MAGNETOSPHERIC INTERCHANGE INSTABILITY IN ANISOTROPIC-PLASMA

We study magnetospheric interchange instability under the assumption t hat the plasma pressure distribution is anisotropic. Previous studies of magnetospheric interchange instability have only considered the cas e of isotropic pressure. We also argue that, under certain circumstanc es, substantial particle energization can accompany outward interchang e motions in rapidly rotating magnetic fields. Our studies of instabil ity treat the plasma as an MHD fluid and deal with two special cases i n which the plasma pressure evolves anisotropically as the interchange motion proceeds. The first case is that of ''fast'' interchange motio ns, where interchange motions can take place rapidly compared with par ticle bounce times. Our analysis uses a small perturbation approach an d takes into account the curved magnetic field, and external forces su ch as gravity or an effective gravity arising from rotation of the sys tem. We contrast this with a second case in which the plasma motion co nserves the adiabatic invariants mu and J, and in both cases consider the implications for a plasma generated by a satellite source in the e quatorial plane of a rapidly rotating, spin-aligned magnetic field. A consequence of fast interchange motions in a corotation-dominated magn etosphere is that the rapid motions will be accompanied by motion of i onized material away from (toward) the equatorial plane as the materia l moves outward (inward). If an outward (inward) interchange motion sh ould 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 plasm a, the less likely it is to remain equatorially confined during outwar d interchange motion, whether it is a diffusive or steady process. We discuss our results in the context of the Jovian magnetosphere.