[Contents] [Index] [Search] [Home] [Up] Prompt Mid-Latitude Electric Field Effects during Severe Geomagnetic Storms

Prompt Mid-Latitude Electric Field Effects during Severe Geomagnetic Storms

In Press: J. Geophys. Res. , 1997.

J. C. Foster Haystack Observatory, Massachusetts Institute of Technology, Westford, Massachusetts 01886
F. J. Rich USAF Phillips Laboratory, Geophysics Directorate, Hanscon AFB, Massachusetts 01731

Abstract

Meridian-plane elevation scans with the Millstone Hill incoherent scatter radar provide evidence of a strong perturbation of the coupled mid-latitude magnetosphere-ionosphere system during the early phases of the November 4, 1993 magnetic storm. A narrow ionospheric trough formed at L=3.5 in the pre-midnight sector, immediately poleward of the Millstone Hill site. The most pronounced radar signature of the developing activity was a brief (20 min) uplifting of the F region plasma equatorward of the trough, such that the peak altitude increased with distance away from the trough. A similar signature had been observed during storm onset on March 20, 1990, and in that event a pronounced topside ionospheric depletion developed in the region far equatorward of the mid-latitude trough and was observed by the radar and the DMSP F9 satellite. During the November 4, 1993 event, the DMSP F10 satellite observed narrow, magnetically conjugate regions of plasma density depletion and strong horizontal and upward plasma velocity (> 1500 m/s) at L=1.5 at the time of the uplifting of the mid-latitude F region observed by the radar. These observations were confined to longitudes near the South Atlantic magnetic anomaly and, in the Nov 1993 case, the perturbation was coincident with the peak of the precipitating particle fluxes associated with inner-belt losses at the anomaly. Both the uplifting of the ionospheric F layer and the triggering of topside density perturbations can be explained in terms of an eastward electric field imposed on the mid and low-latitude ionosphere during the initial stages of the geomagnetic storm. The low-latitude ionospheric perturbations in these events were similar to supersonic equatorial bubbles, triggered by the destabilizing effects of the upward ExB drift associated with the eastward electric field.

Introduction

During geomagnetic disturbances, the electric fields and particle populations which characterize the auroral region expand equatorward and their effects are felt at previously sub-auroral latitudes. Intense convection electric fields appear in the expanded auroral oval (e.g. Yeh et al., [1988]) while transient fields penetrate equatorward of the shielding region near the inner edge of the ring current at mid latitudes (e.g. Gonzales et al., [1983]). A number of important magnetospheric boundaries are found near the auroral/sub-auroral transition (nominally near 60° magnetic latitude) and these result in the ionospheric structure and dynamics which characterize the stormtime ionospheric observations made from Millstone Hill (42.6°N, 288.5°E). The high-altitude plasmapause maps down to the region near the equatorward edge of the (mid-latitude) ionospheric trough and is associated with the boundary between the corotating inner magnetosphere and the strong convection electric fields which drive the ionospheric circulation at auroral latitudes. The equatorward limit of the plasma sheet particle population lies on field lines near the plasmapause and precipitation from the plasma sheet alters the ionospheric conductances, currents and fields. Electric field shielding in the equatorial magnetosphere arises when the inward transport of injected plasma in the time-varying magnetic and convection electric fields, combined with the corotation electric field, results in electron and ion separation and the formation of plasma pressure gradients. In the theoretical treatment of Southwood and Wolf [1978], a northward electric field and rapid sunward convection are seen at subauroral latitudes in the local evening sector when a disturbance results in the penetration of partial ring current ions to a lower L shell than the plasma sheet electrons. The strength of this electric field is inversely related to the latitudinal separation of the particle boundaries. During disturbed conditions, an intense (>100 mV/m) polarization electric field can be set up [Galperin, 1974] and this drives the latitudinally-narrow polarization jet or Sub-Auroral Ion Drifts (SAID) [Spiro et al., 1979]. Associated with the region of strongest convection, a deep, narrow F region trough is formed [Schunk et al., 1976] and this stormtime feature is often nearly overhead at the Millstone site. A non-zero time constant for the buildup (or decay) of electric field shielding at mid latitudes has the consequence that an impulsive increase in the high-latitude cross polar cap electric potential results in the penetration of an eastward electric field to low latitudes in the inner magnetosphere in the pre-midnight sector [Senior and Blanc, 1984]. In investigations of the occurrence of severe radio scintillation at low latitudes during the initial stages of strong geomagnetic storms, Tanaka [1981] identified sudden and short-lived perturbations of the low-latitude ionosphere, confined to the premidnight sector, and concluded that these were due to penetrating eastward electric fields. In the following sections, data from the Millstone Hill incoherent scatter radar and the DMSP spacecraft are used to investigate the prompt ionospheric effects of such eastward electric fields during the early phases of magnetic storms.

