Location, spatial scale and motion of radio wave absorption in the cusp-latitude ionosphere observed by imaging riometers

Journnl of Atmospherrc andSuhrr-Terrestrrai

Pergamon

PII: SOO21-9169(96)00072-A

Phwca, Vol. 59. No. 8. pp. 903-924, 1997 Cc‘1997 Elsewer Science Ltd All nghts reserved. Prmted in Great Bntam 13646826/97 $17.00+0.00

Location, spatial scale and motion of radio wave absorption in the cusp-latitude ionosphere observed by imaging riometers M. Nishino,’ H. Yamagishi,*

P. Stauning,3

T. J. Rosenberg4

and J. A. Holtet’

‘Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Japan ; ‘Upper Atmosphere Division, National Institute of Polar Research, Tokyo, Japan; ‘Solar-Terrestrial Physics Division, Danish Meteorological Institute, Copenhagen, Denmark ; ‘Institute for Physical Sciences and Technology, University of Maryland, Washington, U.S.A. ; ‘Department of Physics, University of Oslo, Oslo, Norway (Receiwd injinalform

3 April 1996; accepd

12 April 1996)

Abstract-Characteristic examples of the location, spatial scale and motion of radio wave absorption events in the cusp-latitude ionosphere are obtained from daytime observations on 18 September and 17 October, 1992, by imaging riometers in the Arctic region. One case observed near local magnetic noon at Ny-Alesund, Svalbard (invariant lat. = 76.1”) displays small-scale absorption events of 100-200 km in extent superposed on large-scale absorption features extending at least 700 km in longitude toward the prenoon sector. Many of the small-scale absorption events show quasi-periodic variations with repetition periods of 3-5 min which correlate well with local magnetic variations. Short-lived, impulsive absorption events (l-3 min duration) found among the quasi-periodic variations corresponded to impulsive magnetic variations observed over a wide range of magnetometer stations. Some of these impulsive events showed northward or northeastward motions. This case is interpreted in terms of the variable precipitation of highenergy substorm electrons. Another characteristic case observed in the noon sector at cusp-latitudes is an event of slowly varying absorption intensities associated with magnetic bays in the cusp and polar cap regions during conditions of strongly negative IMF-B, component (B, zz 0). An interesting feature of this event is the observed antisunward motion of the front-like absorption features extending over 700 km in longitude. From these characteristics the slowly varying absorption intensities are interpreted in terms of E-region ionospheric disturbances related to the east-west oriented DPY currents in the cusp and polar cap. (0 1997 Elsevier Science Ltd. All rights reserved

INTRODUCTION

radars operated at Goose Bay and Halley Station, Antarctica (Greenwald et al., 1990). The riometer (relative ionospheric opacity meter) instrument which measures radio wave absorption in the ionosphere was advanced by the development of the imaging riometer system first deployed at South Pole Station (Detrick and Rosenberg, 1990). The imaging riometer uses a two-dimensional antenna array and Butler matrix phasing circuits to form a number of beams. The instrument operates, typically, at a frequency of 38 MHz within the protected band reserved for radio astronomy. Using, for instance, an 8 x 8 element antenna array this technique enables a spatial resolution of about 20 km near the zenith for determination of the structure of absorption regions. In the high-latitude nighttime sector, absorption events appear to be associated mainly with the excess ionization created in the upper D-region (SCrlOO km) by precipitating aurora1 electrons with energies of a few tens of keV (e.g. Penman et al., 1979).

The dynamic features of the cusp/cleft auroras and their relations to the various changes in the solar wind plasma and the interplanetary magnetic field (IMF) provide us information on solar wind-magnetosphere interactions in the dayside magnetosphere. Midday aurora1 breakup events in the cusp/cleft region associated with bursts of electron precipitation from magnetosheath plasma are considered signatures of transient plasma processes at the magnetopause (Sandholt et al., 1994). Another important sensor of solar wind-magnetosphere coupling is the ionospheric plasma convection in the cusp/cleft and polar cap regions. Information on the plasma convection in the dayside magnetosphere can be derived from existing networks of magnetometers (e.g., Friis-Christensen et al., 1985; Stauning et al., 1994; Clauer et al., 1984) and also from backscatter radars like the two conjugate HF 903

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M. Nishino et al

As a result of the generally larger ionization densities the dayside ionosphere offers a wider group of possible absorption processes. The dominant cause of high-latitude absorption events is still considered to be the variable high-energy electron precipitation. Combining the dayside absorption events observed at Sondre Stromfjord, Greenland (inv. lat. ~74”), with geomagnetic data from the magnetometer chain stations in Greenland, it has been shown that disturbances associated with strong convection shears may cause enhanced and strongly variable precipitation of the eastward drifting high-energy electrons accelerated during substorm activity (Stauning et al., 1995a). Various processes other than high-energy particle precipitation may cause enhanced ionospheric radiowave absorption. Stauning (1984) observed a particular class of absorption events in the daytime sector at Sondre Stromfjord (at cusp-latitude) which could be related to the electron collision frequency increases caused by electron temperature enhancements. These electron temperature increases were associated with E-region plasma instabilities generated by strong ionospheric electric fields. Using observations from South Pole Station (inv. lat. z = - 74”) Rosenberg et al. (1993) have recently revealed another class of unusual dayside absorption events. They have suggested that some of the dayside absorption events observable from South Pole are related to poleward drifting patches of high electron densities in the ionospheric F-region. These events may occur near local magnetic noon when the station is located within or poleward of the dayside cusp. In such cases the precipitation of electrons with keV energies or higher could be considered negligible. In this article we report on coordinated observations using imaging riometers operated at three stations in the polar cusp/cap region and one station in the aurora1 zone. This combination allows us to study the dynamical large-scale behaviour of the absorption processes. The map in Fig. 1 depicts the locations of the four imaging riometers. Invariant latitude contours are indicated in the figure. The geographic coordinates, the invariant latitudes, the UT times for local magnetic noon (12 h Eccentric Dipole Time, EDT) for the four stations, and receiving frequencies and the number of beams for the imaging riometers are listed in Table 1. The three stations, Ny-Alesund (NYA), Danmarkshavn (DMH), and Sondre Stromfjord (STF), are aligned at nearly equal invariant cusplatitudes, while Tjornes (TJO) is located within the aurora1 zone. Using the coordinated observations by the imaging riometers at the four stations we have investigated the

location, the spatial scale, and the motion of dayside absorption events occurring at cusp-latitude during two selected days. By comparison with ground-based geomagnetic data from Svalbard and Greenland and IMF data from the IMP-8 satellite, we have investigated the relations of these events to other disturbances in the solar wind and in the dayside magnetosphere and ionosphere.

