Shear-flow Ballooning, Substorm Onset, and Destablisation of the Stretched Terrestrial Magnetotail: New Evidence and Constraints from Energetic Proton Temperature Anisotropy and Auroral Imaging

The wave processes which occur during the late growth phase of terrestrial substorms appear to provide a powerful diagnostic for the processes which may lead to the destabilization of the magnetotail and hence to substorm expansion phase onset. Here we review new theory which highlights the potential role of pressure anisotropic ballooning modes as a trigger for substorm onset. In this theory, the expansion of the magnetotail naturally generates parallel pressure anisoptropy energetic ion distributions which lower the threshold for the growth of ballooning modes – providing a plausible physical explanation for the transition of the tail from a stable to unstable configuration. We further present observational evidence from geosynchronous orbit in support of the model prediction. Finally, we demonstrate the utility of ground-based auroral observations for probing the dynamics of the near-Earth magnetotail. Auroral observations we present here clearly show a repeatable and characteristic sequence of late growth phase dynamics, including arc brightenings, the formation of auroral beads, and auroral vortex development, all of which occur well in advance of fast Earthward flows in the tail. Indeed, it is only during that later activity that auroral breakup and strong Earthward flows, which we associate with magnetic reconnection further down the tail, are observed together with strong magnetic bays on the ground. The sequence of events is consistent with an inside-to-outside model at substorm expansion phase onset, and where the stretched nightside magnetic field is destabilised by a temperature anisotropic shear-flow ballooning instability in the transition region from dipole to tail-like fields in the near-Earth plasma sheet.