MHD Stability of Magnetotail Configurations With a Bz Hump.

Abstract: Using two‐dimensional magnetohydrodynamic (MHD) simulations, we explore the stability of magnetotail configurations that include a local enhancement of B_z (a “B_z hump”). We demonstrate that hump configurations can become unstable within the constraints of ideal MHD, even when strict boundary conditions v = 0 are imposed at all boundaries, consistent with the constraints of the ideal MHD energy principle. Necessary conditions for instability are that the boundaries on the earthward and high‐latitude sides are far enough away, that the background pressure, equivalent to the lobe pressure, is small enough, and that the B_z enhancement or entropy reduction is strong enough. The unstable evolution then bears strong similarity to the evolution of observed and simulated dipolarized flux bundles and dipolarization fronts, presumed or simulated to be the consequence of reconnection. The B_z humps, which for sufficiently low background pressure correspond to entropy depletions, move faster for lower background density and penetrate closer toward Earth when they are initiated closer to Earth or when the background pressure is lower, similar to 3‐D low‐entropy flux tubes, which are affected by ballooning‐/interchange‐type modes. [ABSTRACT FROM AUTHOR]