INSTRUMENTATION

The basic design and the operating principles for the Imaging Riometer for Ionospheric Studies (IRIS) have been explained by Detrick and Rosenberg (1990). Here we outline briefly the IRIS installed at Ny-Alesund which is of great importance to the study. The antenna system at Ny-Alesund consists of a twodimensional array with 8 x 8 elements each made of a half-wavelength 30 MHz dipole. Spacing between the elements is 0.65 wavelength. The dipole elements are oriented in a direction 9” east of geomagnetic north. The reflector wires on the ground are aligned in the same direction. A phasing Butler matrix system combines the antenna signals to produce 64 narrow beams. Figure 2 shows a central cross-section of the antenna beam-patterns calculated from a linear array composed of eight half-wavelength dipole elements (private communication, Gorokhov, 1995). The -3 dB power cone has a half-angle of about 11”. The eight beams closest to zenith are the main beams for the imaging riometer. Their total field-of-view is within about f45”. The other beams further out are grating lobes of relatively weak power. They are only of importance for the outermost riometer beams (mainly the northernmost ones) during events with strong gradients. Figure 3 shows the celestial projections of the -3 dB beam-power contours for all 64 beams. The intensities of cosmic radio noise received in the 64 beams are recorded within a sampling time of 4 s. As part of the data processing 64 quiet-day (QD) levels (stationary background) are produced from the regular daily variations in the intensity data recorded during an interval of about ten days. The absorption values are obtained by subtraction of the measured cosmic noise intensities from the quiet-day levels. The details are described in the technical report by Nishino et al. (1993). The IRIS systems at Danmarkshavn, Sondre Stromfjord and Tjornes are described in the technical reports by Stauning ef al. (I 992) Stauning and Rosenberg (1992), and Yamagishi et al. (1992), respectively. The magnetometer data used in this study have been collected from Ny-Alesund and from the two arrays

Cusp studies from Svalbard

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50”N

0”

2oow

4oow

Fig. 1. Map showing the locations in the Arctic region of the four imaging riometers observatories used for the study. Invariant latitude contours have been indicated

Table 1. List of imaging Geographic longitude latitude

Station Name (Acronym)

Ny-Alesund (NYA) Danmarkshavn Sdr.Stromfjord Tjornes (TJO)

(DMH) (STF)

78.92N 76.77N 66.99N 66.20N

QUASI-PERIODIC

AND

IMPULSIVE

ABSORPTION

Invariant latitude

11.92E 341.37E 309.05E 342.90E

of meridian chain stations, including the MAGIC cap stations, in Greenland (see Fig. 1).

riometer

76.07 77.29 73.51 66.89

ice

EVENT

Introduction Using photometric observations from the two stations in Svalbard (Ny-Alesund and Longyearbyen), the southward and northward expansions of the midday aurora1 oval in response to the southward and northward turnings of magnetosheath magnetic fields have been reported by Sandholt rt ul. (1985). They also observed events of small-scale cusp auroras (01 630.0 nm), lasting a few minutes, and associated with

and the magnetic in the map.

installattons EDT

Receiving

No. of

noon

frequency

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0831 1017 1406 1144

UT UT UT UT

30 38 38 30

MHz MHz MHz MHz

64 64 49 64

local Pi-type magnetic pulsations, moving poleward at midday from south to north within the field of view of Ny-Alesund. These events could, typically, have repetition periods of about 10 min. Sandholt rt al. (1985) suggested that these auroras were related to impulsive penetration of magnetosheath plasma irregularities across the dayside magnetopause. Similarly Kokubun et al. (1988) observed poleward drifts of transient dayside aurora1 forms. These auroras would last for I-3min and move over spatial ranges of 100 to 300 km at ionospheric level. They would frequently show a quasi-periodic repetition on a time scale of several minutes. The poleward drifting auroras were often associated with impulsive vari-

M. Nishino et al.

906 Main

Lobes 4

5

Bean

Grating Lobe for Beam 1

Grati Lobe Beam

Grating Lobe for Beam 2 etc.

Fig. 2. Cross-section of antenna beam-patterns calculated from a linear array composed of eight elements of halfwavelength dipoles. The outermost beams are undesirable grating lobes.

ations in the magnetic H-components and bursts of pulsations in the PC-1 range. Sandholt et al. (1990) carried out a detailed analysis of the midday amoral breakup phenomena by using

combined observations with meridian-scanning photometers, all-sky aurora1 TV camera and the EISCAT radar. The recurrence period (3-l 5 min), the local time distribution (09-17 MLT), and the lifetime (< 10 min) of the dayside aurora1 breakup events were considered to indicate the important roles of transient magnetopause reconnection processes and associated energy and momentum transfers from the solar wind to the ionosphere in the polar cusp and cleft regions. Below we present a typical case of an interval with dayside absorption events observed near the magnetic noon at Ny-Alesund. Similar to the optical aurora1 events the absorption events display large-scale slow variations as well as small-scale quasi-periodic or impulsive variations. We shall compare the absorption events observed at NYA with the ones observed simultaneously at the other three stations and investigate the relationship with ground geomagnetic variations. Absorption

event on 18 September,

1992

Figure 4 shows stack plots presenting the time varying radio wave absorption intensities derived for all

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Fig. 4. Distributed stack plots of the time-varying absorption intensities at NYA (upper panel) and DMH (lower panel) during 06304853 UT on 18 September 1992. Each plot uses values averaged over 128 s. The absorption intensities are plotted on a scale of 1dB/div. Plot positions are arranged according to beam directions with the magnetic northern beams to the top, and the magnetic western beams to the right.

