Evolution of the low‐latitude geomagnetic storm field and the importance of turbulent diffusion for ring current particle losses

Abstract

An investigation of the temporal evolution of the low‐latitude disturbance field at different magnetic local times for several storms of different activity showed characteristic features for strong, moderate, and weak storms. Strong storms are characterized by a fairly symmietric disturbance field at the beginning of the recovery phase and by about the same recovery decay rate at all magnetic local times (τ ∼ 4–7 hours). For moderate and weak storms the disturbance is strongly asymmetric at the recovery phase beginning, and the recovery decay rates are quite different at different magnetic local times, the fastest recovery (τ ∼ 1–2 hours) being‐recorded in the afternoon/evening sector. We explain our observations assuming that gyroresonant interaction of ring current ions with Alfven ion cyclotron waves is an important loss mechanism for ring current. For strong storms the injection is deep, and the plasmasphere becomes small and symmetric during the main phase, so recovery starts similarly at all longitudes. During the storms of middle and small intensity an eveningside plasmasphere bulge persists at latitudes of the main ring current location, so fast losses of westward drifting ions due to turbulent diffusion occur in that sector; they provide azimuthal asymmetry of magnetic disturbance magnitude and of recovery timescale.