Evaluation of Kinetic Ballooning Instability in the Near-Earth Magnetotail

Abstract

Ballooning instabilities are widely believed to be a possible triggering mechanism for the onset of substorm and current disruption initiation in the near-Earth magnetotail. Yet the stability of the kinetic ballooning mode (KBM) in a global and realistic magnetotail configuration has not been well examined. In this paper, the growth rate of the KBM is calculated from analytical theory for the two-dimensional Voigt equilibrium within the framework of kinetic magnetohydrodynamic (MHD) model. The growth rate of the KBM is found to be strongly dependent on the field line stiffening factor $S$, which depends on the trapped electron dynamics, the finite ion gyroradius, and the magnetic drift motion of charged particles. Furthermore, calculations show that the KBM is unstable in a finite intermediate range of equatorial ${\beta }_{eq}$ values and the growth rate dependence on ${\beta }_{eq}$ is enhanced for larger ${\rho }_i$. The KBM stability is further analyzed in a broad range of $k_y$ for different values of ion Larmor radius ${\rho }_i$ and gradient ratio ${\eta }_j\equiv d\ln (T_j)/d\ln (n_j)$, where $T_j$ is the particle temperature and $n_j$ is the particle density. The KBM is found to be unstable for sufficiently high values of $k_y$, where the growth rate first increases to a maximum value and then decreases due to kinetic effects. The $k_y$ at the maximum growth rate decreases exponentially with ${\rho }_i$. The current sheet thinning is found to enhance the KBM growth rate and the unstable ${\beta }_{eq}$ regime in the near-Earth magnetotail.

24 pages, 9 figures