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M. Nishino ez a/

64 beams of the imaging riometers at Ny-Alesund (NYA) and Danmarkshavn (DMH) during 0630 to 0853 UT. The plots are made from absorption values averaged over 128s and distributed in the figure according to the beam directions with beam NIEl, the north-easternmost direction, placed in the upper left corner (all-sky view). The plots for two of the southern beams (N7E4 and N8E4) of the NYA antenna field are contaminated by local interference noise. Two of the beams in the northern part of the sky (NlE4 and NlE5) are disturbed by ionospheric scintillations in the coherent signals received from strong, discrete radio sources. The absorption events to be discussed below occurred at local times near magnetic noon at NYA (approximately 0830 UT) which is within the prenoon hours at DMH (magnetic noon at DMH is approximately at 1020 UT). It is seen from Fig. 4 that absorption intensities of 0.55l.OdB in magnitudes are observed over the whole field of view of NYA, which indicates a large-scale structure extending outside the field of view. It is also seen that the absorption intensities are strongest at the southeastern and southwestern corners corresponding to the lowest latitude regions (see Fig. 3). Further, the plots indicate the existence of small-scale absorption enhancements, which may have variable amplitudes over the field of view, superposed on the large-scale variations. An interesting feature is that the absorption intensities display long-period variations (about 25 min period) in the western region. From the lower part of the figure it is seen that the absorption intensities at DMH are strongest in the fields at the southeastern corner which comprise the beams directed closest to the NYA station. In order to investigate the location, the spatial-scale and the motion of dayside absorption events, we show in Fig. 5 stack plots of the time-varying absorption intensities for the beams in a central east-west crosssection (N4) passing near the zenith for NYA (uppermost panel) together with the absorption in the EW beams of the second-southernmost row (S35) for DMH (second panel), the absorption in the E-W row of beams through zenith (ZOO) for STF (third panel), and the absorption in a row of beams (N4) near the zenith for TJO (lowermost panel), respectively. The stack plot for NYA and TJO present values averaged during 32s while those for DMH and STF present

30s averages. Note that the beams in the plots for DMH and STF are labelled by the approximate beam angles away from zenith in the N-S plane and in the E-W plane. Also note, that the absorption scales are 1.OdB/div. for the 30 Mhz riometer at NYA. 0.6 dB/div. for the 38 MHz riometers at DMH and STF, and 2.0dB/div. for the riometer at TJO. The different scales provide for the plots the same relative sensitivity to absorption features in the upper three fields for cusp-latitude stations taking into account the usual cf-‘) absorption frequency dependence, while reducing the sensitivity for the aurora1 zone station TJO at the bottom. It is seen from these panels that the absorption events observed at DMH during about 064&0920 UT are weaker but otherwise very similar to those observed at NYA. At STF the absorption events are more extended in time and not so variable in amplitude. The absorption intensities observed at TJO in the aurora1 zone are much stronger and differ in the time variations from the intensities observed at NYA, DMH and STF. An interesting feature seen in Fig. 5 in the absorption intensities recorded at NYA is the quasi-periodic variability. As a result of the shorter averaging time the shorter periods are now more clearly discernible than in the display presented in Fig. 4. A FFT analysis of the absorption variations for specific beams shows a rather broad spectrum with dominant frequencies at about 4mHz, 2 mHz and 0.7 mHz. These quasiperiodic absorptions of several minutes cycles are most pronounced during about 0640 to 0850 UT in the easternmost region at NYA. A particularly remarkable feature is the occurrences of short-lived enhancements of l-3 min duration seen in NYA at about 0650, 0730, 0801, and 0809 UT within the interval of quasi-periodic absorption variations. The short-lived absorption feature (about 1 min duration) at 0730 UT, seen most clearly in the western region and near the zenith, shows northward (poleward) motion. The one at 0809 UT, which is seen most clearly in the southwestern region, shows northeastward motion. The event at 0650 UT, seen in the southwestern region, and the one at 0802 UT, which is most pronounced in the eastern region, show no clear motion. The short-lived absorption features are also seen at some of the other stations. The events at 0650 and

Fig. 5. Stack plots of the time-varying absorption intensities in east--west cross-sections during 060&1200 UT on 18 September, 1992. Beam positions in northhsouth directions are within rows passing near the zenith for NYA (uppermost), STF (second lower), and TJO (lowermost), and in the second-southernmost row for DMH (second upper).

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0730 UT are identified in the eastern region (beams E35 and E50) at DMH as seen in the second panel of Fig. 5. The event at 0801 UT is not identified at DMH in agreement with the observations from NYA which indicate that it was located at the eastern region of NYA farthest away from DMH. The short-lived absorption at 0809 UT is not identifiable in DMH because of an intermittent failure in the data recording. The impulsive event at 0650 is discernible at TJO and is not seen at STF, while the event at 0730 UT is barely visible in the southernmost beams at STF and not identifiable at TJO. The event at 0801 UT is not seen at STF and TJO. The impulsive event at 0809 UT is not seen at STF but appears as a negative spike (reduced absorption) at TJO. During 1000 to 1150 UT, later in the day, when located near magnetic noon (occurring approximately at 1020 UT), very weak absorption features ( < 0.3 dB) with slowly varying intensities were observed at DMH. The time variations observed at STF during this event are much stronger than, but otherwise similar to, the absorption variations seen at DMH. The absorption intensities at TJO, with a strong enhancement occurring at about 1043 UT, have overall amplitudes similar to those of STF but the detailed time variations appear to be completely unrelated. The correlation of the absorption variations to variations in the geomagnetic field is an important element of the analysis of the disturbance events considered here. Figure 6 displays the geomagnetic variations in the horizontal north (H) components observed from the array of magnetometers in Greenland during 060&1200 UT on 18 September, 1992. The locations of these stations were presented in Fig. 1. The curves in the plot in Fig. 6 refer to stations whose acronyms are listed to the left while their invariant latitudes are listed to the right. The upper section of the plots comprise the west-coast stations (THL-NSQ), while the lower sections comprise the east-coast stations (NRD-AMK) and include the ice cap stations (MCNMCE). It is seen clearly that impulsive magnetic events occur at 0648, 0730, 0801, and 0809 UT on a background of moderately disturbed conditions. A more detailed comparison of absorption and magnetic perturbations is enabled in Fig. 7 which shows the time-variations of the absorption intensities in a N-S cross-section at the western part (E7) of

et al.

