Finite ky ballooning instability in the near-Earth magnetotail -: art. no. A11211

[1] Two-dimensional initial value MHD simulations of the linear ideal MHD ballooning instability in the near-Earth magnetotail are presented. The configuration of the magnetotail is modeled by the analytic two-dimensional static equilibrium developed by Voigt. For an intermediate range of plasma beta values ( similar to 1 - 100) at the equatorial plane, the Voigt configuration of the near-Earth magnetotail is shown to be unstable to ballooning modes with finite k(y). ( A similar intermediate range of plasma beta was also obtained in the stability analyses of ballooning modes in the infinite ky limit.) Such a b dependence of the instability arises because of the stabilizing effect of plasma compression involved in the ballooning displacement of flux tubes in the high-beta regime of the magnetotail. The growth rate of the finite k(y) ballooning instability is found to increase with the wave number k(y), approaching a saturated value in the very large k(y) limit. The thinning of the current sheet is found to enhance the regime of unstable b as well as the growth rate of the linear ballooning instability of the near-Earth magnetotail, suggesting a possible scenario for the substorm trigger.