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Polar convection-related ionospheric radiowave absorption processes observed by imaging riometers
Adv. Spcacc Res. Vol. 22, No. 9. pp. 12794288, 8 1998
1998
COSPAR.Published by Elsevia Science Ltd. All rights reserved Printed ia Great Britain 0273-i 177198 $19.00 + 0.00
PII: SO273-1177(98)00172-O
POLAR CONVECTION-RELATED IONOSPHERIC RADIOWAVE ABSORPTION PROCESSES OBSERVED BY IMAGING RIOMETERS
P. Stauning Danish Meteorological Institute, Lyngbyvej 100, DK-2100 Copenhagen, Denmark
ABSTRACT The solar wind-magnetosphere interactions resulting in the transfer of energy and momentum from the solar wind to the Earth’s environment are reflected in the polar ionospheric plasma convection. The imaging riometer technique has added a valuable tool for further investigations of the complex dynamics of the polar plasma convection through the monitoring of convection-associated radiowave absorption events. Such absorption events relate to the enhanced electron collision frequencies resulting from E-region electron heating caused by strong electric fields associated with the convection processes. For such events the imaging riometer observations may provide the detection of convection features with high temporal and spatial resolutions that are not readily available with other observing techniques. In cases of antisunward progressing convection disturbances, which occur during southward interplanetary magnetic field conditions, the velocity vectors derived from monostatic imaging riometer observations assuming E-region absorption processes equal those found for the magnetic perturbations using arrays of magnetometers. It is further demonstrated that .the imaging riometer observations may provide precise identification of the initial region of impact for the ionospheric disturbances associated with the solar windQ 1998 COSPAR. Published by Elsevier Science Ltd. magnetosphere interaction at the front magnetopause. INTRODUCTION The polar ionospheric plasma convection and the associated horizontal electric current systems are coupled through field-aligned currents to the magnetospheric plasma motions and electric fields generated by the solar windmagnetosphere interactions. A comprehensive mapping of the magnetosphere-ionosphere field-aligned currents was enabled by satellite observations of the magnetic perturbations in space (e.g., Armstrong and Zmuda, 1973; Iijima and Potemra, 1976a, b). These observations have revealed the existence of the so-called region 1 and region 2 oppositely directed field-aligned current systems, which appear in the morning and evening sectors of the auroral regions at the border of the polar cap (e.g., Iijima and Potemra, 1976a). In the high-latitude noon sector a different system of field-aligned currents was observed. These currents were termed “cusp currents” by Iijima and Potemra (1976b). The steady state location of the cusp is taken to be the region in the noon sector separating geomagnetic field lines closing on the dayside from field lines swept tailward by the solar wind. Its position depends on the solar wind plasma pressure as well as the magnitude and direction of the interplanetary magnetic field @IF), being more equatorward during negative than during positive IMF B,, and displaced in the E-W direction in response to IMF B, (e.g., Butch, 1973; Newell et al., 1989). Investigations of the time-varying dayside convection patterns in the vicinity of the polar cusp regions have searched for signatures of a direct coupling between the solar wind and the Earth’s magnetosphere and ionosphere. One indication of this coupling is the observed strong relationship between changes in the large-scale dayside ionospheric plasma convection and the strength and orientation of the IMF observed upstream near the magnetopause (e.g., Clauer and Banks, 1986; Greenwald et al., 1990; Potemra et al., 1992). The response of the dayside high-latitude ionosphere to changes in the solar wind is observed within a few minutes of the IMF change encountering the front magnetopause (e.g., Todd et al., 1988). This response time is of the order of the Alfven 1279
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travel time from the magnetopause to the ionosphere suggesting a direct electrical connection between the solar wind and the ionosphere. During intervals of consistently southward IMF the modulations of the polar convective flow generated in consequence of variations in the IMF B, component appear to be progressing in the antisunward direction across the dayside polar cap. The term “progression” relates to the propagation of the modulation patterns, which may differ substantially both in magnitude and in direction from the convection velocities of the ionospheric plasma. The statistical relations between IMF conditions and progressing polar convection disturbances have been discussed by Stauning et al. (1994). Case studies of the progression of IMF Byrelated convection disturbances have been presented, e.g., by Stauning et al. (1995) and Clauer et al. (1995). A model to explain the coupling between IMF B, and the poleward moving ionospheric DPY currents in terms of interplanetary currents flowing along the merged interplanetary and terrestrial field lines connecting to the polar cap was presented by Stauning (1994). INSTRUMENTATION The array of observatories that have supplied the data used in the present study comprises stations in Northern Norway, Iceland, Greenland, and Canada. The illustrative cases presented below use the magnetic observations from the chain of stations at the west coast of Greenland. The acronyms for these stations and their geographic coordinates are listed in Table 1. The table also holds the geomagnetic (dipole) coordinates, the magnetic invariant latitudes, and the universal times for magnetic local noon (in the eccentric dipole system) for the stations. Table 1. Selected Geomagnetic Observatories in Greenland, 1991 Station Name
Station Acronym
Thule
Upernavik UmnWWlaq Godhavn Attu Sdr.Strannfjord Maniitsoq Nuuk Paamiut Narsarsuaq
UMQ GDH ATU STF SKT GHB FHB
NAQ
Geographic Coordinates WI
[“El
11.47 72.78 10.68 69.25 67.93 66.99 65.42 64.11 62.00 61.16
290.11 303.85 307.87 306.47 306.43 309.05 307.10 308.21 310.32 314.56
Geomagnetic Coordinates
Wl
[“El
88.91 351.21 83.49 35.48 81.07 38.18 79.88 32.53 78.62 30.44 71.60 34.33 76.12 28.74 74.19 29.53 72.47 31.03 71.18 36.70
Invariant Latitude
WI 85.72 19.83 77.28 76.27 75.07 73.70 72.60 71.19 68.71 67.03
Etc. Dipole Magn. Noon
WI 1434 1408 1402 1421 1428 1417 1432 1431 1429 1412
The “Imaging Riometer for Ionospheric Studies” (IRIS) used in this analysis is co-located with the magnetometer at the incoherent scatter radar (ISR) station in Sdr. Strnrmljord (STF) at the west coast of Greenland. The 64element antenna array and the beam-forming Butler Matrix system have been supplied by the Institute for Physical Science and Technology at the University of Maryland. The receiving system used in STF is a novel design developed at the Danish Meteorological Institute (DMI). The IRIS system operates at 38 MHz and enables the measurement of cosmic noise power received in 49 narrow beam directions organized in a 1 x 7 matrix of beams. The basic recorded data from the imaging riometer measurements are samples of the intensities of cosmic noise power received within each of the 49 beams. From these data, and the quiet day (QD) reference levels, the ionospheric absorption intensities can be calculated for each beam separately. The basic design and the operating principles for the IRIS have been explained by Detrick and Rosenberg (1990). The average altitude for ionospheric absorption processes is around 90 km during most auroral-type events (e.g., substorms). For electron heating absorption (EHA) events, which are associated with E-region plasma instability processes (Stauning, 1984; Stauning and Olesen, 1989), the dominant height range is 1lo-120 km. Using a selected reference height and image processing techniques enables the absorption intensities measured in the different beam directions to be combined into images of the distribution of absorption intensities over the sky within the antenna field-of-view. The image region is approximately 240 x 240 km in extent for a 90 km reference level.
