Sausage mode instability of thin current sheets as a cause of magnetospheric substorms

Observations have shown that, prior to substorm explosions, thin current sh eets are formed in the plasma sheet of the Earth's magnetotail. This provok es the question, to what extent current-sheet thinning and substorm onsets are physically, maybe even causally, related. To answer this question, one has to understand the plasma stability of thin current sheets. Kinetic effe cts must be taken into account since particle scales are reached in the cou rse of tail current-sheet thinning. We present the results of theoretical i nvestigations of the stability of thin current sheets and about the most un stable mode of their decay. Our conclusions are based upon a non-local line ar dispersion analysis of a cross-magnetic field instability of Harris-type current sheets. We found that a sausage-mode bulk current instability star ts after a sheet has thinned down to the ion inertial length. We also prese nt the results of three-dimensional electromagnetic PIC-code simulations ca rried out for mass ratios up to M-i/m(e) = 64. They verify the linearly pre dicted properties of the sausage mode decay of thin current sheets in the p arameter range of interest.