Observations

The second half of 1993 was marked by a period of recurrent solar-induced activity, with brief geomagnetic storms occurring in response to favorable orientations of a persistent coronal hole at the 27-day solar rotation interval. Figure 1 presents the planetary A index of geomagnetic activity (Ap) for late 1993 which reveals two intervals of solar-produced activity recurring with the 27-day solar rotation period. Using this plot of the Ap index, the radar/tomography (RATE) experiment campaign window was scheduled to overlap our prediction of this recurrent activity. A severe mid-latitude magnetic storm took place early on November 4, 1993 causing the planetary A index (Ap) to reach 77 on that day. Elevation scans were performed with the Millstone Hill incoherent scatter radar along the 285° E meridian and these produced observations of ionospheric plasma density, temperatures, and line of sight velocity over the 180 km - 500 km altitude range and over the magnetic latitude range 49° - 67° with 20-min time resolution. Observations were made through the interval beginning immediately preceding the storm onset and continuing through the subsequent period of rapid ionospheric trough formation and the enhancement of lower-F region ionization at mid latitudes caused by low-energy particle precipitation. A description of the RATE campaign and the intercomparisons of the radar and radiotomography observations through the initial phases of the November 4, 1993 storm are reported by Foster et al. [1994].

Stormtime Ionospheric Perturbation Equatorward of the Trough

Here, we compare and combine the observations of two very similar events in constructing a picture of prompt storm-time mid- and low-latitude effects associated with the impulsive magnetospheric disturbances accompanying sudden solar wind density enhancements. Solar wind density, velocity, and magnetic field parameters are presented in Figure 2 for the March 20-21, 1990 and the November 3-4, 1993 events. Both events were marked by the arrival of an impulsive solar wind density enhancement during an interval of increasing solar wind velocity and accompanied by strongly negative IMF Bz. Sharp density enhancements occurred near 23 UT during both events and, as seen below, produced similar prompt stormtime effects as viewed by the Millstone Hill radar at subauroral latitude in the pre-midnight sector. Radar elevations scans were being performed on both days, but with differing latitude coverage and temporal repeat cycle. A pronounced and unusual effect was observed near 00:30 UT (~19:00 MLT) during each event in which the radar observed a strong uplifting of the F-layer peak altitude in the region equatorward of the sharply-defined mid-latitude ionospheric trough, such that the uplifting was more pronounced at latitudes more equatorward from the trough. For each event, Figure 3 shows an ionospheric trough near 45°, somewhat poleward of Millstone Hill (42.5° geodetic or 55° magnetic latitude), and the well-defined upward slope of the isodensity contours equatorward of that feature. The March, 1990 scans covered a wide expanse of latitude and the uplift of the F layer was observed extending > 15° equatorward of the trough. The November, 1993 scan was of more limited extent but clearly observed the uplift of the F region and, poleward of the trough, an ionization enhancement at altitudes between 200 km and 350 km which accompanied storm-induced particle precipitation between 46° and 51° geodetic latitude (57° - 62° magnetic latitude). This more poleward feature was confirmed to be precipitation-enhanced density, rather than coherent backscatter from E region electric field-produced irregularities (e.g. Foster et al., [1992]), through the radiotomographic analyses presented in Foster et al. [1994]. The pronounced equatorward tilt in the uplifting of the F region was observed in only one scan during the November event, giving a temporal duration of less than ~20 min for that phenomenon. The F peak altitude, hmF2, equatorward of the trough remained elevated (400 km - 450 km) until after 04:00 UT. During the March event, the scan subsequent to that shown in Figure 3 occurred some 45 min later and, at that time, a region of density depletion, with the appearances of a low-latitude ionospheric trough, was seen near the equatorward limit of the Millstone Hill elevation-scan field of view (not shown). The Millstone observations were confined to a N-S plane, however, and no information on the extent of this feature in longitude was obtained. An overflight of the DMSP F9 satellite nearly along the Millstone meridian occurred some 100 min after the observed uplifting of the F layer and both the in situ density probe and the radar scan observed large-scale topside density depletions near 37° magnetic latitude (cf. Figure 4) at that time. Upward vertical and westward horizontal velocities, coincident with the density depletions, were seen with the DMSP ion drift meter.

