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 energize the unmagnetized ions trapped within it via a Fermi acceleration process. We show here that the energy growth rate, affecting mostly the unmagnetized ions, is exponential in time. The Fermi acceleration will give rise to a large enhancement in the cross‐tail ion current density and would be most prominent during the late growth phase. This prediction is quite consistent with previously reported satellite observations which show that the cross‐tail current density intensifies rapidly during the final stage of the growth phase just prior to current disruption.