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Convection-Related Absorption Processes
OBSERVATIONS Poleward Promessing Absorntion Events on 26 Seotember 1991 The first of two selected illustrative cases presents observations from 26 September 1991. Magnetograms for the recordings of the H-components from the array of magnetometer stations at the west coast of Greenland are presented in Figure 1. The acronyms of Table 1 are used to refer to the stations. At and poleward of SKT at an invariant latitude of 72.5” a series of poleward progressing pulsations is observed. The pulsations typically have amplitudes of 200-300 nT. The interval between enhancements is 20-40 minutes. A tilted line has been drawn through a series of successive enhancements to illustrate the progression. For the marked enhancement there is a delay of around 10 min from the southernmost (SKT) to the northernmost (UMQ) station corresponding to an average progression velocity of 0.80 km/s. In Figure 2 we present imaging riometer recordings from the IRIS instrument in Sdr. Stremfjord. The upper panel presents the recordings from the second-westernmost column of 7 beams. Within this panel the traces are arranged in sequence from north (top) to south (bottom). The second panel displays the beams of a central cross section. The middle trace in this panel represents the zenith-directed beam. The third panel displays the second-easternmost column of beams, arranged again in sequence from north (top) to south (bottom). These data represent the recorded signal amplitudes of the cosmic noise power received in some of the individual beams. The signal levels have been normalized such that undisturbed traces are horizontal. Hence absorption is positive downward while upward deflections represent various types of interference adding to the level of cosmic noise. In some cases part of the traces have been blanked at the times of anticipated scintillations which may occur when either of the strong, discrete galactic radio sources (e.g. Cassiopeia A or Cygnus A) enters the respective beams. The lowermost panel displays the geomagnetic H, E (eastward) and 2 (vertical) components recorded by the co-located magnetometer. MAGNETOMETER
H
CHAtN
SEP 26
1991
I. 10
II
12
13
UT
I4
15
16
Fig. 1. Example of poleward progressing convection disturbances observed in the H-components recorded at the Greenland west-coast chain of stations during 1000 - 1600 UT on 26 September 1991. Note that the stations are not equidistantly positioned in the plot.
12
I
.
.
..a.....t.....I II
UT
11
Fig 2. Upper three panels: Recordings of cosmic noise signal intensities in selected IRIS beams for the events observed during 1200-I 600 UT on 26 September 199 1. Bottom panel: Geomagnetic recordings from the colocated magnetometer in Sdr. Stromfjord.
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It is seen from the IBIS traces in the upper three panels that there is a weak downward deflection (an absorption event) at around 1400-1420 UT. Further one may notice the phase shirts making the event occur further and further delayed further poleward. The delays correspond to a poleward progression which is emphasized by the tilted arrow in the middle panel. Corresponding to the time of the absorption event as seen in the zenith-directed beam the geomagnetic H-component displays a pronounced peak. In Figure 3 the data from all beams are presented in the form of absorption intensities (reduction in signal relative to the QD level). There is a field for the time-varying absorption intensities for each beam. The fields have been positioned to represent the direction of the beams such that the upper left field represents the data from the northwesternmost beam and so on. The absorption intensities are defined by the vertical scale to the left. The time marks along the horizontal axes depict 5-min intervals. Date and time interval for the recordings are shown at the upper right comer. The figure presents an absorption event of approximately 0.25 dB peak intensity where absorption features are progressing from south to north as indicated by the tilted line.
Fig. 3. Absorption intensities recorded by the__IRIS imaging riometer in Sdr. Stromfjord during 1400-1430 . . UT on 26 September 1991. The progression of the absorption enhancements is emphasised by the arrow line. In order to more precisely calculate the progression velocity vector a correlation program has been developed. The principles of the calculations are shown in Figure 4. The left diagram displays the original data in the form of absorption intensities. The data for the various beams have been grouped in panels such that all traces from the northernmost row of beams have been plotted in the upper field and so on downward. There is an absorption scale for each panel to the left of the diagram. A tilted arrow has been drawn to emphasise the poleward progression of the absorption enhancement. In the lower part of the left diagram the geomagnetic recordings have been displayed with a trace for each component (H, E, and Z). Baselines (I-&,E,, and Z,) are indicated by the dashed lines. Assuming that the disturbances occur in the E-region at 120 km of altitude and propagate across the field of view in the form of progressing tints, the program calculates the best fit to the progression velocity vector. The result is printed in the upper right comer of the right diagram in Figure 4. In this case the program has derived a progression velocity of 0.85 km/s in a direction 16” east of geomagnetic north. Using the calculated velocity vector, and the distances from the ionospheric projections of the individual beams to the centre of the field, the corresponding delays from the assumed progression have been calculated. The resulting delays have then been imposed on the recorded data and the resulting traces are displayed in the right diagram of Figure 4. It is clear from the figure, that the absorption enhancements are now aligned. The vertical dashed line is for guidance only.
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Convection-Related Absorption Recesses IRIS
36
f4hr
Sdr.
Stronf)orO
26
SEPTEI49ER
1991
IRIS
36
Uh?.
Ser.
Stromfjord
1412
26
KPTEM6ER
1991
UT
1442
Fig. 4. Diagram to describe the calculation of progression velocities. Upper part of left diagram: Raw imaging riometer data. The progression is emphasised by the arrow line. Lower part: Recordings of individual geomagnetic components. Upper part of right diagram: Absorption traces shifted by anticipated delays using calculated progression velocity vector. Lower part: Total horizontal geomagnetic perturbation. In the right diagram the numerical value of the total horizontal magnetic deflection has been plotted. The absorption enhancements referred to the centre of the field are coincident with the peak in the total horizontal geomagnetic perturbation. In addition, the progression velocity derived from the monostatic absorption observations (0.85 km/s) is in reasonable agreement with the average progression velocity (0.80 km/s) derived from the array of geomagnetic recordings presented in Figure 1. This agreement justifies the choice of an altitude of 120 km (E®ion) as the reference height for the absorption progression calculations.
MAGNETOMETER
H
CHAIN
SEP 3
1991
ProR-ressinp Absomtion Events on 3 SeDtember 1991 For the progressing events occurring on 3 September 1991 the available H-component recordings for the west coast are displayed in Figure 5 for the time interval from 1000 to 1600 UT. The poleward progressing disturbances are seen in the recordings from STF at an invariant latitude of 73.7“ and further poleward. There is, however, a large gap in latitude from STF equatorward to NAQ at 67” invariant latitude. Hence is not possible from these data to determine the precise onset time and initial location for the poleward progression.
LNl---
y.cx._&.c
N,O
___..........
_..........
-.
__-:
--
l 10
11
12
13
“I
14
lb
m
nr
IS
Fig. 5. Stack plot of geomagnetic H-components for 1000-1600 UT on 3 September 1991. The line emphasises the poleward progressing features.
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P. Stauning
Even in cases with much closer station coverage the uncertainty in determining the start of progression from the geomagnetic recordings is at least 100-200 km since the magnetometer is inherently sensitive to ionospheric currents over a range at least that wide. Figure 6 displays, for the time interval 1330-1400 UT on 3 September 1991, the absorption traces for the individual beams in the same format as that used for Figure 3. The traces of the fields in the upper right comer have been blanked due to anticipated scintillations. Several progressing absorption enhancements are observed. The one occurring at 1340-1350 UT has been marked by a tilted arrow corresponding to the previously used signature to emphasise the poleward progression. There is, however, another remarkable feature in this case. The events start from a latitude slightly south of the centre of the field of view. The events are seen with full strength in the middle row of panels (beams) but are only just discernible in the traces from the row below. In the two equatormost rows the events are not seen at all. In the central part of the antenna field-of view the angular separation between adjacent beams is approximately 13”. Assuming again an altitude of 120 km for the anticipated E-region processes, the distance between the centres of the ionospheric projections of beams is approximately 30 km. Hence it seems possible to define the starting location for the progressing disturbances with an accuracy better than 30 km for this case where the onset occurs within the IRIS field-of-view.