The similarities of the March, 1990 and the November, 1993 events prompted an examination of the region equatorward of the radar field of view for evidence of low- or mid-latitude density depletions following the uplift of the F layer on November 4, 1993. The DMSP F10 satellite was well positioned and was making a pre-midnight equatorial crossing from south to north at 320° east longitude (~35° east of the Millstone Hill meridian) exactly at the time of the F layer uplifting (00:30 UT) shown in Figure 3. Figure 5 presents the DMSP F10 observations of magnetically conjugate, order of magnitude, topside density depletions and large (~1500 m/s) horizontal and vertical ion flows near L=1.25 (+/- 26° MLAT110, magnetic latitude determined at the 110 km altitude point on the field line through the satellite). At the top of the figure, the smooth, rounded increase in the electron-detector number flux indicates the effect of energetic ion impact on the satellite at 840 km altitude as it flies through the low-altitude radiation belt environment associated with the South Atlantic magnetic anomaly (SAA). It is noteworthy that the field line associated with the topside density and velocity perturbation is nearly coincident with the peak of these energetic proton fluxes (- 25° MLAT110) which serve to define the extent of the SAA-related effects at the 320° E longitude of the satellite pass. There is a data gap over that portion of the northern hemisphere DMSP F10 orbit which is coincident with the Millstone Hill field of view. A limited longitudinal extent for the topside density perturbation observed by DMSP F10 is indicated by a near-simultaneous pass of DMSP F8 which crossed the equator at 00:38 UT at 265° longitude (near 18 MLT and 20° west of the Millstone Hill meridian) and observed no density or velocity perturbation at mid, low, or equatorial latitudes.

Evidence for an Eastward Electric Field

Both the uplifting of the ionospheric F layer and the triggering of topside density perturbations can be explained in terms of an eastward electric field imposed on the mid and low-latitude ionosphere during the initial stages of the geomagnetic storm. The Arecibo incoherent scatter radar (18.3°N, 293.25°E) was performing a high- elevation angle azimuth scan during the March 21, 1990 event and line of sight Doppler velocity observations near 00 UT shown in Figure 6 indicate a strong northward component to the plasma motion at that time. Buonsanto and Foster [1993] analyzed the radar observations at Arecibo and Millstone Hill and found that at Arecibo an upward excursion in hmF2 accompanied an upward and northward perpendicular ion velocity near 00:00 UT, indicative of an eastward electric field at Arecibo's latitude. After 01:00 UT, surges in the equatorward neutral wind resulted in a downward and southward perpendicular ion motion. Details of these analyses were provided in that earlier paper.

At midlatitudes, the effect of an eastward electric field is to uplift the ionosphere perpendicular to the magnetic field line, while at the same time downward redistribution of the plasma occurs parallel to the field line (cf. Maruyama [1990]). The net effect for an equatorward-looking incoherent scatter radar viewing such a region at low elevation angle is the observation of plasma motion toward the radar. Millstone Hill elevations scans to the South at the time of the F-layer uplifting observed a strong ion velocity component directed toward the radar whose magnitude increased with decreasing latitude (Figure 7). This inferred electric field effect had temporal duration of ~ 1 hour, with maximum effect near 38° magnetic latitude at ~00:30 UT. The topside perturbations observed by the radar and the DMSP satellite subsequent to this were concentrated near 37° magnetic latitude (cf. Figure 4) suggesting that these were topside ionospheric effects of the penetrating stormtime electric field responsible for the uplifting of the F layer at mid and low latitudes.