the NYA imaging riometer field-of-view (upper panel) and the magnetic H, D and Z-components observed at NYA (lower panel). It is readily seen that some of the impulsive, short-lived absorption features have a northward (poleward) motion, as mentioned in the description of Fig. 5. It should be noted that the shortlived absorption at about 0730 coincides with a large bipolar impulsive magnetic change in the H-component and smaller variations in the D- and Z-components at NYA. Such magnetic deflections could indicate the overhead passage of localized fieldaligned currents (Kokubun et al., 1988). The quasi-periodic absorption variations are seen to correspond in structure and frequency to similar or fluctuations in the magnetic variations components. There is not, however, a direct one-toone correspondence between peaks except for the above mentioned impulsive magnetic events. The close association of the absorption intensities and the magnetic variations is demonstrated in Fig. 8 which presents a coherency analysis between the absorption at a specific beam (N4E6) and the northward magnetic component (H) for NYA. From the figure we notice a correlation coefficient of about 0.8 at frequencies of about 335mHz (recurrence time of 335min) during 0730 to 0900 UT. In the following discussion we focus on the quasiperiodic and short-lived absorption events occurring during about 0640 to 0920 UT.

The above observational results indicate that absorption processes are active during 0600&1200 UT on 18 September, 1992, within a region extending (at least) in invariant latitude from 66.9” (TJO) to 77.3” (DMH) and over longitudes (STF-NYA) separated by almost 6 h in local magnetic time. Within this region we can identify large-scale features which are seen simultaneously at two or more stations in the group and small-scale features which may display spatial variations and motions over the field of view of a single station. In Figs 4 and 5, we have seen that the long-period (l&30 min) absorption features could be identified both in NYA and DMH. Such events would have a spatial-scale of at least 700 km in longitude and 200 km in latitude based on the separation of the sta-

Fig. 6. Magnetograms during 0600P1200 UT on 18 September, 1992, of the geomagnetic H-components for the Greenland West-coast stations (THL-NAQ) and East-coast stations (NRD-AMK) including the ice cap stations (MCN-MCE). Station acronyms are listed to the left while their invariant latitudes are listed to the right at the positions of the base-lines (QL) for the respective traces. For DMH marked with an asterix (*) the day-average level has been used instead of QL.

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GREENLAND MAGNETOMETER

H

CHAIN

SEP 18

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Fig. 7. Time variations during 0600-0930 UT of the absorption intensities in a north-south at the western part of the NYA field (upper panel) and the magnetic H, D and Z-components 18 September, 1992.

tions and including the distributions over the antenna fields at both locations. Small-scale absorption features of the scale size of 1O&200 km are also observed. They are most pronounced in the southeastern and southwestern regions of the NYA antenna field-ofview. The stack-plots of the time-variations of the absorption features indicate that the weak, small-scale

1

9:oo

cross-section at NYA on

absorption events with intensities of about 0.3 dB are superposed on the large-scale events, and that many of them show poleward motions. The impulsive absorption variations are apparently small-scale features since they may have variable intensities over the field-of-view of a single station. Some of them, however, like the above-mentioned

Cusp studies from Svalbard and Greenland

Nv-Alesund

6

1992.9.18

9

7 UNIVERs8AL

IO

TIME

Fig. 8. Time variation during 06OGlOOO UT on 18 September, 1992, of the coherency between absorption at a specific beam (N4E6) and the magnetic H-component at NYA. Coherency coefficients given by the grey scale to the right.

events at 0648,0730,0801 and 0809 UT on 18 September, 1992, occur at several stations at about the same time in association with magnetic impulsive variations which are observed over wide ranges in latitude and longitude. These features are consistent with a model where the absorption events are caused by the variable precipitation of electrons which have been accelerated previously to high energies during substorm activity in the night sector. These electrons are subsequently drifting eastward to the daytime sector in clouds of quasi-trapped high-energy electrons and gradually lost due to precipitation. The quasi-trapped electrons are constrained to the ‘closed’ magnetospheric field regions while the magnetically ‘open’ polar regions would be void of such particles. The location of the boundary between the open and closed magnetospheric regions depends on local time and solar wind conditions. One would expect that the gross behaviour of such absorption events would be similar at the four stations considered here since they are all located in the high-latitude morning and day sectors at the time in question. Also one would expect the absorption intensities to be strongest at aurora1 zone latitudes (TJO) and weakest at the very high aurora1

the are

oval latitudes (DMH). These features are confirmed by the observations. The detailed temporal and spatial variations in the electron precipitation and in the associated absorption intensities would most likely depend on more localized processes. These could be of many different kinds like pitch-angle scattering by VLF wave-particle interactions or plasma turbulences, or general mirror point changes due to longitudinal electric fields or largescale magnetospheric changes. The observations presented in Figs 7 and 8 indicate that the precipitation causing the quasi-periodic absorption variations is modulated by waves in the PC-~ range. The interaction of the waves with the highenergy electron population may take place in distant regions of the magnetosphere. Since the (near relativistic) particles causing ionospheric absorption of radio waves move much faster than the magnetic disturbances propagating with the Alfven velocity then the direct coincidence of the two features is rapidly lost. However, the frequency spectra and the gross behaviours may remain correlated, like the observations presented above have demonstrated for a typical example case. On this basis we suggest that the large-scale absorp-