Fig. 6. Absorption intensities in all IBIS beams during 1330-1400 UT on 3 September 1991.The poleward progression of a selected absorption enhancement occurring at 1340-1350 UT has been emphasised (same feature as that marked in Figure 5).
Summary of Observations of Polar Progressinn Absorotion Events In Figure 7 we present the distribution of the absorption peak amplitudes based on approximately 80 events observed during 19 July 1991, to 3 1 December 1992. It is seen that the distribution peaks at amplitudes of approximately 0.3 dB. No event larger than 0.5 dB was observed within the data interval. The lower part of the distribution, that is, the occurrence frequencies for events at or below 0.1 dB, is more questionable. It is difftcult to establish with certainty whether the very weak events are real ionospheric absorption events and have the progression characteristics of this class of events. Hence the weakest events might have been disregarded. It is quite possible that the amplitude distribution is, in fact, steadily increasing toward the smallest absorption intensities.
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Convection-Related Absorption Processes DAYTIME
POLAR
PROGRESSING
0.5
ABSORPTION
Figure 8 presents the derived progression velocity vectors for all the cases observed during the above interval. The horizontal time axis depicts universal time, Note again, that for Sdr. Strsmfjord, magnetic noon is around 1400 UT. The vertical axis displays a scale for the progression velocity values. The “dots” are placed at a position in the plot corresponding to the time of the observation and the numerical value of the calculated progression velocity. A short bar extends from each dot in the calculated direction of progression. Geomagnetic north is upward in the diagram. Two signatures are used fur the dots. One, where the dot is filled with an asterix (*), depicts absorption events where the progression is evident in all beams. Such events start equatorward of Sdr. Strmnfjord and progress poleward over the station. The other signature, where the dot is open, indicates events seen in the upper beams only. These latter events have their onset within the field-of-view of the imaging riometer in Sdr. Strmnijord. The median level for the calculated velocities as indicated by the horizontal dotted line is 0.71 km/s. This velocity is substantially lower than typical ionospheric plasma convection velocities for the events in question.
EVENTS
i.0
dG
Fig. 7. Statistical amplitude distribution of peak absorption intensities for the cases of poleward progressing absorption events observed during the interval from 19 July 1991 to 31 December 1992. Absorption
I
2.001
V
I
Data
I
I
r
Progressioii
1 I
do
Period:
19/07
t it b
1991
* -
Yelocities.
I
1 I
31/12
I
1992
I
I
Sdr.
I
j
Reference
Stromfjord
1 1 1 1 I
height:
120 km
d
km/s
1.50
i
If
i
I
d
l.OO,-
___ cf cl’“‘” kn’s; evaI_
medim
0.50,-
d
o.ooj 10 ’
’
’
’
’
!
12
’
’
’
’
’
’
14
’ ’ UT
’
’
’
! 16
’
’
’
’
’
1
16
Fig. 8. Summary plot of the distribution of progression velocity vectors calculated for events observed during the interval fkom 19 July 1991, to 3 1 December 1992. The horizontal time axis is universal time. Local magnetic noon for Sdr. Stnamfjord is approximately at 1420 UT. The median direction of progression is indicated by the dotted line tilted to a direction 11 degrees east of geomagnetic north. One may note, that geomagnetic north for Sdr. Str0mfjord is in a direction 40 degrees west of geographic north while invariant north is in a direction 27 degrees west of geographic north. Hence the median progression direction is close to the invariant north direction.
1286
P.Stauning
DISCUSSION A model to explain the progressing absorption events has two parts. One should explain how the interplanetary conditions influence the polar ionospheric plasma convection such that during southward interplanetary magnetic fields the variations in the IMF B, component are transposed to polar geomagnetic perturbations. At times the variations in the H-components observed from stations at the dayside polar cap poleward of the cusp region are almost congruent with the IMF B, variations observed from interplanetary satellites taking into account the appropriate propagation delays and scale factors (Stauning, 1994; Stauning et al, 1994, 1995). A model for the interactions was suggested by Stauning (1994). One important assumption used in this work is based on the observation that the variations in the magnitude of the interplanetary field are rather small compared to the variations in the individual components. The field changes its direction rather than its magnitude. This implies that the currents associated with the field changes most likely are field-aligned. Another assumption is that the interplanetary magnetic fields are embedded in the streaming solar wind such that phase fronts are perpendicular to the flow. In cases with a dominant north-south IMF B, component these assumptions imply that changes in the IMP B, component are associated with sheets of interplanetary currents extended in the Y-Z plane and flowing in the field (Z-) direction. When the Earth’s magnetosphere intersects such current sheets two different modes are possible. In case of a “closed” magnetosphere (no merging) the interplanetary currents are diverted around the magnetopause and will flow mainly in the magnetosheath. Since the magnetopause is not a perfect insulator, part of the current system may reach the terrestrial field lines forming the cusp region. The latter current filaments may enter the cusp region in one hemisphere, cross the cusp at ionospheric altitude and flow from the equatorward edge of the cusp to the opposite hemisphere along the closed field lines of the inner magnetosphere. Here they may, correspondingly, cross the cusp within the ionosphere and flow out along field lines at the poleward edge as depicted by Clauer and Banks (1986). In this model the ionospheric Pedersen currents and thus the imposed horizontal electric fields are north-south oriented. Hence the ionospheric Hall currents, the so-called DPY currents (Wilhjelm at el., 1978), will flow in the east-west direction and generate perturbations in the northward Hcomponent. For constant IMF B, and solar wind plasma parameters this model predicts stationary DPY currents since the ionospheric current systems are constrained to the cusp region. Accordingly the perturbations in the geomagnetic field measured from ground observatories will display stationary, that is, non-progressing patterns. The alternative mode which may occur during southward IMF B, is the case of an “open” magnetosphere where the polar geomagnetic field is connected (merged) with the interplanetary field (Dungey, 1961) such that electric currents may flow along the merged field lines. In this case the interplanetary currents will flow into the magnetosphere along the merged field lines and continue down to the polar cap ionosphere. The currents may flow to the polar cap of the other hemisphere either within the ionosphere or along the closed field lines of the inner magnetospheric regions and then leave from the opposite polar cap along the merged magnetic field lines returning to the interplanetary medium. The field-aligned currents which enter the polar cap ionosphere are locally continued in the form of horizontal Pedersen currents. Due to the limited ionospheric Pedersen conductivities these currents impose large-scale electric fields on the ionosphere. These horizontal electric fields drive the convective plasma flows which, in turn, cause the Hall currents responsible for the geomagnetic perturbations. For cases where the interplanetary field is dominantly southward (IMF B,
Convection-Related Absorption Processes
1287