During the November 3-4, 1993 event, the Millstone Hill radar scans did not extend so far equatorward and, other than the uplifting of the F layer, the low-latitude effects of an eastward penetration electric field were not observed directly. However, a low/mid-latitude chain of ionosondes in Japan (135°E meridian) provided evidence of a concentration of storm-induced eastward electric field near the ~25° magnetic latitude where the topside perturbations were observed in the American sector by DMSP F10 (cf. Figure 5). F-layer virtual height was scaled from summary plots of the Japanese-chain observations [Ionospheric Data in Japan for November 1993, 45 #11, Com. Research Lab., Tokyo, 1994] and the perturbation height plotted in Figure 8 for stations spanning the magnetic latitude range 16° - 35°. Uplifting was observed between 00:30 UT and 02:30 UT on November 4, 1993 such that the magnitude of the perturbation increased with decreasing latitude to a maximum near 23°N magnetic latitude. Although the virtual-height observations do not differentiate between the effects of an eastward electric field or of an equatorward surge of the neutral wind, Tanaka [1986] has analyzed the storm-time ionospheric response seen along this chain of stations and provides criteria for differentiating between the effects of electric fields and winds. The effect of a penetrating eastward magnetospheric electric field is to uplift the F layer, producing a simultaneous increase in h'F at mid latitudes, while enhancing the equatorial anomalies at sub-auroral latitudes, producing an increase in foF2 at slightly later times. The equatorial anomalies are created as an eastward electric field uplifts the equatorial ionosphere, which subsequently diffuses down the magnetic field to produce density enhancements at somewhat higher latitudes. The response to winds propagating equatorward from a high-latitude disturbance, however, is characterized by increases in h'F that travel from north to south with noticeable time dispersions and no change in foF2 or total electron content. For the November, 1993 event (cf. Figure 8) increases in foF2 of 25% - 50% were observed along the station chain, with the greatest increase occurring near Yamagawa, consistent with an enhancement of the equatorial anomaly by an eastward electric field. The perturbations in virtual height shown in the figure, however, suggest a N-S temporal dispersion, consistent with the effect of a storm-generated wind or topside ionospheric disturbance, TID. We have found the mid-latitude ionospheric perturbations to be associated with density irregularities with spatial scale sizes of tens of meters to > 100 km. In situ ion density was sampled at a 24 Hz rate by the DMSP satellites as they crossed the mid-latitude perturbation regions with 7.45 km s-1 orbital speed, providing information on the spectral characteristics of the irregularities with spatial scale sizes > 600 m. Figure 9 presents the 24-Hz density observations obtained by DMSP F-9 as it crossed the perturbed region near 37°L on March 21, 1990 at 02:25 UT, some two hours after the destabilizing effects of the penetrating eastward electric field (cf. Figure 6). Spectral power as a function of spatial wavenumber is presented below indicating an approximate -1.5 slope for irregularities with scale sizes between 0.6 km - 60 km. These spectral characteristics are consistent with those reported for irregularities associated with equatorial spread F [Livingston et al., 1981; Hysell and Kelley, 1997] and indicate the importance of such stormtime processes as a cause of mid-latitude scintillations.

Discussion

The topside perturbations reported above for the November 3-4, 1993 event have many similarities with the supersonic equatorial plasma bubbles reported by Aggson et al. [1992], who provide a description and references related to the characteristics of such field-aligned density depletions. Plasma bubbles at equatorial latitudes develop in the bottomside of the F layer where plasma density gradients are unstable to the growth of Rayleigh-Taylor turbulence and Aggson et al. [1992] report the occurrence of bubbles with updrafting velocities as large as ~2 km/s. A difference is that the low and mid-latitude topside perturbations reported here were seen at latitudes (L > 1.2) usually stable to the Rayleigh-Taylor processes. Muldrew [1980] examined the occurrence frequency and characteristics of topside ionization ducts and concluded that only the ducts in the region L < 1.2 are associated with bubbles. He also reported that the occurrence of ducts for L > 1.2 tends to increase in the local time range between 20 and 22 hours, which is the preferred local time interval for the occurrence of disturbance-induced eastward electric fields [Senior and Blanc, 1984]. Maruyama [1990] has investigated the stability of the low-latitude F layer to the gravitational force (Rayleigh-Taylor) and the effects of the ExB instability and finds the region L > 1.17 to be stable to the gravitational term, but to be unstable to the effects of an eastward electric field. These findings lead us to suggest that the prompt ionospheric perturbations in the November 1993 and March 1990 events are the effects of the destabilization of the mid-latitude ionosphere by a storm-induced eastward electric field penetrating the inner magnetosphere.