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tion variations observed during this event are the combined result of the variations in the source mechanism (substorm activity) and the magnetospheric configuration, primarily the variable boundary between the closed and the open field regions. The small-scale variations are seen as the results of disturbances which locally modulate the precipitation of the eastward drifting high-energy electrons. Some of these disturbances could well be impulsive magnetic variations, which may expand over a wide region, while other could be more localized to occur, for instance, at or close to the cusp region. This behaviour is similar to that of dayside cusp amoral features. For the slow, large-scale variations it should be noted that the aurora1 observations by Sandholt et al. (1985) have shown that the cusp aurora (630.0nm) occurring at the boundary between closed and open magnetospheric regions will move northward or southward depending on IMF Bz orientation and strength. Sandholt et al. (1990) observed transient aurora1 arcs near the midday at Ny-Alesund. The 557.7nm emission was strongly enhanced in events with a peak intensity of 15 kR, from which they derived a peak value of about 1 keV in the average energy for the precipitating electron flux. They concluded from their analysis that the event gave strong evidence in favour of a transient, intermittent reconnection process at the dayside magnetopause and associated energy and momentum transfer to the ionosphere in the polar cusp and cleft regions. Fasel et al. (1992) observed poleward moving dayside aurora1 transients of the 557.7 nm emissions near the magnetic noon at Longyearbyen, Svalbard. These events had several cycles of intensity variations. They inferred from the multiple brightenings of the discrete arcs that electrons can be accelerated to keV energies as the field-line currents associated with Alfven waves encounter the high altitude ionosphere leading to quasi-periodic brightenings of the transient aurora1 forms. Lockwood et al. (1990) presented observations of a transient aurora1 event in the dayside ionosphere at magnetic noon at Ny-Alesund. Embedded within the widening band of 630.0nm emission they observed two intense, short-lived transient 557.7nm arc fragments with rays which persisted for about 2 min each. The green line photometer observed strong luminosity of about 10 kR. Recently it has been suggested that the reconnection may take place in a series of discontinuous events, that is, in a ‘pulsating cusp mode’ (e.g. Smith et al., 1992). Lockwood et al. (1993) presented coordinated radar and satellite observations of a series of discrete,

poleward-moving plasma structures that were consistent with the pulsating-cusp model. Lockwood and Wild (1993) surveyed the recurrence rate of FTEs by using the magnetopause observations by the ISEE satellites. They found that the distribution of the intervals between FTE signatures had a mode with a preferred value of 3min, and suggested that the recurrence frequency spectrum could arise from fluctuations in the IMF &-component. These optical aurora1 observations have close resemblance to the absorption features described above. The particle energies involved in the cusp auroras (few hundreds of eV-few keV), however, are much less than those required (few tens-few hundreds of keV) to produce significant radiowave absorption intensities. We believe that the above-mentioned absorption features are associated with the variable high-energy electron precipitation in a region equatorward of the cusp proper while the softer precipitation associated with the cusp auroras occur within and poleward of the cusp region. However, because of the continuity in the ionospheric and magnetospheric plasma convection many of the dynamic features will be carried through the cusp region and will be seen with similar appearances on both sides. Further, the geomagnetic variations associated with convection features and other possible disturbances within the cusp will spread and may modulate processes like electron precipitation within a wider region. Accordingly, the absorption features displayed through imaging riometer observations may monitor the dynamic properties of the cusp region as seen from the equatorward side.

DPY-RELATED,

SLOWLY VARYING ABSORPTION

EVENT

Introduction The cusp/cleft red auroras observed at Ny-Alesund around magnetic noon typically show westward or eastward motion depending on the orientation of the IMF B,-component. The motion is in the same direction as the ionospheric convection (Sandholt et a/., 1993). From simultaneous prenoon and postnoon observations of aurora1 activity, using all-sky TV cameras at NYA and DMH, Sandholt et al. (1994) have further demonstrated that auroras, depending on the sign of IMF B,,, may drift eastward or westward across the noon meridian. This motion also corresponds to the prenoon-postnoon asymmetry in plasma convection related to IMF B, polarity (e.g. Cowley et al., 1991). cusp-latitude absorption events Concerning observed by imaging riometers, Stauning et al. (1993)

Cusp studies from Svalbard and Greenland Stauning et al. (1995a), Stauning et al. (1995b) and Clauer et al. (1995) observed absorption events with a quasi-periodic nature (15-30 min repetition periods) occurring near magnetic noon at Sondre Stromfjord in association with similar quasi-periodic variations in the IMF B,-component as observed from the IMP8 satellite in the solar wind. During conditions of strongly negative IMF B, (around - 1OnT) the absorption events showed poleward progression at about 0.551.0 km/s. The absorption events were closely resembling the large-amplitude disturbances in the magnetic field, particularly in the H-components, observed at successively higher and higher latitudes at the meridian chain stations in Greenland. They have interpreted the progressing magnetic disturbance events as the poleward propagation of DPY current intensifications associated with enhancements in the IMF B,-component. They further suggested that the absorption events, similar to the case studied by Stauning (1984) were caused by electron collision frequency increases caused by electron heating in the large ionospheric electric fields associated with strong DPY currents flowing in the cusp region. The intensities of these currents, and hence the ionospheric electric field strengths, depend largely on the IMF Bycomponent, but they are intensified during negative IMF B,. They are eastward in the northern hemisphere for positive IMF B,. The location of the DPY currents depends primarily on IMF B,. They occur more poleward and in a more narrow region for IMF B, positive. modulations in During negative IMF B, conditions, the IMF By-related DPY currents may progress poleward (antisunward) as observed in the associated magnetic variations (e.g., Stauning et al., 1994) Here, we present a DPY-related dayside absorption event with slowly varying features observed at NYA and DMH in the cusp-region. The substorm-related absorption intensities observed simultaneously at STF and TJO are also presented. Further, the magnetic variations observed at Greenland chain stations and Ny-Alesund, and the IMF data recorded by the IMP8 satellite, are shown in order to discuss the dynamics of this ionospheric disturbance. Absorption

event on 17 October, 1992

For the interval from 0900 to 1400 UT on 17 October, 1992, Fig. 9 presents stack plots of the timevarying absorption intensities observed in the beams of E-W cross-sections passing through or near zenith of the respective antenna fields. The uppermost to lowermost panels show the absorption variations for NYA, DMH, STF, and TJO, respectively. The values plotted are 30 or 32s absorption intensity averages.