altitude decreases the denser neutral atmosphere will brake primarily the ions while the electrons continue to move freely under the influence of the electric (and geomagnetic) fields. At still lower altitudes both the ion and the electron drift motions are stopped by collisions with the neutrals. In the intermediate region, which happen to be in the E-region at an altitude of 1lo-120 km, the ions are almost completely at rest in the neutral atmosphere while the electrons still move almost unimpeded. In this situation the presence of an electric field will cause a differential motion between the ions and the electrons. When this relative motion exceeds the local ion-acoustic velocity, Farley-Buneman plasma instabilities may be generated (e.g., D’Angelo, 1976). Such plasma instabilities may act as colliding particles (plasmons) to accomplish a frictional transfer of electric field energy into kinetic energy of the electron population with the result that the electrons are heated to temperatures much higher than those resulting from ordinary Joule heating (Robinson, 1986; Stauning and Olesen, 1989). At such elevated temperatures the electrons collide more frequent with the neutrals. Hence the electron-neutral collision frequencies are increased as the result of increasing electric field strengths above the threshold of -25 mV/m in the polar E-region (e.g., D’Angelo, 1976) for the onset of unstable Farley-Buneman ion-acoustic plasma waves. At present there is no general consensus concerning the proper theoretical description of the plasma physical processes involved. There is, however, abundant experimental evidence that enhanced ionospheric electric fields do cause enhanced E-region temperatures, for instance in Schlegel and St.-Maurice (1981); Stauning (1984); Jones et al. (1991). The possible association between enhanced electric fields and absorption of cosmic noise radio waves was first suggested by D’ Angelo (1976). He, however, explained the observed reduction in the signal level of received cosmic noise as the result of backscatter into space due to E-region electron density irregularities associated with the ion-acoustic plasma waves. The first experimental evidence of the association between strong ionospheric electric fields, enhanced E-region electron temperatures, and cosmic noise absorption observed on riometers was presented by Statming (1984). In this work a close agreement was found between measured absorption intensities and calculated absorption values based on incoherent scatter radar observations of E-region electron densities and electron temperatures for a disturbance event occurring on August 12, 1983. The observed and the calculated cosmic noise intensities varied in close agreement with the ionospheric electric field strengths deduced from the observed ion drift velocities. The association between the enhanced electron temperatures and the large electric fields in the ionosphere was substantiated by model calculations for this event (Stauning and Olesen, 1989) based on the theoretical methods developed by Robinson (1986). The above model, in two parts, connects the variations in IMF B, occurring during southward interplanetary fields with the occurrences of poleward progressing absorption events. CONCLUSIONS Imaging riometer observations have provided a new tool for investigations of polar convection disturbances. For the class of poleward progressing convection disturbance events, which occur as the result of variations in the B, component of the interplanetary magnetic field (IMF) during conditions of strongly southward IMF, the absorption features are easily identified from their progressing character. Such absorption events are usually quite weak, of the order of 0.2-0.3 dB at 38 MHz, contrary to commonly observed types of amoral absorption events which may often amount to several dB at this frequency. The progressing absorption events are related to the E-region electron heating resulting from strong electric fields which trigger Farley-Buneman ion-acoustic plasma instabilities. At the electron densities of a normal daytime E-region, the increase in electron-neutral collision frequencies associated with such elevated electron temperatures may cause increased absorption of cosmic noise radio waves to generate events observable by riometers. Using the imaging riometer techniques enables the precise determination of the progression velocity vector and also the accurate determination of the location of the disturbed ionospheric region. Such observations may, for instance, define the precise region of impact for the disturbances conveyed to the ionosphere from the magnetopause regions where the interactions of the Earth’s magnetosphere with the interplanetary medium are initiated. ACKNOWLEDGEMENTS We gratefully acknowledge the use of the Greenland magnetometer data supplied in a readily accessible form by 0. Rasmussen, DMI. The IRIS imaging riometer in Sdr. Stromfjord is operated in collaboration between DMI and the University of Maryland which is gratefully acknowledged. We, furthermore, gratefully acknowledge the careful
1288
P. Stauning
operation of the imaging riometer by the staff at the ISR radar station in Sdr. Stromfjord, managed for National Science Foundation by SRI International. The IRIS data handling assistance provided by S. Henriksen, DMI, is greatly appreciated. Support for the DMI operation of IRIS imaging riometers has been provided by the Danish Council for Natural Sciences.
REFERENCES Armstrong, J. C., and A. J. Zmuda, Triaxial magnetic measurements of field-aligned currents at 900 km in the auroral region: Initial results, J. Geophys. Res., 78, 6802, 1973. Bumh, J. L., Rate of erosion of dayside magnetic flux based on a quantitative study of the dependence of polar cusp latitude on the interplanetary magnetic fteld, Radio Sci., 8, 955, 1973. Clauer, C. R., and P. M. Banks, Relationship of the interplanetary electric field to the high-latitude ionospheric electric field and currents: Observations and model simulation, J. Geophys. Res., 91, 6959, 1986. Clauer, C.R., P. Stauning, T.J. Rosenberg, E. Friis-Christensen, P.M. Miller, and R.J. Sitar, Observations of a solar wind driven modulation of the dayside ionospheric DPY current system. J. Geophys. Res., 100, 7697-7713, 1995. D’Angelo, N., Gn Riometers, J. Geophys. Res., 81, 5581, 1976. Debick, D. L., and T. J. Rosenberg, A phased-array radiowave imager for studies of cosmic noise absorption, Radio Sci., 25, 325, 1990. Dungey, J. W., Interplanetary magnetic field and the aurora1 zones, Phys. Rev. Lett., 6, 47, 1961. Greenwald, R. A.,, K. B. Baker, J. M. Ruohoniemi, J. R. Dudeney, M. Pinnock, N. Mattin, J. M. Leonard, and R. P. Lepping, Simultaneous conjugate observations of dynamic variations in high-latitude dayside convection due to changes in IMF B,, J. Geophys. Res. 95, 8057, 1990. Iijima, T., and T. A. Potemra, The amplitude distribution of field-aligned currents at northern high latitudes observed by TRIAD, J. Geophys. Res., 81, 2165, 1976a. Iijima, T., and T. A. Potemra, Field-aligned currents in the dayside cusp observed by Triad, J. Geophys. Res., 81, 5971, 1976b. Jones, B., P. J. S. Williams, K. Schlegel, T. R. Robinson, and I. Haggstrom, Interpretation of enhanced electron temperatures measured in the aurora1 E-region during the ERRRIS campaign, Ann. Geophysicae, 9, 55, 1991. Newell, P. T., C.-I. Meng, D. G. Sibeck, and R. P. Lepping, Some low-altitude Cusp Dependencies on the interplanetary magnetic field, J. Geophys. Res., 94, 8921, 1989. Potemra, T. A., R. E. Erlandson, L. J. Zanetti, R. L. Amoldy, J. Woch, and E. Friis-Christensen, The dynamic cusp, J. Geophys. Res., 97, 2835, 1992. Robinson, T.R., Towards a self-consistent nonlinear theory of radar-aurora1 backscatter, J. Atmos. Terr. Phys., 48, 417, 1986. Schlegel, K., and J. P. St.-Maurice, Anomalous heating of the polar E-region by unstable plasma waves, J. Geophys. Res., 86, 1447, 1981. Stauning, P., Absorption of cosmic noise in the E-region during electron heating events. A new class of riometer absorption events, Geophys. Res. Lett., 11, 1184, 1984. Stauning, P., Coupling of IMF B, variations into the polar ionospheres through interplanetary field-aligned currents, J. Geophys. Res., 99, 17,309-17,322, 1994. Stauning, P., and J. K. Olesen, Observations of the unstable plasma in the disturbed polar E-region, Phys. Scripta, 40, 325, 1989. Stauning, P., CR. Clauer, T.J Rosenberg, E. Friis-Christensen, and R. Sitar, Observations of solar-wind-driven progression of interplanetary magnetic field Bv-related dayside ionospheric disturbances. J. Geophys. Res., 100, 7567-7585, 1995. Stauning, P., E. Friis-Christensen, 0. Rasmussen, and S. Vennerstrom, Progressing polar convection disturbances: Signature of an open magnetosphere, J. Geophys. Res., 99, 11,303, 1994. Todd, H., S. W. H. Cowley, M. Lockwood, D. M. Willis, and H. Liihr, Response time of the high latitude dayside ionosphere to sudden changes in the north-south component of the IMF, Wilhjelm, J., E. Friis-Christensen, and T. A. Potemra, The relationship between ionospheric and field-aligned currents in the dayside cusp, J. Geophys. Res., 83, 5586, 1978.
