Near Earth Plasma Sheet Penetration and Geomagnetic Disturbances

Abstract

The plasma sheet has a distinct inner boundary that moves inward during periods of, and as a result of, enhanced convection. This leads to dramatic increases in plasma pressure and cross-tail current in the near-Earth (equatorial radial distances r ∼ 6-10 RE) region of the nightside magnetosphere that are associated with geomagnetic activity. During quiet times, the plasma sheet remains beyond r ∼ 10 R E and precipitation from the plasma sheet is weak and peaks at invariant latitudes A ≥ 68°. Weak auroral disturbances, here called separatrix disturbances, are occasionally seen to move equatorward from near the magnetic separatrix. During the growth phase o isolated substorms, the plasma sheet moves into the near-Earth night side region leading to large increases in plasma pressure. Precipitation expands equatorward from its quiet-time location to A = 64°-67°. At the onset of the substorm expansion phase, active aurora initiates within the inner plasma sheet at an invariant latitude ∼5-6° equatorward of the separatrix. The region of active aurora then moves several degrees in latitude poleward during the expansion phase. This corresponds to a large tailward motion of the equatorial region that maps to the active aurora from its initial location in the near-Earth region. After the expansion phase, the plasma sheet precipitation re-forms well poleward of its location at onset an near where it is located during quiet times, implying a significant decrease in near-Earth plasma sheet pressure. Particle injections are juxtaposed with a decrease in plasma pressure within the inner plasma sheet during the expansion phase of su storms. Separatrix disturbances are seen during all phase of substorm activity. During convection bays, the plasma sheet remains within the near-Earth region wit enhanced pressures. Auroral activity during convection bays is dominated by separatrix disturbances that can be quite strong (ground magnetic perturbations up to ∼300 nT) and move towards the inner plasma sheet from near the separatrix. Poleward moving active aurora that characteri.ze.