915

During about 0900-l 100 UT, while NYA was located in the postnoon sector, and both of STF and TJO were located at prenoon sector, the absorption intensities observed at DMH in the noon sector display slowly varying features with peak intensities of about 0.3 dB. The absorption intensities seen at NYA at this time were very weak and disturbed by local interference. The absorption intensities at STF were also very weak except for the impulsive event at 1025 UT. The slowly varying absorption event observed at TJO in the auroral zone with intensities of 0.5-l.OdB before 1000 UT seems to be different from the absorption events occurring at cusp-latitudes and is most likely substorm-related. In a time-series of absorption images for DMH during about 0900-l 100 UT (not shown) it appears that the absorption varies uniformly in the northern beams of the DMH antenna field, which indicates a large-scale absorption event gradually weakening equatorward. This event does not extend to the NYA field-of-view at slightly lower latitudes than DMH. During about 1130-1230 UT, while both NYA and DMH were located in the postnoon sector, TJO close to noon, and STF still in the prenoon sector, slowly varying absorption features were again observed at DMH. These features are now clearly recognized in the display from NYA. The absorption displays for STF and TJO are very like each other and, again, quite different from the displays for DMH and NYA. In a time-series of absorption images for this timeinterval (not shown) it appears that the absorption features extend over the whole field-of-view of DMH. The intensities, however, are strongest in the northern beams. The peak intensities vary from about O.lL 0.2 dB in the southern beams to more than 0.3 dB in the northern beams. From the events of corresponding absorption features observed at NYA and DMH around 1200 UT, we have investigated the spatial scale and motion of the absorption features. Figure 10 shows on expanded time scales compared to Fig. 9 a set of plots of the absorption intensities at NYA (upper panels) and DMH (lower panels). The left section displays absorption for N-S cross-sections and the right displays EW cross-sections. From this figure we can see two large-scale absorption enhancements occurring around 1200 and 12 10 UT, respectively. From Fig. 10 it is clear, that the absorption enhancements are moving through the field-of-views for both stations. DMH is leading. From the DMH recordings the apparent S-to-N velocity is estimated to 0.76 km/s and the W-to-E velocity to 1.I2 km/s using a reference altitude of 120 km (E-region) for the absorption processes. Assuming that the variations

E8

E7

ES

ES

E4 E3

E2

El

-5

m-

Cusp studies from Svalbard

OCT

NYA :

917

and Greenland

17 1992

0.7

km/s

:

DMH

0.76

1.12 km/s

km/s

N35 N21 NO7

E21

so7

E07

s21

wo7

S35

w21

s50

w35 12

UT

12 ‘UT

Fig. IO. Detailed stack plots of the time varying absorption intensities at NYA (upper panels) and DMH (lower panels) during 1130-1230 UT on 17 October, 1992. The left panels display absorption intensities in nor&south cross-sections. The right panels display the absorptiofi in east-west cross-sections. The

propagation of frontal absorption features through the fields of view are indicated by tilted arrows.

are caused by front-like

disturbances the propagation is in a direction 34,. east of magnetic north at a velocity of 0.63 km/s. As a result of the stronger level of interfering disturbances it is a little more difficult to assess the vel-

ocity at NYA. We have estimated an eastward component of 0.7 km/s at about 1210 UT. Further, there is a delay of about 3min from DMH to NYA (central fields). Figure 11 displays the time variations of the H-

Fig. 9. Stack plots of the time-varying absorption intensities in east-west cross-sections during 0900-1400 UT on 17 October, 1992. Beam positions in north-south directions are within rows passing through or near the zenith for NYA (uppermost), DMH (second upper), STF (second lower), and TJO (lowermost panel).

918

M. Nishino

et a/

GREENLAND MAGNETOMETER

CHAIN

H

OCT

17

1992 INVL

THL

85.4

KUV

81.2

UPN

79.4

UMQ

76.8

GDH

75.9

ATU

74.6

STF

73.2

SKT

72.1

GHB

70.6

FHB

68.1

NAQ

66.4

NRD

80.9

DMH *

77.2

MCN

77.6

MCW

76.8

MCG

76.6

DNB

75.2

SC0

71.7

AMK

69.5

nT

,

9

10

11

UNIVERSAL

12

TIME

13

14

Cusp studies from Svalbard and Greenland components observed at the magnetometer chain stations in Greenland and at Ny-Alesund during 09001400 UT. One should observe that for DMH (marked with an asterix) the day-average value has been used instead of the QL value. It is seen that a range of the northern stations, that is, UMQ-ATU at the west coast of Greenland (upper part of Fig. ll), DMHDNB at the east coast (lower part) and NYA at Svalbard (bottom panel) display the large-amplitude, relatively slow variations in the H-components characteristic for DPY current variations. The DPYrelated perturbations are particularly large and extend farthest equatorward at around 093&1015 and 114tL 1215 UT. At lower latitudes the faster variations are more pronounced. One may note, among others, the impulsive magnetic event at 1025 UT which is also seen clearly in the absorption display for STF in Fig. 9. The time-intervals of the absorption events observed at DMH as shown in Figs 9 and 10 are represented by horizontal bars marked with the text ‘CNA’ in the middle part of the figure. There is a close relationship between the absorption events and the magnetic variations in H at DMH around 1000 UT and at about 1100 UT close to magnetic noon, and also around 1200 UT in the post-noon sector. The peak perturbations during these intervals are about 200 nT at 1000 UT, 100 nT at around 1100 UT, and 300 nT close to 1200 UT. A detailed comparison of the absorption variations in the central beams of DMH, as displayed in Figs 9 and 10, and the magnetic variations displayed in Fig. 11 shows a consistent peak-to-peak correlation of perturbations through these intervals. This tendency is also evident from a comparison of the variations in absorption and in the magnetic H-component observed at NYA, particularly distinct around 1200 UT. The interplanetary magnetic field was monitored by the IMP-8 satellite which at this time was located upstream of the dayside magnetopause at around X = 34 Re, Y = 1 Re, Z = -4 Re, which is a favourable position for investigations of the response of the magnetosphere to conditions in the solar wind. In the IMF data shown in Fig. 12, the B,-component fluctuates around zero with rather small amplitudes ( i 5 nT) during 0900 to 1300 UT. Averaged over some tens of minutes the IMF B, level was negative during

