Investigation of Kinetic Ballooning Instability in 2D Harris Sheet Equilibrium With Finite Normal B z Field

Abstract

Substorm onset is believed to be a fundamental process for magnetic and plasma energy transport in both magnetosphere and ionosphere. In this regard, the ballooning instability is considered to be one of the key trigger mechanism for the substorm onset scenario in the near-Earth plasma sheet. Here, the kinetic ballooning instability (KBI) in the near-Earth magnetotail is analyzed using the two-dimensional (2D) generalized Harris sheet and Voigt equilibrium models. The corresponding kinetic ballooning mode is unstable in the intermediate range of perpendicular wave number (ky) and the equatorial beta (βeq). We further note that the growth rate of the ballooning mode reduces significantly with the increase in electron-to-ion temperatures ratio (Te/Ti) and wave number ky. The kinetic ballooning mode is found to be most unstable in a thin current sheet region at the equatorial location xe ∼ (8.5 − 10)RE, where the ballooning drive term (βeq/LpRc) is dominant on stiffening effect due to minimum in normal magnetic field. Because of the stabilizing effect as caused by the field line stiffening and the strong field line stabilization, the ballooning mode is stable (marginally unstable) close (away) from the Earth. The magnitude of the KBI growth rates as a function of the equatorial beta βeq and location xe for a wide and thin current sheets using the Voigt equilibrium model are smaller than the Harris sheet counterpart, which are used for modeling the near-Earth plasma sheet configuration for the slow and fast substorm growth phases, respectively. The results suggest that the excitation of KBI through the local current sheet thinning may also be an effective trigger mechanism for the substorm onset in the near-Earth magnetotail. This article is protected by copyright. All rights reserved.