Energy Storage and Release in the Magnetotail as a Function of the Position and the Substorm Magnitude

The present study investigates the energy storage and release processes of substorms as a function of both position and substorm intensity in a model independent manner. As an index of the substorm intensity, we use the amplitude of the positive bay observed at mid and low latitudes. As an index of the energy storage and release in the magnetotail, we use the total pressure measured by GEOTAIL. We have performed a superposed epoch analysis of the total pressure for each substorm group binned in terms of the position and the substorm intensity. We have found that (1) the energy stored in the near tail region (-15~R_E< X_GSM<-6~R_E and -8~R_E< Y_GSM< 8~R_E) is larger than that in any other region, (2) both magnitude and rate of energy storage in the near tail region strongly depend on the substorm intensity, (3) the start of the energy release of small substorms in the near tail region tends to be delayed from the first Pi2 onset. These results suggest that the energy storage and release processes mainly occur in the near region where the current disruption process plays a main role.