Ballooning Instability in the Magnetotail: MHD Simulations

Recent progress in observations and modeling of magnetotail dynamics has strengthened the case of ballooning instability as a trigger mechanism for substorm onset. We report our findings on the ballooning instability of the near-Earth magnetotail from initial-boundary value MHD simulations. The simulations focus on the stability of analytic 2D static magnetotail equilibria developed by Voigt. The linear stage of the pressure-driven instability has been extensively studied by examining the parametric dependence of the growth rate of a single Fourier eigenmode in 2D MHD simulations on the wavenumber k_y, plasma β , and the thin current sheet width. The thinning of the current sheet is found to effectively enhance the regime of unstable β as well as the growth rate of the linear ballooning instability of the near-Earth magnetotail, suggesting a new scenario for the substorm trigger within the context of the linear theory. To study the nonlinear behavior, particularly the possible explosive growth of the ballooning instability that can cause current disruption at near-Earth distances, 3D MHD simulations with sufficiently high resolution along all three coordinate directions have been developed in the PETSc framework. Results from linear and nonliner simulations will be presented.