FILAMENTARY CURRENT STRUCTURES IN THE POSTNOON SECTOR - OBSERVATIONS FROM UARS

During an intense geomagnetic storm (Kp 7+) that began at approximatel y 1830 UT on October 1, 1991, the UARS satellite encountered the daysi de postnoon auroral oval. On two consecutive crossings of the northern hemisphere between 2040 and 2240 UT, the vector magnetometer detected region 1 and 2 Birkeland and ionospheric currents in the postnoon sec tor. Low-energy electron events were observed near 1400 MLT within a n arrow portion of the region 1 current system. Simultaneous magnetic fi eld measurements revealed the presence of intense (approximately 20 mu A/m2) bipolar filament current structures embedded in the auroral oval . The upward-directed currents were associated with the more concentra ted region of precipitating electrons. Ions associated with the more i ntense flux of low-energy electrons exhibited a dispersion signature t ypical of an ion velocity filter. The dispersion, aligned along the or bit, exhibited higher-energy ions at lower latitudes and earlier local times. The colocation of filament currents and ion dispersion signatu res at such late postnoon local times is not consistent with typical E x B ''cusp'' dispersions. These features more likely result from days ide boundary wave phenomena.