in the high latitude ionosphere. Rev. geophys. Space Phys. I7,99-134. Ion-neutral momentum coupling near discrete highlatitude ionospheric features. J. geophys. Res. 86, 11299-11321. Advances in ionospheric physics: roles, relevance and predictions in the system of solar-terrestrial plasmas. Rev. geophys. Suppl., 721-728. Coupling mechanisms in the lower ionospheric-thermospheric system and manifestations in the formation and dynamics of intermediate and descending layers. J. atmos. terr. Phys., 57, 1483-1496. Winds in the ionosphere-a review. J. atmos. terr. Phys. 57,1681&1714. Control of the seasonal longitudinal occurrence of equatorial scintillations by the longitudinal gradient in integrated E region Pedersen conductivity. J. geophys. Res. 90,447. High-latitude F region irregularities: a review and synthesis. Rev. Geophys. 26,7 19-760. The electrodynamic, thermal, and energetic character of intense sun-aligned arcs in the polar cap. J. geophys. Res. 96, 137991400. Interplanetary magnetic field dependency of stable sunaligned polar cap arcs. J. geophys. Res. 99, 62476272. Experimental evidence for the formation and entry of patches into the polar cap. Radio Sci. 29, 167-194. A model of the magnetospheric electric convection field. J. geophys. Res. 83,2695-2699. F layer ionization patches in the polar cap. J. geophys. Res. 89, 1683-1694. and dynamics.

J.

1986

Polar cap F layer patches: structure geophys. Rex 91, 12121-12129.

1989

Rocket measurements within a polar cap arc: plasma, particle, and electric circuit parameters. J. geophys. Res. 94,6692-6712.

1995

Models of high-latitude electric potentials derived with a least error fit of spherical harmonic coefficients. J. geophys. Res. 100, 19595-19607. Recent work on mid-latitude and equatorial sporadicE. J. atmos. terr. Phys. 51,401424.

R. W. Schunk

1574 Wilkinson

P. J., Szuszczewicz

E. P. and Roble R. G.

Yeh K. C., Lin K. H. and Conkright

R. 0.

Yeh K. C., Ma S. Y., Liu K. H. and Conkright Zalesak

S. T. and Ossakow

Zalesak

S. T., Ossakow

1992

1992 R. 0.

1994 1980

S. L.

S. L. and Chaturvedi

and J. J. Sojka

P. K.

1982

Zanetti L. J., Potemra T. A., Erlandson R. E., Bythrow P. F., Anderson B. J., Murphree J. S. and Marklund G. T. Zhu L., Sojka J. J. Schunk R. W. and Crain D. J.

1990

Zhu L., Sojka J. J., Schunk

R. W. and Crain D. J.

1994a

Zhu L., Sojka J. J., Schunk

R. W. and Crain D. J.

1994b

Zhu L., Sojka J. J., Schunk

R. W. and Crain D. J.

1996

1993

Measurements and modeling of intermediate, descending and sporadic layers in the lower ionosphere: results and implications for global-scale ionospherithermospheric studies. Geophys. Res. Lett. 19, 9598. The global behavior of the March 1989 ionospheric storm. Can J. Phys. 70,532-543. Global ionospheric effects of the October 1989 geomagnetic storm. J. geophys. Res. 99,6201-6218. Nonlinear equatorial spread F: spatially large bubbles resulting from large horizontal scale initial perturbation. J. geophys. Res. 85,2131. Nonlinear equatorial spread F: the effect of neutral winds and background Pedersen conductivity. J. geophys. Res. 87, 15 1. Polar region Birkeland current, convection, and aurora for northward interplanetary magnetic field. J. geophys. Res. 95,5825-5833. A time-dependent model of polar cap arcs. J. geophys. Res. 98,6139-6150. Theoretical study of polar cap arcs: time-dependent model and its applications. Radio Sci. 29,283-292. Model study of multiple polar cap arcs: occurrence and spacing. Geophys. Res. Letf. 21, &199652. A model-observation study of multiple polar cap arcs. J. geophys. Res., 101,323-333