1998
COSPAR.Published by Elsevia Science Ltd. All rights reserved Printed ia Great Britain 0273-i 177198 $19.00 + 0.00
PII: SO273-1177(98)00172-O
POLAR CONVECTION-RELATED IONOSPHERIC RADIOWAVE ABSORPTION PROCESSES OBSERVED BY IMAGING RIOMETERS
P. Stauning Danish Meteorological Institute, Lyngbyvej 100, DK-2100 Copenhagen, Denmark
ABSTRACT The solar wind-magnetosphere interactions resulting in the transfer of energy and momentum from the solar wind to the Earth’s environment are reflected in the polar ionospheric plasma convection. The imaging riometer technique has added a valuable tool for further investigations of the complex dynamics of the polar plasma convection through the monitoring of convection-associated radiowave absorption events. Such absorption events relate to the enhanced electron collision frequencies resulting from E-region electron heating caused by strong electric fields associated with the convection processes. For such events the imaging riometer observations may provide the detection of convection features with high temporal and spatial resolutions that are not readily available with other observing techniques. In cases of antisunward progressing convection disturbances, which occur during southward interplanetary magnetic field conditions, the velocity vectors derived from monostatic imaging riometer observations assuming E-region absorption processes equal those found for the magnetic perturbations using arrays of magnetometers. It is further demonstrated that .the imaging riometer observations may provide precise identification of the initial region of impact for the ionospheric disturbances associated with the solar windQ 1998 COSPAR. Published by Elsevier Science Ltd. magnetosphere interaction at the front magnetopause. INTRODUCTION The polar ionospheric plasma convection and the associated horizontal electric current systems are coupled through field-aligned currents to the magnetospheric plasma motions and electric fields generated by the solar windmagnetosphere interactions. A comprehensive mapping of the magnetosphere-ionosphere field-aligned currents was enabled by satellite observations of the magnetic perturbations in space (e.g., Armstrong and Zmuda, 1973; Iijima and Potemra, 1976a, b). These observations have revealed the existence of the so-called region 1 and region 2 oppositely directed field-aligned current systems, which appear in the morning and evening sectors of the auroral regions at the border of the polar cap (e.g., Iijima and Potemra, 1976a). In the high-latitude noon sector a different system of field-aligned currents was observed. These currents were termed “cusp currents” by Iijima and Potemra (1976b). The steady state location of the cusp is taken to be the region in the noon sector separating geomagnetic field lines closing on the dayside from field lines swept tailward by the solar wind. Its position depends on the solar wind plasma pressure as well as the magnitude and direction of the interplanetary magnetic field @IF), being more equatorward during negative than during positive IMF B,, and displaced in the E-W direction in response to IMF B, (e.g., Butch, 1973; Newell et al., 1989). Investigations of the time-varying dayside convection patterns in the vicinity of the polar cusp regions have searched for signatures of a direct coupling between the solar wind and the Earth’s magnetosphere and ionosphere. One indication of this coupling is the observed strong relationship between changes in the large-scale dayside ionospheric plasma convection and the strength and orientation of the IMF observed upstream near the magnetopause (e.g., Clauer and Banks, 1986; Greenwald et al., 1990; Potemra et al., 1992). The response of the dayside high-latitude ionosphere to changes in the solar wind is observed within a few minutes of the IMF change encountering the front magnetopause (e.g., Todd et al., 1988). This response time is of the order of the Alfven 1279
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travel time from the magnetopause to the ionosphere suggesting a direct electrical connection between the solar wind and the ionosphere. During intervals of consistently southward IMF the modulations of the polar convective flow generated in consequence of variations in the IMF B, component appear to be progressing in the antisunward direction across the dayside polar cap. The term “progression” relates to the propagation of the modulation patterns, which may differ substantially both in magnitude and in direction from the convection velocities of the ionospheric plasma. The statistical relations between IMF conditions and progressing polar convection disturbances have been discussed by Stauning et al. (1994). Case studies of the progression of IMF Byrelated convection disturbances have been presented, e.g., by Stauning et al. (1995) and Clauer et al. (1995). A model to explain the coupling between IMF B, and the poleward moving ionospheric DPY currents in terms of interplanetary currents flowing along the merged interplanetary and terrestrial field lines connecting to the polar cap was presented by Stauning (1994). INSTRUMENTATION The array of observatories that have supplied the data used in the present study comprises stations in Northern Norway, Iceland, Greenland, and Canada. The illustrative cases presented below use the magnetic observations from the chain of stations at the west coast of Greenland. The acronyms for these stations and their geographic coordinates are listed in Table 1. The table also holds the geomagnetic (dipole) coordinates, the magnetic invariant latitudes, and the universal times for magnetic local noon (in the eccentric dipole system) for the stations. Table 1. Selected Geomagnetic Observatories in Greenland, 1991 Station Name
Station Acronym
Thule
Upernavik UmnWWlaq Godhavn Attu Sdr.Strannfjord Maniitsoq Nuuk Paamiut Narsarsuaq
UMQ GDH ATU STF SKT GHB FHB
NAQ
Geographic Coordinates WI
[“El
11.47 72.78 10.68 69.25 67.93 66.99 65.42 64.11 62.00 61.16
290.11 303.85 307.87 306.47 306.43 309.05 307.10 308.21 310.32 314.56
Geomagnetic Coordinates
Wl
[“El
88.91 351.21 83.49 35.48 81.07 38.18 79.88 32.53 78.62 30.44 71.60 34.33 76.12 28.74 74.19 29.53 72.47 31.03 71.18 36.70
Invariant Latitude
WI 85.72 19.83 77.28 76.27 75.07 73.70 72.60 71.19 68.71 67.03
Etc. Dipole Magn. Noon
WI 1434 1408 1402 1421 1428 1417 1432 1431 1429 1412
The “Imaging Riometer for Ionospheric Studies” (IRIS) used in this analysis is co-located with the magnetometer at the incoherent scatter radar (ISR) station in Sdr. Strnrmljord (STF) at the west coast of Greenland. The 64element antenna array and the beam-forming Butler Matrix system have been supplied by the Institute for Physical Science and Technology at the University of Maryland. The receiving system used in STF is a novel design developed at the Danish Meteorological Institute (DMI). The IRIS system operates at 38 MHz and enables the measurement of cosmic noise power received in 49 narrow beam directions organized in a 1 x 7 matrix of beams. The basic recorded data from the imaging riometer measurements are samples of the intensities of cosmic noise power received within each of the 49 beams. From these data, and the quiet day (QD) reference levels, the ionospheric absorption intensities can be calculated for each beam separately. The basic design and the operating principles for the IRIS have been explained by Detrick and Rosenberg (1990). The average altitude for ionospheric absorption processes is around 90 km during most auroral-type events (e.g., substorms). For electron heating absorption (EHA) events, which are associated with E-region plasma instability processes (Stauning, 1984; Stauning and Olesen, 1989), the dominant height range is 1lo-120 km. Using a selected reference height and image processing techniques enables the absorption intensities measured in the different beam directions to be combined into images of the distribution of absorption intensities over the sky within the antenna field-of-view. The image region is approximately 240 x 240 km in extent for a 90 km reference level.
