Effects of Interchange Instability on the Dynamics of the Ring Current During September 25, 1998 Magnetic Storm

The established picture of ring current decay during magnetic storms is a combination of losses due to drift through the dayside magnetopause in the main or early recovery phase followed by a slower decay due to charge exchange of ring current particles with the geocorona. We present numerical simulations of the inner magnetosphere for the September 25, 1998 magnetic storm suggesting the possibility of an additional loss process resulting from interchange instability in the inner magnetosphere following a decrease in the plasma sheet pressure. We use the Rice Convection Model with the outer boundary set at geosynchronous orbit. Measurements of low-energy (less than 50 keV) particles by Los Alamos geosynchronous satellites are used to estimate the plasma distribution on the outer boundary of the model. The simulation shows that when the plasma pressure on the boundary drops a factor of ~5 in the main phase of the storm, the plasma sheet becomes interchange unstable. We present results showing rapid loss of the storm-time ring current caused by strong ripple-like electric fields associated with the instability. The ring-current particles are adiabatically de-energized as they move out, so the result of the interchange is a decrease of total particle energy in the inner magnetosphere. This process may play an important role in the rapid recovery of Dst during this storm.