Theoretical Study On Kinetic Ballooning Instability In The Near-Earth Magnetotail

Substorm onsets are believed to be a fundamental process for magnetic and plasma energy transport in both magnetosphere and ionosphere.It has been sug-gested over three decades that ballooning instability in the near-Earth plasma sheet could play a significant role in the substorm onset scenario.It is still not clear whether the ballooning instability plays a leading role in the substorm onset dy-namics or has only a marginal part.In particular,the ballooning instabilities have been proposed to be a possible triggering mechanism for the onset of substorm and current disruption initiation in the near-Earth magnetotail.Whereas many agree that the plasma sheet regions can be linearly unstable to the ideal MHD balloon-ing mode,however,it has become clear that the kinetic effects significantly modify the stability criterion from the ideal MHD model.Yet the stability of the kinetic ballooning mode(KBM)in a global and realistic magnetotail configuration has not been well examined.In this dissertation,we evaluate the stability of the ballooning mode in different current sheet configurations of the near-Earth magnetotail based on an analytical kinetic theory.In this study,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 ?eq and only marginally unstable at higher ?eq regime for higher Te/Ti 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 ?eq regime and stabilizes KBM in the higher ?eq regime.The kinetic ballooning instability of a general type of two-dimensional(2D)kinetic sheet equilibria,the so-called generalized Harris sheet equilibria with a deep Bz minimum region is further analyzed.The KBM 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(?eq/LpRc)becomes maximum due to the minimum in the normal magnetic field Bz there.Because of the strong field line stabilization and stabilizing effect through the stiffening factor S,the KBM is stabilized close to the Earth and away from the Earth.This suggests the local current sheet thinning may be also an effective trigger mechanism for the onset of KBM instability in the near-Earth magnetotail.