Wind observations pertaining to current disruption and ballooning instability during substorms

The westward propagation of wave disturbances associated with current disruption is observed in the near-Earth plasma sheet by the Wind satellite. By analyzing the time delay between earthward and tailward flux enhancements of energetic ions, the propagation velocity is estimated to be several hundred kilometers per second. A large anisotropy between the duskward and dawnward fluxes of energetic ions is observed to persist until the local onset of a current disruption. This anisotropy is consistent with an earthward density gradient which is significantly reduced after the magnetic fluctuations that accompany the current disruption cease. The reduction process is impulsive and bursty, suggesting that the underlying dynamics is nonlinear. The westward propagation of the unstable wave disturbances, the radial density gradient and its subsequent reduction support the drift ballooning instability as a possible mechanism for triggering substorms.