Ballooning instability of a thin current sheet in the high‐Lundquist‐number magnetotail

Two‐dimensional simulations of the magnetotail in the high‐Lundquist‐number regime indicate the slow growth of thin current sheets and an impulsive intensification of the cross‐tail current density at near‐Earth distances during a short interval just before the onset of the expansion phase, consistent with multi‐satellite observations. Such a two‐dimensional magnetotail, symmetric along y and containing a thin current sheet, is found to be unstable to a symmetry‐breaking, ideal compressible ballooning instability with high wave number along y. The linear instability is demonstrated by numerical solutions of the ideal ballooning eigenmode equation for a sequence of two‐dimensional thin current sheet configurations in the impulsive growth phase. Line‐tied boundary conditions at the ionosphere are imposed, and shown to play a crucial role in the stability analysis. It is suggested that the ideal ballooning instability, which has strong spatial variation along y, provides a possible mechanism for disrupting the cross‐tail current at onset.