Observational evidence for a kinetic ballooning instability during substorm

A theory of kinetic ballooning instability has been proposed to explain the trigger of substorm expansion phase. It results from the energy release of nonuiform plasma pressure with gradient along the direction of the magnetic field curvature. Recent plasma observations also show the possible evidence for a kinetic ballooning instability. In this study, we investigate the wave activities around the onset of substorm expansion phase based on the THEMIS satellites observations and evolution of auroral activities during substorm. Pi 2 low frequency perturbation prior to current disruption can be identified in the magnetic fluctuations. When Pi 1 high frequency perturbation is also excited, it enters the turbulent state. During the late growth phase, the auroral arc is forming with an azimuthally-spaced structure with high mode number and growing with a linear growth rate. The theory of kinetic ballooning instability can explain the physical mechanism of Pi 2 instability excited prior to the current disruption, the properties of substorm onset arc, and the cause of eventual arc breakup.