Midnight radial profiles of the quiet and growth-phase plasma sheet: The Geotail observations

Abstract

To evaluate the plasma sheet transition from weak to strong convection, we investigate ion moments and magnetic field observed by Geotail. The Geotail data from 1995 to 1997 within the area -9 RE ≥ XGSM ≥ -30 RE and |YGSM| ≤ 5 RE were sorted for periods of quiet times and the substorm growth phase, which gives a better indication of convection strength than do global indexes. We find that the overall growth-phase ion pressure is a factor of ∼1.55 higher than the quiet-time ion pressure. Density is higher and temperature is lower during quiet times. Ions drift mainly earthward and duskward, and the drift speed is about twice stronger during the growth phase. The duskward drift is dominated by ions' diamagnetic drift. The |BX| in the lobes increases strongly, while Bz near the current sheet at small radial distance decreases as convection increases, indicating that field lines become more stretched during the growth phase because of the earthward penetration of the plasma sheet. The increase in the lobe magnetic field is sufficient to maintain force balance in the z direction with the enhanced ion pressure in the plasma sheet. Intervals of negative Bz are observed during both quiet and growth-phase conditions. The observed negative Bz intervals near the current sheet are short and are likely caused by perturbations, while those in the lobes are substantially longer and are likely associated with field line flaring. Copyright 2004 by the American Geophysical Union.