Following suggestion that substorm breakup might be caused by an interchange or ballooning instability, several MHD and gyro-kinetic stability analysis have been performed for plasma sheet magnetic field geometries. However, the stochastic ion dynamics in the highly stressed, thin high-&bgr; near earth plasma sheet violate the locality requirements of MHD and the invarience of the magnetic moment, &mgr;, required by gyro-kinetic theories. In this paper we develop a new linear Vlasov kinetic theory (for low frequency modes &ohgr;<&ohgr;b, &ohgr;b being the bounce frequency) which includes the dynamics of stochastic ions.