A different perspective on low-latitude phenomena similar to those observed during the events reported here has been provided by the ISIS-1 observations of a low-latitude SAR arc and equatorial density trough made during the August, 1972 great magnetic storm. Shepherd et al. [1976] report a 1-kR narrow 630.0 nm emission feature observed at -27° invariant latitude as ISIS-2 flew along the meridian of the magnetic anomaly (66°W) at 1400 km altitude. This feature corresponded with a sharply-defined electron temperature peak of ~4000°K (from a background of 2000°K) observed by Brace et al. [1974] and a topside electron density trough whose origin was suggested to be the inner of two plasmasphere shells observed along that orbit. It is possible that such an inner-plasmasphere boundary marks the equatorward extent of strong electric field penetration and suggests that this earlier observation was also related to electric field penetration near the magnetic anomaly. Brace et al. [1974] interpreted their observations in terms of an equatorial trough at L=1.23 of possible limited longitudinal and/or temporal extent.

The great magnetic storm of March 13-14, 1989 produced a pronounced, widespread reduction of the topside ionosphere near the SAA [Greenspan et al., 1991]. DMSP satellites at 840 km altitude observed a 10x - 50x reduction in the equatorial topside ionosphere between +/- 20° magnetic latitude, accompanied by strong upward and westward ion drifts, while near-equatorial ground-based ionosondes in Brazil observed the rapid uplifting and disappearance of the F layer and E layer enhancement due to particle precipitation near the magnetic anomaly. Those authors attribute these drastic storm-time effects to a strong enhancement of the equatorial ion fountain by an eastward electric field in the dusk sector. Greenspan et al. [1991] note an apparent localization of these effects near the SAA and suggest the enhanced conductivity and conductivity gradients near the anomaly contribute to the development of the eastward electric fields and a concentration of the storm-time topside perturbations in that region.

Conclusions

The Millstone Hill Radar and DMSP satellite observations of mid- and low-latitude ionospheric perturbations during the geomagnetic disturbances of November 3-4, 1993 and March 20-21, 1990 are consistent with the findings of the earlier studies referenced above and reconfirm and further define the effects related to the occurrence on an enhanced eastward electric field in the pre-midnight sector during early phases of strong storms. We observe a prompt uplifting of the ionospheric F layer at latitudes equatorward of the mid-latitude trough, which is more pronounced at more-equatorward latitudes. An effect of the eastward electric field is to destabilize the F layer [Maruyama, 1990], producing localized perturbations of the mid and low-latitude topside ionosphere which have the appearance of a low-latitude density trough in the radar observations and signatures similar to equatorial plasma bubbles when seen by the satellites. Other authors have reported strong scintillations associated with such events (e.g. Tanaka, 1981) and the depletion of the low-latitude topside ionosphere by the enhancement of the equatorial ion fountain by the storm-induced eastward electric field [Greenspan et al., 1991]. Upward and westward ion velocities characterize the depleted regions and these signatures persist in the low-latitude trough long after the relaxation of the causative eastward electric field.

Penetrating eastward electric fields produce significant stormtime space weather effects at mid and low latitudes. These observations, and reports of similar phenomena in the published literature, indicate that such effects are often pronounced at, and perhaps limited to, longitudes near the South Atlantic magnetic anomaly. The role of the SAA in defining the equatorward extent of penetrating electric fields and the perturbation of the thermal plasmasphere and energetic radiation belts [e.g. Foster et al., 1997, this issue], is suggested for further investigation in order to understand better the prompt effects associated with impulsive magnetic disturbances.

