THEORY OF BALLOONING-MIRROR INSTABILITIES FOR ANISOTROPIC PRESSURE PLASMAS IN THE MAGNETOSPHERE

A kinetic-MHD eigenmode analysis of ballooning-mirror instabilities is performed for anisotropic pressure plasmas in the magnetosphere. The energetic particle kinetic effects and the coupling between the balloo ning and mirror modes are taken into account. Without energetic trappe d particle kinetic effects the ballooning-mirror modes with symmetric field-aligned structure of parallel perturbed magnetic field deltaB(pa rallel-to) and electrostatic potential PHI have lower beta instability threshold than the antisymmetric modes. Pressure anisotropy with (P(p erpendicular-to)/P(parallel-to) > 1) reduces the beta threshold for ba llooning-mirror instabilities. In the limit that the wave frequency is smaller than the energetic trapped particle magnetic drift frequency, the symmetric ballooning-mirror mode is completely stabilized by the energetic trapped particle kinetic effects. However, the antisymmetric ballooning-mirror mode is only weakly influenced by the energetic tra pped particle kinetic effects and has the lowest beta threshold. For s ymmetric modes the energetic trapped particles experience a bounce-ave raged wave structure due to their rapid bounce motion, and their nonad iabatic kinetic pressure response cancels with their fluid pressure re sponse so that they do not contribute to the mode stability. Physicall y, the energetic trapped particles precess very rapidly across the B o ver arrow pointing right field, and their motion becomes very rigid wi th respect to low-frequency symmetric MHD perturbations. For antisymme tric modes the energetic trapped particle kinetic pressure response fr om the northern hemisphere cancels with that from the southern hemisph ere in a bounce period, and thus the instability beta thresholds is ma inly determined by t,he energetic particle fluid free energy. The fiel d-aligned perturbed magnetic field structure of the antisymmetric mode changes from a ballooning mode with dominant transverse magnetic fiel d components at P(perpendicular-to)/P(parallel-to) = 1 to a mixed ball ooning-mirror mode with comparable transverse and compressional compon ents near the equator as P(perpendicular-to)/P(parallel-to) increase. With large equatorial plasma beta (beta(parallel-to) greater-than-or-e qual-to 0(1)) and pressure anisotropy (P(perpendicular-to)/P(parallel- to) > 1) the field-aligned wave structure of antisymmetric ballooning- mirror mode resembles the multisatellite observation of a long lasting compressional Pc 5 wave event during November 14-15, 1979 [Takahashi et al., 1987].