Evidence of Kinetic Ballooning Instability Causing Substorm Onset

The evidence of kinetic ballooning instability (KBI) prior to substorm onset is presented to support KBI as the cause of substorms. KBI is destabilized when the free energy associated with the plasma pressure gradient that is in the same direction as the magnetic field curvature can overcome the stabilizing energy of the magnetic field line tension and particle kinetic effects. We demonstrated that KBI is most unstable in the strong cross-tail current region magnetic field lines and the KBI parallel electric field accelerates electrons along the magnetic field lines into the ionosphere to produce the substorm onset arc. We describe the expected features of KBI in the magnetosphere and ionosphere such as the mode structure, the wave frequency and growth rate, and the auroral arcs. Then, we show the observational evidence of KBI by examining the auroral onset arc features, and Pi2 waves in the plasma sheet. In particular, we show the correlation between the substorm onset arcs and the Pi2 pulsations in terms of wave structure, propagation, and the exponential growth of arc intensity and Pi2 wave amplitude.