Kinetic Ballooning Instability of the Near-Earth Magnetotail in Voigt Equilibrium

TL;DR

This study investigates the kinetic ballooning mode stability in the near-Earth magnetotail using a Voigt equilibrium model, revealing how kinetic effects and current sheet conditions influence instability growth.

Contribution

It provides the first detailed analysis of KBM stability in a realistic magnetotail configuration incorporating kinetic effects and finite ion gyroradius.

Findings

KBM growth rate depends on magnetic field line stiffening factor S.

KBM is unstable within a specific range of equatorial beta.

Finite ion gyroradius and trapped electrons tend to stabilize KBM.

Abstract

For a long time, ballooning instabilities have been 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, stability of the KBM is evaluated for the two-dimensional Voigt equilibrium of the near-Earth magnetotail based on an analytical kinetic theory of ballooning instability in the framework of kinetic magnetohydrodynamic (MHD) model, where the kinetic effects such as the finite gyroradius effect, wave-particle resonances, particle drifts motions are included usually through kinetic closures. The growth rate of the KBM strongly depends on the magnetic field line stiffening factor $S$, which is in turn determined by the effects of the trapped electrons, the finite ion gyroradius, and the magnetic drift motion of charged particles. The KBM is unstable in a finite intermediate range of equatorial $\beta_{eq}$ and only marginally unstable at higher $\beta_{eq}$ regime for higher $T_e/T_i$ values. The finite ion gyroradius and the trapped electron fraction enhance the stiffening factor that tends to stabilize the KBM in the magnetotail far away from Earth. On the other hand, the current sheet thinning destabilizes KBM in the lower $\beta_{eq}$ regime and stabilizes KBM in the higher $\beta_{eq}$ regime.