A New Substorm Onset Mechanism: Increasingly Parallel Anisotropic Ballooning

The nature of the physical processes active in magnetospheric substorms has been a source of debate since their early characterization in the ionosphere by Akasofu (1964). The onset of the substorm expansion phase in near-Earth space is commonly linked to the formation of ballooning instabilities. We study the destabilization of these ballooning instabilities from a novel perspective by examining the effect of changing pressure anisotropy, with a particular focus on anisotropies that become increasingly parallel to the local magnetic field in the magnetotail, focussing on the transition region between dipolar and tail-like fields at the edges of the plasmasheet. We conduct this analysis in three parts. First, we adopt the ballooning stability model of Chan et al. (1994) to examine the behaviour of the instability threshold under conditions of varying plasma β, pressure gradient, and pressure anisotropy. In combination with the increasing plasma β and increasing pressure gradients that occur naturally during the substorm growth phase, we find that a change in the pressure anisotropy from an initially perpendicular anisotropic configuration toward more parallel anisotropy is destabilizing. In the second part, we trace particles through a two-dimensional magnetotail magnetic field model as they drift around Earth from a less stretched, early growth phase tail state to a more stretched state shortly before substorm onset. We find that, as a direct consequence of growth phase tail stretching, conservation of the first and second adiabatic invariants acts in combination with drift shell splitting to produce more unstable, increasingly parallel anisotropy in a radially localized onset region in the near-Earth tail. Thirdly, we compare our results and our onset hypothesis with ground-based and in-situ observations. The predictions from our onset model show good agreement with seemingly disparate observed phenomena, including auroral beads on the onset arc, proton auroras surrounding the onset arc, and in-situ observations of increasingly parallel anisotropic pressures prior to substorm onset. The required increasingly parallel anisotropic ballooning occurs as a natural consequence of growth phase tail stretching and offers a self-consistent explanation for the observed sequence of substorm events.