Stability of magnetotail equilibria with a tailward Bz gradient

Abstract

Analytical properties of two-dimensional magnetotail equilibria with a region of magnetic flux accumulation (a “Bz hump”) are considered with respect to tearing and interchange stability. Since the former mode does not initially involve a change in topology, we denote it the Magnetic Flux Release Instability (MFRI). The region earthward of the Bz peak is found most unstable to both modes due to the presence of a tailward Bz gradient there. Stability is studied as a function of proximity of the Bz accumulation region to Earth. Interchange is found more difficult to stabilize closer to Earth as the level of the background pressure required for stabilization is higher there. At the same time, the kinetic MFR mode has the largest potential for destabilization farther in the tail as long as the current sheet is sufficiently thin to allow Landau dissipation, while the tearing (MFRI) Cd parameter is sufficiently large. An MHD analog of the kinetic MFR mode is allowed still farther from Earth, and the regions of kinetic and MHD instability may overlap dependent on the parameters of the equilibrium. We provide analytical results describing the MFR stability as a function of a number of key parameters, including the level of current sheet stretching and its length, as well as the amplitude and the scale size of the Bz gradient.