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Convection-Related Absorption Processes
OBSERVATIONS Poleward Promessing Absorntion Events on 26 Seotember 1991 The first of two selected illustrative cases presents observations from 26 September 1991. Magnetograms for the recordings of the H-components from the array of magnetometer stations at the west coast of Greenland are presented in Figure 1. The acronyms of Table 1 are used to refer to the stations. At and poleward of SKT at an invariant latitude of 72.5” a series of poleward progressing pulsations is observed. The pulsations typically have amplitudes of 200-300 nT. The interval between enhancements is 20-40 minutes. A tilted line has been drawn through a series of successive enhancements to illustrate the progression. For the marked enhancement there is a delay of around 10 min from the southernmost (SKT) to the northernmost (UMQ) station corresponding to an average progression velocity of 0.80 km/s. In Figure 2 we present imaging riometer recordings from the IRIS instrument in Sdr. Stremfjord. The upper panel presents the recordings from the second-westernmost column of 7 beams. Within this panel the traces are arranged in sequence from north (top) to south (bottom). The second panel displays the beams of a central cross section. The middle trace in this panel represents the zenith-directed beam. The third panel displays the second-easternmost column of beams, arranged again in sequence from north (top) to south (bottom). These data represent the recorded signal amplitudes of the cosmic noise power received in some of the individual beams. The signal levels have been normalized such that undisturbed traces are horizontal. Hence absorption is positive downward while upward deflections represent various types of interference adding to the level of cosmic noise. In some cases part of the traces have been blanked at the times of anticipated scintillations which may occur when either of the strong, discrete galactic radio sources (e.g. Cassiopeia A or Cygnus A) enters the respective beams. The lowermost panel displays the geomagnetic H, E (eastward) and 2 (vertical) components recorded by the co-located magnetometer. MAGNETOMETER
H
CHAtN
SEP 26
1991
I. 10
II
12
13
UT
I4
15
16
Fig. 1. Example of poleward progressing convection disturbances observed in the H-components recorded at the Greenland west-coast chain of stations during 1000 - 1600 UT on 26 September 1991. Note that the stations are not equidistantly positioned in the plot.
12
I
.
.
..a.....t.....I II
UT
11
Fig 2. Upper three panels: Recordings of cosmic noise signal intensities in selected IRIS beams for the events observed during 1200-I 600 UT on 26 September 199 1. Bottom panel: Geomagnetic recordings from the colocated magnetometer in Sdr. Stromfjord.
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It is seen from the IBIS traces in the upper three panels that there is a weak downward deflection (an absorption event) at around 1400-1420 UT. Further one may notice the phase shirts making the event occur further and further delayed further poleward. The delays correspond to a poleward progression which is emphasized by the tilted arrow in the middle panel. Corresponding to the time of the absorption event as seen in the zenith-directed beam the geomagnetic H-component displays a pronounced peak. In Figure 3 the data from all beams are presented in the form of absorption intensities (reduction in signal relative to the QD level). There is a field for the time-varying absorption intensities for each beam. The fields have been positioned to represent the direction of the beams such that the upper left field represents the data from the northwesternmost beam and so on. The absorption intensities are defined by the vertical scale to the left. The time marks along the horizontal axes depict 5-min intervals. Date and time interval for the recordings are shown at the upper right comer. The figure presents an absorption event of approximately 0.25 dB peak intensity where absorption features are progressing from south to north as indicated by the tilted line.
Fig. 3. Absorption intensities recorded by the__IRIS imaging riometer in Sdr. Stromfjord during 1400-1430 . . UT on 26 September 1991. The progression of the absorption enhancements is emphasised by the arrow line. In order to more precisely calculate the progression velocity vector a correlation program has been developed. The principles of the calculations are shown in Figure 4. The left diagram displays the original data in the form of absorption intensities. The data for the various beams have been grouped in panels such that all traces from the northernmost row of beams have been plotted in the upper field and so on downward. There is an absorption scale for each panel to the left of the diagram. A tilted arrow has been drawn to emphasise the poleward progression of the absorption enhancement. In the lower part of the left diagram the geomagnetic recordings have been displayed with a trace for each component (H, E, and Z). Baselines (I-&,E,, and Z,) are indicated by the dashed lines. Assuming that the disturbances occur in the E-region at 120 km of altitude and propagate across the field of view in the form of progressing tints, the program calculates the best fit to the progression velocity vector. The result is printed in the upper right comer of the right diagram in Figure 4. In this case the program has derived a progression velocity of 0.85 km/s in a direction 16” east of geomagnetic north. Using the calculated velocity vector, and the distances from the ionospheric projections of the individual beams to the centre of the field, the corresponding delays from the assumed progression have been calculated. The resulting delays have then been imposed on the recorded data and the resulting traces are displayed in the right diagram of Figure 4. It is clear from the figure, that the absorption enhancements are now aligned. The vertical dashed line is for guidance only.
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Convection-Related Absorption Recesses IRIS
36
f4hr
Sdr.
Stronf)orO
26
SEPTEI49ER
1991
IRIS
36
Uh?.
Ser.
Stromfjord
1412
26
KPTEM6ER
1991
UT
1442
Fig. 4. Diagram to describe the calculation of progression velocities. Upper part of left diagram: Raw imaging riometer data. The progression is emphasised by the arrow line. Lower part: Recordings of individual geomagnetic components. Upper part of right diagram: Absorption traces shifted by anticipated delays using calculated progression velocity vector. Lower part: Total horizontal geomagnetic perturbation. In the right diagram the numerical value of the total horizontal magnetic deflection has been plotted. The absorption enhancements referred to the centre of the field are coincident with the peak in the total horizontal geomagnetic perturbation. In addition, the progression velocity derived from the monostatic absorption observations (0.85 km/s) is in reasonable agreement with the average progression velocity (0.80 km/s) derived from the array of geomagnetic recordings presented in Figure 1. This agreement justifies the choice of an altitude of 120 km (E®ion) as the reference height for the absorption progression calculations.
MAGNETOMETER
H
CHAIN
SEP 3
1991
ProR-ressinp Absomtion Events on 3 SeDtember 1991 For the progressing events occurring on 3 September 1991 the available H-component recordings for the west coast are displayed in Figure 5 for the time interval from 1000 to 1600 UT. The poleward progressing disturbances are seen in the recordings from STF at an invariant latitude of 73.7“ and further poleward. There is, however, a large gap in latitude from STF equatorward to NAQ at 67” invariant latitude. Hence is not possible from these data to determine the precise onset time and initial location for the poleward progression.
LNl---
y.cx._&.c
N,O
___..........
_..........
-.
__-:
--
l 10
11
12
13
“I
14
lb
m
nr
IS
Fig. 5. Stack plot of geomagnetic H-components for 1000-1600 UT on 3 September 1991. The line emphasises the poleward progressing features.
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Even in cases with much closer station coverage the uncertainty in determining the start of progression from the geomagnetic recordings is at least 100-200 km since the magnetometer is inherently sensitive to ionospheric currents over a range at least that wide. Figure 6 displays, for the time interval 1330-1400 UT on 3 September 1991, the absorption traces for the individual beams in the same format as that used for Figure 3. The traces of the fields in the upper right comer have been blanked due to anticipated scintillations. Several progressing absorption enhancements are observed. The one occurring at 1340-1350 UT has been marked by a tilted arrow corresponding to the previously used signature to emphasise the poleward progression. There is, however, another remarkable feature in this case. The events start from a latitude slightly south of the centre of the field of view. The events are seen with full strength in the middle row of panels (beams) but are only just discernible in the traces from the row below. In the two equatormost rows the events are not seen at all. In the central part of the antenna field-of view the angular separation between adjacent beams is approximately 13”. Assuming again an altitude of 120 km for the anticipated E-region processes, the distance between the centres of the ionospheric projections of beams is approximately 30 km. Hence it seems possible to define the starting location for the progressing disturbances with an accuracy better than 30 km for this case where the onset occurs within the IRIS field-of-view.
