Ballooning Instabilty of Thin Current Sheets: A Possible Mechanism for Substorm Onset

It is widely believed that thin current sheets are formed in the growth phase of a substorm in the Earth's magnetotail. We present new results on the linear and nonlinear ballooning instability of thin current sheets in which the magnetic field lines are line-tied. It is shown that thin sheets become spontaneously linearly unstable to ballooning instabilities in an intermediate range of plasma beta (~1-10) at near-Earth distances. Nonlinearly, the modes grow exponentially to form fingers that penetrate into neighboring regions, eventually developing into a mushroom-type structure. Strong shear flows develop, and the pressure gradient accumulates at the stagnation point of the flow, producing a shock structure. This nonlinear behavior is different from what is predicted by asymptotic theories of nonlinear ballooning, and has important consequences for the problem of substorm onset. We will compare these results with observations from Wind and Cluster.