UARS observations of Birkeland currents and Joule heating rates for the November 4, 1993, storm

Abstract

Magnetic field and particle observations from the Upper Atmosphere Research Satellite particle environment monitor (UARS/PEM) are used to estimate field‐aligned currents, electron precipitation energy flux, ionospheric conductivities, and Joule heating rates during the main phase of the November 4, 1993, geomagnetic storm. From 0300 to 1200 UT on November 4 the auroral oval expanded equatorward of 65° magnetic latitude (MLAT), and UARS encountered the polar cap on seven consecutive passes during the storm main phase. These passes provide data appropriate to determine field‐aligned currents and estimate ionospheric Joule heating. For this storm, UARS sampled the midnight to dawn sector in the northern hemisphere and the noon to dusk sector in the southern hemisphere. The maximum net currents on the day side and nightside are comparable and reach 1 A/m for several hours. The average Joule heating rates are comparable at midnight, early morning, and noon, where they are 9.2, 6.6, and 7.7 GW/h, respectively, but have a strong peak in the late afternoon, where they are 25.6 GW/h. In contrast, the electron precipitation energy deposition is highest near midnight at 5.6 GW/h but drops to less than half this level to 2.4 GW/h and 1.9 GW/h in the early morning and at dusk, respectively, but is very small near noon, only 0.24 GW/h. The Joule to particle energy deposition rate ratio thus varies by roughly an order of magnitude with local time, being over 40 near noon, about 20 at dusk, 3 near dawn, and 2 at midnight. The hemispherical Joule and electron precipitation heating rates, H J and H elec , are estimated to have been 290 GW and 50 GW, respectively, giving H J / H elec = 4.5 and H J + H elec = 340 GW. Differences between these averages and assimilative mapping of ionospheric dynamics (AMIE) results, H J = 200 GW and H elec = 80 GW, reflect time variability during the storm and are largely resolved when AMIE results only at the times of UARS passes are considered.