Fig. 6. Absorption intensities in all IBIS beams during 1330-1400 UT on 3 September 1991.The poleward progression of a selected absorption enhancement occurring at 1340-1350 UT has been emphasised (same feature as that marked in Figure 5).
Summary of Observations of Polar Progressinn Absorotion Events In Figure 7 we present the distribution of the absorption peak amplitudes based on approximately 80 events observed during 19 July 1991, to 3 1 December 1992. It is seen that the distribution peaks at amplitudes of approximately 0.3 dB. No event larger than 0.5 dB was observed within the data interval. The lower part of the distribution, that is, the occurrence frequencies for events at or below 0.1 dB, is more questionable. It is difftcult to establish with certainty whether the very weak events are real ionospheric absorption events and have the progression characteristics of this class of events. Hence the weakest events might have been disregarded. It is quite possible that the amplitude distribution is, in fact, steadily increasing toward the smallest absorption intensities.
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Convection-Related Absorption Processes DAYTIME
POLAR
PROGRESSING
0.5
ABSORPTION
Figure 8 presents the derived progression velocity vectors for all the cases observed during the above interval. The horizontal time axis depicts universal time, Note again, that for Sdr. Strsmfjord, magnetic noon is around 1400 UT. The vertical axis displays a scale for the progression velocity values. The “dots” are placed at a position in the plot corresponding to the time of the observation and the numerical value of the calculated progression velocity. A short bar extends from each dot in the calculated direction of progression. Geomagnetic north is upward in the diagram. Two signatures are used fur the dots. One, where the dot is filled with an asterix (*), depicts absorption events where the progression is evident in all beams. Such events start equatorward of Sdr. Strmnfjord and progress poleward over the station. The other signature, where the dot is open, indicates events seen in the upper beams only. These latter events have their onset within the field-of-view of the imaging riometer in Sdr. Strmnijord. The median level for the calculated velocities as indicated by the horizontal dotted line is 0.71 km/s. This velocity is substantially lower than typical ionospheric plasma convection velocities for the events in question.
EVENTS
i.0
dG
Fig. 7. Statistical amplitude distribution of peak absorption intensities for the cases of poleward progressing absorption events observed during the interval from 19 July 1991 to 31 December 1992. Absorption
I
2.001
V
I
Data
I
I
r
Progressioii
1 I
do
Period:
19/07
t it b
1991
* -
Yelocities.
I
1 I
31/12
I
1992
I
I
Sdr.
I
j
Reference
Stromfjord
1 1 1 1 I
height:
120 km
d
km/s
1.50
i
If
i
I
d
l.OO,-
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medim
0.50,-
d
o.ooj 10 ’
’
’
’
’
!
12
’
’
’
’
’
’
14
’ ’ UT
’
’
’
! 16
’
’
’
’
’
1
16
Fig. 8. Summary plot of the distribution of progression velocity vectors calculated for events observed during the interval fkom 19 July 1991, to 3 1 December 1992. The horizontal time axis is universal time. Local magnetic noon for Sdr. Stnamfjord is approximately at 1420 UT. The median direction of progression is indicated by the dotted line tilted to a direction 11 degrees east of geomagnetic north. One may note, that geomagnetic north for Sdr. Str0mfjord is in a direction 40 degrees west of geographic north while invariant north is in a direction 27 degrees west of geographic north. Hence the median progression direction is close to the invariant north direction.
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DISCUSSION A model to explain the progressing absorption events has two parts. One should explain how the interplanetary conditions influence the polar ionospheric plasma convection such that during southward interplanetary magnetic fields the variations in the IMF B, component are transposed to polar geomagnetic perturbations. At times the variations in the H-components observed from stations at the dayside polar cap poleward of the cusp region are almost congruent with the IMF B, variations observed from interplanetary satellites taking into account the appropriate propagation delays and scale factors (Stauning, 1994; Stauning et al, 1994, 1995). A model for the interactions was suggested by Stauning (1994). One important assumption used in this work is based on the observation that the variations in the magnitude of the interplanetary field are rather small compared to the variations in the individual components. The field changes its direction rather than its magnitude. This implies that the currents associated with the field changes most likely are field-aligned. Another assumption is that the interplanetary magnetic fields are embedded in the streaming solar wind such that phase fronts are perpendicular to the flow. In cases with a dominant north-south IMF B, component these assumptions imply that changes in the IMP B, component are associated with sheets of interplanetary currents extended in the Y-Z plane and flowing in the field (Z-) direction. When the Earth’s magnetosphere intersects such current sheets two different modes are possible. In case of a “closed” magnetosphere (no merging) the interplanetary currents are diverted around the magnetopause and will flow mainly in the magnetosheath. Since the magnetopause is not a perfect insulator, part of the current system may reach the terrestrial field lines forming the cusp region. The latter current filaments may enter the cusp region in one hemisphere, cross the cusp at ionospheric altitude and flow from the equatorward edge of the cusp to the opposite hemisphere along the closed field lines of the inner magnetosphere. Here they may, correspondingly, cross the cusp within the ionosphere and flow out along field lines at the poleward edge as depicted by Clauer and Banks (1986). In this model the ionospheric Pedersen currents and thus the imposed horizontal electric fields are north-south oriented. Hence the ionospheric Hall currents, the so-called DPY currents (Wilhjelm at el., 1978), will flow in the east-west direction and generate perturbations in the northward Hcomponent. For constant IMF B, and solar wind plasma parameters this model predicts stationary DPY currents since the ionospheric current systems are constrained to the cusp region. Accordingly the perturbations in the geomagnetic field measured from ground observatories will display stationary, that is, non-progressing patterns. The alternative mode which may occur during southward IMF B, is the case of an “open” magnetosphere where the polar geomagnetic field is connected (merged) with the interplanetary field (Dungey, 1961) such that electric currents may flow along the merged field lines. In this case the interplanetary currents will flow into the magnetosphere along the merged field lines and continue down to the polar cap ionosphere. The currents may flow to the polar cap of the other hemisphere either within the ionosphere or along the closed field lines of the inner magnetospheric regions and then leave from the opposite polar cap along the merged magnetic field lines returning to the interplanetary medium. The field-aligned currents which enter the polar cap ionosphere are locally continued in the form of horizontal Pedersen currents. Due to the limited ionospheric Pedersen conductivities these currents impose large-scale electric fields on the ionosphere. These horizontal electric fields drive the convective plasma flows which, in turn, cause the Hall currents responsible for the geomagnetic perturbations. For cases where the interplanetary field is dominantly southward (IMF B,
Convection-Related Absorption Processes
1287
