PROMPT MIDLATITUDE ELECTRIC-FIELD EFFECTS DURING SEVERE GEOMAGNETIC STORMS

Meridian-plane elevation scans with the Millstone Hill incoherent scat ter radar provide evidence of a strong perturbation of the coupled mid -latitude magnetosphere-ionosphere system during the early phases of t he November 4, 1993 magnetic storm. A narrow ionospheric trough formed at L=3.5 in the pre-midnight sector, immediately poleward of the Mill stone Hill site. The most pronounced radar signature of the developing activity was a brief (20 min) uplifting of the F region plasma equato rward of the trough, such that the peak altitude increased with distan ce 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 m id-latitude trough and was observed by the radar and the DMSP F9 satel lite. During the November 4, 1993 event, the DMSP F10 satellite observ ed narrow magnetically conjugate regions of plasma density depletion a nd strong horizontal and upward plasma velocity(> 1500 m/s) at L=1.5 a t the time of the uplifting of the mid-latitude F region observed by t he radar. These observations were confined to longitudes near the Sout h Atlantic magnetic anomaly and, in the Nov 1993 case, the perturbatio n was coincident with the peak of the precipitating particle fluxes as sociated with inner-belt losses at the anomaly. Both the uplifting of the ionospheric F layer and the triggering of topside density perturba tions can be explained in terms of an eastward electric field imposed on the mid and low-latitude ionosphere during the initial stages of th e geomagnetic storm. The low-latitude ionospheric perturbations in the se events were similar to supersonic equatorial bubbles, triggered by the destabilizing effects of the upward ExB drift associated with the eastward electric field.