Observational test of the interchange stability associated with near-tail dipolarizations

The interchange instability is a pressure gradient-driven instability. Formally speaking, the interchange mode can be considered to be a special case of the more general ballooning mode in the sense that the perpendicular displacement (as multiplied by the magnetic field strength) of the interchange mode is defined to be constant along the magnetic field line while that of a ballooning mode can be localized to the “bad curvature” region (i.e. the equatorial plane). Since the radial pressure profile in the near-earth tail is expected to become steeper prior to the substorm onset, instability of either interchange or ballooning type mode may be expected to occur. An observational test is generally non-trivial and difficult for the ballooning mode. For the interchange mode, however, the classic analytic criterion has long been known and thus using this criterion an observational test of the instability can be more feasible. The only difficulty has been how to evaluate the flux tube volume, for which Wolf et al. [2006] have suggested a useful formula. Using the formula, we have evaluated the interchange criterion for a set of pair-dipolarization events that are radially aligned. We found that the near-tail configuration before and during a dipolarization is likely stable against the interchange mode. Using a number of more dipolarizations, we have performed an analysis on evaluating the interchange criterion in a statistical manner and reached the results that support the conclusion of the interchange stability. This seems to suggest to us that the interchange instability is unlikely a candidate to trigger dipolarizations in association with substorms. But, the ballooning instability, the mode structure of which can be localized near the equatorial plane, remains a possibility responsible for dipolarizations.