Wind Observations Pertaining to Current Disruption and Ballooning Instability during Substorms

In this paper, the drift ballooning instability as a possible substorm trigger mechanism is examined. Two features of the drift ballooning instability are the westward propagation of wave perturbations and the presence of a pressure gradient. Energetic ion (75 keV - MeV) and the magnetic field data from the Wind spacecraft observe time delays between earthward and tailward flux enhancements of ions at the onset of current disruption. We interpret the time delays as signatures of westward propagation, and estimate accordingly the propagation speeds to be several hundred kilometers per second. Clear signatures of westward propagation are present in most current disruption events observed ~ 8-10 R_E. For events observed beyond 10 R_E, it is often difficult to identify time delays. This radial distance dependence suggests that the initially unstable region is within 10 R_E. For an event observed at ~ 8 R_E, anisotropy of ion fluxes observed between the duskward and dawnward directions persists until the time of the onset of the current disruption. The evolution of this anisotropy is consistent with a radial density gradient that is impulsively reduced during the abrupt magnetic fluctuations. The instability that triggers the current disruption, and hence the substorm, has to be able to relax the pressure gradient (which is proportional to the density gradient assuming an isothermal process). The impulsive reduction of the pressure gradient and the fact that its corresponding magnetic fluctuations have δ B/B exceeding 1 indicate that the process is nonlinear. We conclude that these observations support the drift ballooning instability as a possible mechanism to trigger substorms, but a nonlinear theory of the instability is called for.