altitude decreases the denser neutral atmosphere will brake primarily the ions while the electrons continue to move freely under the influence of the electric (and geomagnetic) fields. At still lower altitudes both the ion and the electron drift motions are stopped by collisions with the neutrals. In the intermediate region, which happen to be in the E-region at an altitude of 1lo-120 km, the ions are almost completely at rest in the neutral atmosphere while the electrons still move almost unimpeded. In this situation the presence of an electric field will cause a differential motion between the ions and the electrons. When this relative motion exceeds the local ion-acoustic velocity, Farley-Buneman plasma instabilities may be generated (e.g., D’Angelo, 1976). Such plasma instabilities may act as colliding particles (plasmons) to accomplish a frictional transfer of electric field energy into kinetic energy of the electron population with the result that the electrons are heated to temperatures much higher than those resulting from ordinary Joule heating (Robinson, 1986; Stauning and Olesen, 1989). At such elevated temperatures the electrons collide more frequent with the neutrals. Hence the electron-neutral collision frequencies are increased as the result of increasing electric field strengths above the threshold of -25 mV/m in the polar E-region (e.g., D’Angelo, 1976) for the onset of unstable Farley-Buneman ion-acoustic plasma waves. At present there is no general consensus concerning the proper theoretical description of the plasma physical processes involved. There is, however, abundant experimental evidence that enhanced ionospheric electric fields do cause enhanced E-region temperatures, for instance in Schlegel and St.-Maurice (1981); Stauning (1984); Jones et al. (1991). The possible association between enhanced electric fields and absorption of cosmic noise radio waves was first suggested by D’ Angelo (1976). He, however, explained the observed reduction in the signal level of received cosmic noise as the result of backscatter into space due to E-region electron density irregularities associated with the ion-acoustic plasma waves. The first experimental evidence of the association between strong ionospheric electric fields, enhanced E-region electron temperatures, and cosmic noise absorption observed on riometers was presented by Statming (1984). In this work a close agreement was found between measured absorption intensities and calculated absorption values based on incoherent scatter radar observations of E-region electron densities and electron temperatures for a disturbance event occurring on August 12, 1983. The observed and the calculated cosmic noise intensities varied in close agreement with the ionospheric electric field strengths deduced from the observed ion drift velocities. The association between the enhanced electron temperatures and the large electric fields in the ionosphere was substantiated by model calculations for this event (Stauning and Olesen, 1989) based on the theoretical methods developed by Robinson (1986). The above model, in two parts, connects the variations in IMF B, occurring during southward interplanetary fields with the occurrences of poleward progressing absorption events. CONCLUSIONS Imaging riometer observations have provided a new tool for investigations of polar convection disturbances. For the class of poleward progressing convection disturbance events, which occur as the result of variations in the B, component of the interplanetary magnetic field (IMF) during conditions of strongly southward IMF, the absorption features are easily identified from their progressing character. Such absorption events are usually quite weak, of the order of 0.2-0.3 dB at 38 MHz, contrary to commonly observed types of amoral absorption events which may often amount to several dB at this frequency. The progressing absorption events are related to the E-region electron heating resulting from strong electric fields which trigger Farley-Buneman ion-acoustic plasma instabilities. At the electron densities of a normal daytime E-region, the increase in electron-neutral collision frequencies associated with such elevated electron temperatures may cause increased absorption of cosmic noise radio waves to generate events observable by riometers. Using the imaging riometer techniques enables the precise determination of the progression velocity vector and also the accurate determination of the location of the disturbed ionospheric region. Such observations may, for instance, define the precise region of impact for the disturbances conveyed to the ionosphere from the magnetopause regions where the interactions of the Earth’s magnetosphere with the interplanetary medium are initiated. ACKNOWLEDGEMENTS We gratefully acknowledge the use of the Greenland magnetometer data supplied in a readily accessible form by 0. Rasmussen, DMI. The IRIS imaging riometer in Sdr. Stromfjord is operated in collaboration between DMI and the University of Maryland which is gratefully acknowledged. We, furthermore, gratefully acknowledge the careful
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operation of the imaging riometer by the staff at the ISR radar station in Sdr. Stromfjord, managed for National Science Foundation by SRI International. The IRIS data handling assistance provided by S. Henriksen, DMI, is greatly appreciated. Support for the DMI operation of IRIS imaging riometers has been provided by the Danish Council for Natural Sciences.
REFERENCES Armstrong, J. C., and A. J. Zmuda, Triaxial magnetic measurements of field-aligned currents at 900 km in the auroral region: Initial results, J. Geophys. Res., 78, 6802, 1973. Bumh, J. L., Rate of erosion of dayside magnetic flux based on a quantitative study of the dependence of polar cusp latitude on the interplanetary magnetic fteld, Radio Sci., 8, 955, 1973. Clauer, C. R., and P. M. Banks, Relationship of the interplanetary electric field to the high-latitude ionospheric electric field and currents: Observations and model simulation, J. Geophys. Res., 91, 6959, 1986. Clauer, C.R., P. Stauning, T.J. Rosenberg, E. Friis-Christensen, P.M. Miller, and R.J. Sitar, Observations of a solar wind driven modulation of the dayside ionospheric DPY current system. J. Geophys. Res., 100, 7697-7713, 1995. D’Angelo, N., Gn Riometers, J. Geophys. Res., 81, 5581, 1976. Debick, D. L., and T. J. Rosenberg, A phased-array radiowave imager for studies of cosmic noise absorption, Radio Sci., 25, 325, 1990. Dungey, J. W., Interplanetary magnetic field and the aurora1 zones, Phys. Rev. Lett., 6, 47, 1961. Greenwald, R. A.,, K. B. Baker, J. M. Ruohoniemi, J. R. Dudeney, M. Pinnock, N. Mattin, J. M. Leonard, and R. P. Lepping, Simultaneous conjugate observations of dynamic variations in high-latitude dayside convection due to changes in IMF B,, J. Geophys. Res. 95, 8057, 1990. Iijima, T., and T. A. Potemra, The amplitude distribution of field-aligned currents at northern high latitudes observed by TRIAD, J. Geophys. Res., 81, 2165, 1976a. Iijima, T., and T. A. Potemra, Field-aligned currents in the dayside cusp observed by Triad, J. Geophys. Res., 81, 5971, 1976b. Jones, B., P. J. S. Williams, K. Schlegel, T. R. Robinson, and I. Haggstrom, Interpretation of enhanced electron temperatures measured in the aurora1 E-region during the ERRRIS campaign, Ann. Geophysicae, 9, 55, 1991. Newell, P. T., C.-I. Meng, D. G. Sibeck, and R. P. Lepping, Some low-altitude Cusp Dependencies on the interplanetary magnetic field, J. Geophys. Res., 94, 8921, 1989. Potemra, T. A., R. E. Erlandson, L. J. Zanetti, R. L. Amoldy, J. Woch, and E. Friis-Christensen, The dynamic cusp, J. Geophys. Res., 97, 2835, 1992. Robinson, T.R., Towards a self-consistent nonlinear theory of radar-aurora1 backscatter, J. Atmos. Terr. Phys., 48, 417, 1986. Schlegel, K., and J. P. St.-Maurice, Anomalous heating of the polar E-region by unstable plasma waves, J. Geophys. Res., 86, 1447, 1981. Stauning, P., Absorption of cosmic noise in the E-region during electron heating events. A new class of riometer absorption events, Geophys. Res. Lett., 11, 1184, 1984. Stauning, P., Coupling of IMF B, variations into the polar ionospheres through interplanetary field-aligned currents, J. Geophys. Res., 99, 17,309-17,322, 1994. Stauning, P., and J. K. Olesen, Observations of the unstable plasma in the disturbed polar E-region, Phys. Scripta, 40, 325, 1989. Stauning, P., CR. Clauer, T.J Rosenberg, E. Friis-Christensen, and R. Sitar, Observations of solar-wind-driven progression of interplanetary magnetic field Bv-related dayside ionospheric disturbances. J. Geophys. Res., 100, 7567-7585, 1995. Stauning, P., E. Friis-Christensen, 0. Rasmussen, and S. Vennerstrom, Progressing polar convection disturbances: Signature of an open magnetosphere, J. Geophys. Res., 99, 11,303, 1994. Todd, H., S. W. H. Cowley, M. Lockwood, D. M. Willis, and H. Liihr, Response time of the high latitude dayside ionosphere to sudden changes in the north-south component of the IMF, Wilhjelm, J., E. Friis-Christensen, and T. A. Potemra, The relationship between ionospheric and field-aligned currents in the dayside cusp, J. Geophys. Res., 83, 5586, 1978.