BALLOONING-MIRROR INSTABILITY AND INTERNALLY DRIVEN PC-4-5 WAVE EVENTS

A kinetic-MHD field-aligned eigenmode stability analysis of low freque ncy ballooning-mirror instabilities has been performed for anisotropic pressure plasmas in the magnetosphere. The ballooning mode is mainly a transverse wave driven unstable by pressure gradient in the bad curv ature region. The mirror mode with a dominant compressional magnetic f ield perturbation is excited when the product of plasma beta and press ure anisotropy (P-perpendicular to/P-parallel to > 1) is large. In the limit that the wave frequency is smaller than the energetic trapped p article magnetic drift frequency, which is usually much smaller than t he energetic trapped particle bounce frequency, the energetic trapped particles experience the bounce-averaged wave structure due to their r apid bounce motion. For modes with north-south symmetric field-aligned structure of parallel perturbed magnetic field the energetic trapped particle kinetic pressure response is finite and cancels with their fl uid pressure response so that the symmetric mode is stable. Physically the energetic trapped particles precess very rapidly across the ($) o ver right arrow B field, and their motion becomes very rigid with resp ect to low frequency symmetric MHD perturbations. For antisymmetric mo des the energetic trapped particle kinetic pressure response from the northern hemisphere cancels with that from the southern hemisphere in a bounce period, and the instability beta threshold of the antisymmetr ic mo de is determined by the energetic particle fluid free energy. Pr essure anisotropy with P-perpendicular to/P-parallel to > 1 reduces th e beta(parallel to) threshold. The antisymmetric mode changes from a b allooning mode with dominant transverse magnetic field components at P -perpendicular to/P-parallel to = 1 to a hybrid ballooning-mirror type mode with comparable transverse and compressional magnetic field comp onents near the equator as P-perpendicular to/P-parallel to increases. With large equatorial plasma beta (beta(parallel to) greater than or equal to O(1)) and pressure anisotropy (P-perpendicular to /P-parallel to > 1) the field-aligned wave structure of antisymmetric ballooning- mirror mode resembles the multisatellite observations of a long lastin g compressional Pc 5 wave event during November 14-15, 1979 (Takahashi et al., 1987). From the AMPTE/CCE particle and magnetic field data ob served during Pc 4-5 wave events we compute the ballooning-mirror inst ability parameters and perform a correlation study with the theoretica l instability threshold. We find that compressional Pc 5 waves approxi mately satisfy the ballooning-mirror instability condition, and transv erse Pc 4-5 waves are probably related to resonant ballooning instabil ities with small pressure anisotropy.