EVOLUTION OF THE LOW-LATITUDE GEOMAGNETIC STORM FIELD AND THE IMPORTANCE OF TURBULENT-DIFFUSION FOR RING CURRENT PARTICLE LOSSES

An investigation of the temporal evolution of the low-latitude disturb ance field at different magnetic local times for several storms of dif ferent activity showed characteristic features for strong, moderate, a nd weak storms, Strong storms are characterized by a fairly symmetric disturbance field at the beginning of the recovery phase and by about the same recovery decay rate at all magnetic local times (tau similar to 4-7 hours), For moderate and weak storms the disturbance is strongl y asymmetric at the recovery phase beginning, and the recovery decay r ates are quite different at different magnetic local times, the fastes t recovery (tau similar to 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 a n important loss mechanism for ring current. For strong storms the inj ection is deep, and the plasmasphere becomes small and symmetric durin g: the main phase, so recovery starts similarly at all longitudes. Dur ing the storms of middle and small intensity an eveningside plasmasphe re bulge persists at latitudes of the main ring current location, so f ast losses of westward drifting ions due to turbulent diffusion occur in that sector; they provide azimuthal asymmetry of magnetic disturban ce magnitude and of recovery timescale.