MAGNETOSPHERIC INTERCHANGE INSTABILITY IN ANISOTROPIC-PLASMA

We study magnetospheric interchange instability under the assumption that the plasma pressure distribution is anisotropic. Previous studies of magnetospheric interchange instability have only considered the case of isotropic pressure. We also argue that, under certain circumstances, substantial particle energization can accompany outward interchange motions in rapidly rotating magnetic fields. Our studies of instability treat the plasma as an MHD fluid and deal with two special cases in which the plasma pressure evolves anisotropically as the interchange motion proceeds. The first case is that of ''fast'' interchange motions, where interchange motions can take place rapidly compared with particle bounce times. Our analysis uses a small perturbation approach and takes into account the curved magnetic field, and external forces such as gravity or an effective gravity arising from rotation of the system. We contrast this with a second case in which the plasma motion conserves the adiabatic invariants mu and J, and in both cases consider the implications for a plasma generated by a satellite source in the equatorial plane of a rapidly rotating, spin-aligned magnetic field. A consequence of fast interchange motions in a corotation-dominated magnetosphere is that the rapid motions will be accompanied by motion of ionized material away from (toward) the equatorial plane as the material moves outward (inward). If an outward (inward) interchange motion should be slowed such that it is no longer rapid compared with particle bounce times, particles will resume bounce motion, but with increased (reduced) parallel energy. In practice, It is likely that lower energy particles in a distribution will violate the longitudinal invariant, J, during interchange motion, whereas particles of higher energy will conserve J. Thus our work implies that the lower the energy of a plasma, the less likely it is to remain equatorially confined during outward interchange motion, whether it is a diffusive or steady process. We discuss our results in the context of the Jovian magnetosphere.