ROLE OF FERMI ACCELERATION IN EXPLOSIVE ENHANCEMENT OF CROSS-TAIL CURRENT IN LATE SUBSTORM GROWTH-PHASE

During the substorm growth phase, thinning of the plasma sheet can ene rgize the unmagnetized ions trapped within it via a Fermi acceleration process. We show here that the energy growth rate, affecting mostly t he unmagnetized ions, is exponential in time. The Fermi acceleration w ill give rise to a large enhancement in the cross-tail ion current den sity and would be most prominent during the late growth phase. This pr ediction is quite consistent with previously reported satellite observ ations which show that the cross-tail current density intensifies rapi dly during the final stage of the growth phase just prior to current d isruption.