Acknowledgments

A portion of this work was conducted while JF was a Visiting Professor at the Solar-Terrestrial Environment Laboratory of Nagoya University. We acknowledge helpful discussions with R. Nakamura, T. Maruyama, M. Buonsanto, and members of the Atmospheric Sciences Group at the MIT Haystack Observatory. Arecibo radar data were obtained through the CEDAR/incoherent scatter database at NCAR. Millstone Hill observations and analysis are supported by the National Science Foundation.

References

Aggson, T. L., W. J. Burke, N. C. Maynard, W. B. Hanson, P. C. Anderson, J. A. Slavin, W. R. Hoegy, and J. L. Saba, Equatorial Bubbles updrafting at supersonic speeds, J. Geophys. Res., 97, 8581-8590, 1992.
Brace., L. H., E. J. Maier, J. H. Hoffman, J. Whitteker, and G. G. Shepherd, Deformation of the night side plasmasphere and ionosphere during the August 1972 geomagnetic storm, J. Geophys. Res., 79, 5211-5218, 1974.
Buonsanto, M. J., and J. C. Foster, Effects of Magnetospheric Electric Fields and Neutral Winds on the Low-Latitude Ionosphere during the March 20-21, 1990 Storm, J. Geophys. Res., 98, 19133-19140, 1993.
Foster, J. C., D. Tetenbaum, C. F. del Pozo, J. -P. St. Maurice, and D. R. Moorcroft, Aspect Angle Variations in Intensity, Phase Velocity, and Altitude for High-Latitude 34-cm E Region Irregularities, J. Geophys. Res., 97, 8601-8617, 1992.
Foster, J. C., M. J. Buonsanto, J. M. Holt, J. A. Klobuchar, P. Fougere, W. Pakula, T. D. Raymund, V. E. Kunitsyn, E. S. Andreeva, E. D. Tereshchenko, and B. Z. Khudukon, Russian-American Tomography Experiment, Int. J. Imaging Sys. Tech, 5, 148-159, 1994.
Galperin, Y., V. N. Ponomarev, and A. G. Zosimova, Plasma convection in the polar ionosphere, Ann. Geophys., 30, 1-7, 1974.
Gonzales, C. A., M. C. Kelley, R. A. Behnke, J. F. Vickery, R. Wand, and J. Holt, On the latitudinal variations of the ionospheric electric field during magnetospheric disturbances, J. Geophys. Res., 88, 9135-9144, 1983.
Greenspan, M. E., C. E. Rasmussen, W. J. Burke, and M. A. Abdu, Equatorial density depletions observed at 840 km during the great magnetic storm of March 1989, J. Geophys. Res., 96, 13931-13942, 1991.
Hysell, D. L., and M. C. Kelley, Decaying equatorial F region plasma depletions, J. Geophys. Res., 102, 20007-20017, 1997.
Livingston, R. C., C. L. Rino, J. P. McClure, and W. B. Hanson, Spectral characteristics of medium-scale equatorial F region irregularities, J. Geophys. Res., 86, 2421-2428, 1981.
Maruyama, T., ExB instability in the F-region at low- to midlatitudes, Planet. Space Sci., 38, 273-285, 1990.
Muldrew, D. B., The formation of ducts and spread F and the initiation of bubbles by field-aligned currents, J. Geophys. Res., 85, 613-6252, 1980.
Schunk, R. W., P. M. Banks, and W. J. Raitt, Effects of electric fields and other processes upon the nighttime high-latitude F layer, J. Geophys. Res., 80, 3121, 1976.
Shepherd, G. G., L. L. Cogger, and J. R. Burrows, Mid-latitude auroras and SAR arcs observed from the Isis 2 spacecraft during the August 1972 geomagnetic storm, J. Geophys. Res., 81, 4597-4602, 1976.
Senior, C., and M. Blanc, On the control of magnetospheric convection by the spatial distribution of ionospheric conductivities, J. Geophys. Res., 89, 261-284, 1984.
Southwood, D. J., and R. A. Wolf, An assessment of the role of precipitation in magnetospheric convection, J. Geophys. Res., 83, 5227-5232, 1978.
Spiro, R. W., R. A. Heelis, and W. B. Hanson, Rapid subauroral ion drifts observed by Atmospheric Explorer C, Geophys. Res. Lett., 6, 657-660, 1979.
Tanaka, T., Severe ionospheric disturbances caused by the sudden response of evening subequatorial ionospheres to geomagnetic storms, J. Geophys. Res., 86, 11335-11349, 1981.
Tanaka, T., Low-latitude ionospheric disturbances: results for March 22, 1979, and their general characteristics, Geophys. Res. Lett., 13, 1399-1402, 1986.
Yeh, H.-C., J. C. Foster, F. J. Rich, and W. Swider, Storm-time electric field penetration observed at mid-latitude, J. Geophys. Res., 96, 5707-5721, 1991.