919

091&1000 UT, positive during 100&1050, negative during 1050-1200, and then positive from 120&1300 UT. The IMF &-component was close to - 6 nT during around 09Ot%lO30 and 1130 to 1200 UT. During the remaining time within 090&1300 UT the IMF B,component was close to zero. Discussion

Stauning (1984) and Stauning et al. (1993, 1995a, 1995b) have revealed that some dayside cusp-latitude absorption events can be associated with DPY-current perturbations in the ionospheric E-region and thus related to the varying IMF conditions which modulate the DPY current systems. Such events are quite different from the more usual absorption events related to the variable precipitation of high-energy particles (most frequently substormelectrons). Their primary cause is the enhanced electron collision frequencies in an E-region of usual daytime electron densities heated through unstable ionacoustic plasma waves in strong electric fields. The electric field strength must be higher than about 25 mV/m to initiate the plasma instabilities. However, field strengths greater than about 50 mV/m are needed to create absorption intensities discernible on riometers (Stauning, 1984). In cases where the events extend uniformly over large regions (tens to hundreds of km) such electric fields will generate strong ionospheric currents detectable by ground magnetometers. Thus, contrary to most precipitation-related daytime absorption events, in such cases there will be a one-toone correspondence between absorption and magnetic perturbation intensities. Another important difference between the two types of absorption relate to the latitudinal intensity distributions. The precipitation-related daytime absorption events are usually most intense at the centre of the aurora1 zone where the substorm-related highenergy electrons have the shortest drift paths from the acceleration regions at the night-side. The quasitrapped high-energy electron fluxes decrease as one approaches the boundary between the ‘closed’ inner magnetospheric regions and the ‘open’ polar cap regions. In most events the precipitated fluxes relate to the intensities of the trapped fluxes. Hence the precipitation and thus the absorption intensities decrease, as one moves poleward from the central aurora1 zone,

t at the Fig. 11.Time variations during 09OtL1400 UT on 17 October, 1992, of magnetic H-components meridian chain stations in Greenland. Station acronyms and their invariant latitudes are listed to the left and right of the magnetic variations, respectively. The intervals of the absorption events are represented by horizontal bars marked ‘CNA’ in the middle part. The lower panel shows time-variation of the Hcomponent at NYA.

920

M. Nishino ef al.

OCT17

CNA

1992

CNA

CNA

20

r

10 F 9

________

0

S

UT (hour) Fig. 12. Time-variations of the three components (B,, B, and BJ of the interplanetary magnetic field (IMF) observed from the IMP-S satellite during 0900-1300 UT on 17 October, 1992. The intervals of the absorption events observed at DMH are represented by horizontal bars marked ‘CNA’.

and disappear upon entering the cusp and polar cap regions. Contrary to this behaviour the electron heating absorption intensities will usually increase with latitude from the dayside aurora1 regions through the cusp to the polar cap regions, since in these regions

the solar wind-magnetosphere interactions may create strong electric fields in the ionosphere. Most prominent among such sources are the electric fields associated with the IMF-related DPY disturbances. The relations between the IMF data and the DPY

Cusp studies from Svalbard and Greenland geomagnetic perturbations observed in the dayside polar region during 0900-1330 on 17 October, 1992, are in agreement with previous models and statistical results obtained, for instance, by Friis-Christensen (1984) and Friis-Christensen et al. (1985). In the present case the ionospheric DPY currents are generally westward, giving negative perturbations in the Hcomponents at cusp-latitudes (see Fig. 1 I), corresponding to the dominantly negative IMF B, level (see Fig. 12). The location of the ionospheric DPY currents (approximately the cusp location) is more equatorward and the latitudinal width of the current system is larger during intervals of negative IMF B,. The complicated dynamical behaviour of the DPY current system during changes in the IMF conditions has been modelled by Stauning et al. (1994). They have found, among others, that the IMF By-related patterns in the ionospheric DPY currents, observed through the associated magnetic perturbations, would be stationary (i.e. just time-varying) during IMF B,> 0 or IMF B,zO, while such patterns would be progressing poleward (antisunward) from the cusp region across the polar cap during conditions of IMF B,< 0. A further complication is the time delay between measuring IMF field parameters at the IMP-8 position and observing the IMF related effects in the polar ionosphere. We may coarsely estimate the propagation delay according to the procedure used by Stauning et ul. (1995b). The distance from the IMP-8 position (XZ 34 Re) to the bow shock in front of the magnetopause, anticipated at 12 Re, is 22 Re. Using a solar wind velocity of V,, = 500 km during this event, the propagation time is estimated to be about 5 min. The propagation times from the shock to the magnetopause subsolar point and from the magnetopause to the ionospheric footpoint in the cusp region are assumed to be 4min and 2 min, respectively. The total propagation delay is now about 11 min. In addition there is the delay associated with the progression of the disturbances from the cusp region to the location considered. During this event the IMF B,-component is close to zero most of the time. Hence the location of the cusp region is very close to the latitude of DMH at 77.3” inv. lat. In summary we expect a delay of about 12 min between IMF variations at IMP-8 and related changes in the ionosphere above DMH. The expectation is that the absorption features seen at DMH shall resemble the IMF B, variations taking into account the above delay and with the added modifications introduced by the IMF B, variations. This is confirmed by the observations. We observe fluctuating absorption intensities in the northern beams at DMH during 09OG1300 UT except 090&

