Ballooning Instabilities in the Plasma Sheet in Conjunction with Auroral Wave Structures

Abstract:

Ballooning instability in the near-Earth plasma sheet transition region has been suggested to be a plausible trigger leading to azimuthally-aligned auroral wave structures (auroral beading) and subsequent substorm onset expansion. Previous observational studies on ballooning instability have been focused on the subsequent particle and field oscillations. However, the initiation of the instability has never been validated, and its connection to the auroral beading is also to be evaluated observationally. We take advantage of the THEMIS multi-spacecraft and ground All-Sky-Imager and Multi-Spectrum-Imager observation conjunctions to examine whether the plasma and field conditions in the transition region prior to the substorm/pseudo-breakup onset is unstable against ballooning mode within MHD regime. The instability criterion is calculated using observations from radially aligned THEMIS probes in the near-Earth plasma sheet. We found that the ballooning instability criterion is violated locally in the transition region about one Alfvenic transit time prior to the auroral beading initiation. The instability is associated with enhanced field-aligned currents (FAC) on top of the background currents, which provides energy source for the auroral brightening. These observations support the idea that the ballooning instability in the near-Earth plasma sheet is a plausible trigger for the auroral beading. We also found evidence in some cases that earthward moving flow channels from middle tail to the near-Earth region precedes the instability, indicating that the earthward penetrating plasma may lead to the violation of the instability criterion. The observed magnetic field profile prior to the instability initiation provides an initial condition for the numerical simulations, which may assist further understanding of the plasma sheet conditions leading to instability for every event.