Figure Captions

Figure 1. Recurrent geomagnetic activity apparent in the mid-1993 auroral activity index Ap was used to pre-schedule the Millstone Hill radar (RATE) experiment for the interval of the November 3-4, 1993 magnetic storm. Two intervals of solar-induced activity repeating with the 27-day solar-rotation period are indicated by Xs and Os.

Figure 2. Solar wind parameters for disturbance intervals in March 1990 and November 1993 were very similar. In each event, increasing solar wind velocity was accompanied by a density enhancement and strongly southward IMF Bz near 23 UT.

Figure 3. Millstone Hill radar elevations-scan observations of the latitude/altitude variation of mid-latitude F region density reveal a strong uplifting of the F layer at latitudes equatorward of the storm-enhanced mid-latitude ionospheric trough (near 45° latitude) during both events approximately 90 min after the disturbance onset. Iso-density contours (log10 (Ne m-3)) are shown with 0.1 spacing. In both cases, the altitude of the F layer maximum increases with increasing distance equatorward from the trough.

Figure 4. A pronounced low-latitude topside ionosphere density depletion was observed by the Millstone Hill radar and by the DMSP F-9 satellite more than 15° equatorward of the normal mid-latitude trough during the March, 1990 event. Upward and westward ion velocity were observed by DMSP.

Figure 5. DMSP F-10 observed magnetically-conjugate topside density and strong velocity perturbations at +/- 26° magnetic latitude, coincident in time with the uplifting of the ionosphere observed by the Millstone Hill radar (cf. Figure 3) during the November, 1993 event. The response of the electron flux detector to energetic proton bombardment as the satellite traverses the South Atlantic magnetic anomaly is shown in the top panel and indicates that the field line of the topside perturbation lay near the peak of the effect of the anomaly (-26° magnetic latitude).

Figure 6. Arecibo radar line of sight Doppler velocity observations during the March, 1990 event indicate a strong northward plasma flow at and shortly after 00 UT on March 21, 1990, indicative of the effect of an eastward electric field (cf. Buonsanto and Foster, [1993]). Positive velocity is directed away from the radar.

Figure 7. Millstone Hill radar line of sight Doppler velocities observed looking far equatorward at low elevation angles measured a strong velocity toward the radar (poleward and downward) which indicates the effect of an eastward electric field. The toward velocity increases in magnitude equatorward of the radar (at 55° invariant latitude), reaching a maximum near 38° invariant, the approximate location of the `low-latitude trough' density depletion shown in Figure 4.

Figure 8. Increases in ionospheric virtual height observed by a low-mid latitude chain of ionosondes in Japan (135°E meridian) provide evidence of a concentration of storm-induced eastward electric field near the ~25° magnetic latitude where the topside perturbations were observed in the American sector by DMSP F-10 (cf. Figure 5). Figure 9. High-resolution (24 Hz, ~300 m) total density measurements observed by DMSP F-10 crossing the ionospheric perturbation seen near 38° in Figure 4. Spectral power as a function of spatial wavenumber determined for the 86-sec interval of data indicated indicates an approximate -1.5 slope for irregularities with scale sizes between 0.6 km - 60 km.

http://www.haystack.edu/jcf/papers/slot.htm -- Revised: November 20, 1997
E-mail: jcf@hyperion.haystack.edu