921

0920, when IMF B,is strongly positive (cusp poleward of DMH). The absorption variations track closely the variations in IMF B, such that the intervals of IMF B, <<< - 6 nT (delayed approximately 12 min) corresponds to intervals of absorption intensities A~0.3dB. The intervals of IMF B, near zero, for instance 1008-1016, 1040-1044, 105661126, 12055 1226 UT, which transforms to about 1020-1028, 105221056, 1108~-1138, and 1217-1238 UT for the expected effects at DMH, correspond to intervals of very small (nearly zero) absorption intensities. During the intervals of negative IMF B, like, for instance, during 114&l 146 and 11541202 UT, which transform to 1152-I 158 and 120661214 UT we observe an equatorward extension of the absorption features (to the southern beams at DMH) and also a clear poleward (antisunward) progression of the absorption patterns. Another interesting feature derived from the observations is the motion of the absorption region. From the above discussion of the display in Fig. 10 we derived for the enhancement seen at about 12 10 UT a progression velocity of 0.63 km/s in a direction 34” east of North. At this time DMH is 2 h past noon in local magnetic time. Accordingly the angle between magnetic north and the antisunward direction is about 30” so the direction of the progression is consistent with a general antisunward motion of the phase planes of interactions between the ionosphere and the interplanetary medium as suggested by Stauning et al. (1995a, 1995b). At the same time (1210 UT) NYA is around 3.5 h past local magnetic noon. Accordingly, the angle between magnetic north and antisunward direction is around 52”. Antisunward motion at NYA is now much closer to eastward direction. This is consistent with the local absorption observations within the fieldof-view of NYA. Further, the distance between DMH and NYA projected to the antisunward direction is around 170 km. Accordingly, the observed delay of 3min from DMH to NYA corresponds to a progression velocity of 0.94 km/s in reasonable agreement with the velocity of 0.63 km/s determined locally at DMH. It should be noted that these coarse calculations have assumed that both antenna fields are aligned in a magnetic N-S direction identical to invariant N-S direction. Corrections of the small inaccuracies involved here will not, however, change the results significantly. From the magnetic variations displayed in Fig. 11 we believe, that at the time around 1200 UT the boundary between the ‘closed’ aurora1 field regions and the ‘open’ polar cap region is located between

922

M. Nishino

STF (inv. lat. = 73.5”) and ATU (inv. lat. = 74.6”) at the West coast, around DNB (inv. lat. = 75.2”) at the East coast, and equatorward of NYA (inv. lat. = 76.1”). Equatorward of this boundary we observe the quasi-periodic pulsating or impulsive variations characteristic of the closed magnetospheric regions. On the poleward side we observe the slowly varying DPY disturbances. This hypothesis is confirmed by the absorption variations seen around 1200 UT since the intensities at TJO and STF resemble the aurora1 precipitation-related event discussed previously in case 1, while the absorption variations observed at DMH and NYA can be explained by DPY-related fronts of absorption progressing antisunward from the cusp region located at approximately 74” inv. lat. into the polar cap.

CONCLUSIONS

We have investigated the location, the spatial scale and the motion of radio wave absorption events occurring in the cusp-latitude ionosphere as observed by imaging riometers in the polar region. We have presented and discussed separate cases of absorption events related to the variable precipitation of substorm-related high-energy electrons, and IMF-related electron heating absorption events associated with DPY disturbances, respectively. The main characteristics of the precipitation-related absorption event occurring near but equatorward of the cusp region at magnetic noon as observed at NyAlesund and other imaging riometer stations are the following: Small-scale absorption events with dimension of 100-200 km are observed at NYA near local noon mostly at the lower-latitude regions. They are superposed on large-scale events where coherent variations are seen to extend at least 700 km in longitude toward the prenoon sector. Some of the small-scale absorption events show quasi-periodic variations which correlate well in frequency spectra and general intensity structures to the local magnetic variations, most pronounced at repetition periods of 3-5 min. Short-lived (I-3 min duration), impulsive absorption events found during intervals of quasi-periodic variations show northward or northeastward motion, and coincide with impulsive magnetic variations observed at a wide range of magnetometer stations. These characteristics quasi-periodic and

lead us to conclude impulsive absorption

that the events

et al

observed near the magnetic noon may be caused by modulations of the precipitation of high-energy electrons by disturbance processes originating in the outer magnetosphere or in the magnetosheath plasma. These absorption processes, as observed by imaging riometers, behave in many respects similarly to other phenomena, like optical aurora1 emissions, observed in the polar cusp ionosphere. The main characteristics of a different case where the absorption, most likely, was caused by electron heating associated with the IMF-related DPY disturbances observed in the noon sector at and poleward of the cusp latitude, are the following: has a front-like 1. The slowly varying absorption structure extending over (at least) 700 km in longitude. 2. The absorption events show antisunward motion (poleward at noon, eastward in the postnoon sector). 3. The absorption events are associated with IMF Byrelated magnetic bays in the north horizontal (H) component observed at stations in the cusp and polar cap regions (negative bay during negative IMF By). 4. The locations of the DPY disturbances and thus of the absorption features depend on the sign and magnitude of the IMF B, component. (More poleward for positive IMF BJ 5. During negative IMF B, conditions the absorption patterns may progress in the antisunward direction from the cusp region across part of the dayside polar cap. This phase motion is not necessarily identical with the physical plasma convection. If further works sustain these conclusions, then the coordinated observations using arrays of imaging riometers in the polar regions will be very useful for detecting dayside transient events occurring in the outer magnetosphere, and at the magnetopause interfacing to the solar wind, and for examining important aspects of the polar cap plasma convection.

Acknowledgements-We are pleased to acknowledge the encouragement of A. Egeland, University of Oslo, and S. Kokubun, STE Laboratory, Nagoya University. We gratefully acknowledge the Norwegian Polar Research Institute for incessant observations at Ny-Alesund and we greatly appreciate the careful operation of riometers and magnetometers by the staffs at the other various observing stations. We also acknowledge A. Brekke and T. Hansen, University0 of Tromso, for the use of magnetic field data from Ny-Alesund, 0. Rasmussen, DMI and CR. Clauer, University of Michigan, for the use of Greenland magnetic data, and we acknowledge gratefully R. Lepping at NASA Goddard Space Flight Center, Maryland, for the use of IMP-

Cusp studies from Svalbard 8 IMF data. We very much appreciate the assistance received from S. Henriksen, DMI, in the processing of imaging riometer data from Danmarkshavn and Sondre Stromtjord.

923

and Greenland

The development of the Ny-Alesund absorption the result of help from H. Ohta, graduate student of Engineering, Nagoya University, Japan.

features is of Faculty

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