Investigation of Kinetic Ballooning Instability in 2D Harris Sheet Equilibrium with Finite Normal BZ Field.

Substorm onset is believed to be a fundamental process for magnetic and plasma energy transport in both magnetosphere and ionosphere.Ballooning instability is one of the key trigger mechanism to examine the substorm onset scenario in the near-Earth plasma sheet. For this subject,the kinetic ballooning instability (KBI) in the near-Earth magnetotail is analyzed using the general type of two-dimensional (2D) kinetic sheet equilibria, the so-called generalized Harris sheet equilibria. The kinetic ballooning mode is unstable in the intermediate range of perpendicular wave number (ky) and the equatorial beta (eq). The growth rate of the ballooning mode reduces signicantly with the increase in electron and ion temperatures ratio (Te=Ti) and wave number ky. The kinetic ballooning mode is found to be most unstable in the thin current sheet region at the equatorial location xe ~(9-10)RE, where the ballooning drive term (βeq/LpRc) is dominant on stiffening effect due to the minimum in the normal magnetic field Bz there.Because of the stabilizing effect through the field line stiffening factor and the strong field line stabilization, the ballooning mode is stable close to the Earth and becomes marginally unstable away from the Earth. This suggests the local current sheet thinning may be also an effective trigger mechanism for the onset of ballooning instability in the near-Earth magnetotail.