Magnetospheric activity during undriven recovery

A significant fraction of magnetospheric and geomagnetic activity is a direct result of variations in the external solar wind energy input, which fluctuates on time scales that are similar to the time scales of internal magnetospheric phenomena. To isolate the part of the magnetospheric activity signal that is due to internal magnetospheric processes, we consider the magnetosphere during undriven recovery. A recovery event is defined to occur when the magnetosphere is first in an excited state after which time the solar wind energy input is suddenly reduced for at least thirty minutes or longer. The statistics of these events are compared with that from non--events. For the events we find power--law behavior in both the lifetime of bursts above a decay curve and the power spectrum of geomagnetic variations. The functional form of the probability of large changes in the ground magnetic field is also found to follow a power law. The relaxation time for the events has a broad distribution that is not correlated with the initial activity level, and the power law exponent of the undriven recovery events is near that of the non--event intervals. The results from the actual events are compared to magnetospheric and geomagnetic activity signals extracted from MHD simulations of relaxation events.