GROUND-BASED STUDIES OF IONOSPHERIC CONVECTION ASSOCIATED WITH SUBSTORM EXPANSION

The instantaneous patterns of electric fields and currents in the high -latitude ionosphere are deduced by combining satellite and radar meas urements of the ionospheric drift velocity, along with ground-based ma gnetometer observations for October 25, 1981. For this purpose, an upd ated version of the assimilative mapping of ionospheric electrodynamic s technique has been used. These global patterns are unobtainable from any single data set. The period under study was characterized by a re latively stable southward interplanetary magnetic field (IMF), so that the obtained electric field patterns do reflect, in general, the stat e of sustained and enhanced plasma convection in the magnetosphere. Du ring one of the satellite passes, however, an intense westward electro jet caused by a substorm into the satellite (DE 2) and radar (Chatanik a, Alaska) field of view in the premidnight sector, providing a unique opportunity to differentiate the enhanced convection and substorm exp ansion fields. The distributions of the calculated electric potential for the expansion and maximum phases of the substorm show the first cl ear evidence of the coexistence of two physically different systems in the global convection pattern. The changes in the convection pattern during the substorm indicate that the large-scale potential distributi ons are indeed of general two-cell patterns representing the southward IMF status, but the night-morning cell has two positive peaks, one in the midnight sector and the other in the late morning hours, correspo nding to the substorm expansion and the convection enhancement, respec tively.