CONVECTIVE INSTABILITIES OF ELECTROMAGNETIC ION-CYCLOTRON WAVES IN THE OUTER MAGNETOSPHERE

The path-integrated linear growth of electromagnetic ion cyclotron wav es in the outer (L greater than or equal to 7) magnetosphere is invest igated using a realistic thermal plasma distribution with an additiona l anisotropic energetic ring current H+ to provide free energy for ins tability. The results provide a realistic simulation of the recent AMP TE observations. For conditions typical of the dayside magnetosphere, high plasma beta effects reduce the group velocity and significantly i ncrease the spatial growth rates for left-hand polarized instabilities just below the helium gyrofrequency Omega(He+), and on the unguided m ode above Omega(He+) but below the cross over frequency omega(cr). Rel atively high densities, typical of the afternoon local time sector, fa vor these low group velocity effects for predominantly field-aligned w aves. Lower densities, typical of those found in the early morning loc al time sector, increase the group velocity but allow strong convectiv e instabilities at high normalized frequencies well above Omega(He+). These waves are reflected in the magnetosphere and can exist for sever al equatorial transits without significant damping. They are left-hand polarized only on the first equatorial crossing and become linearly p olarized for the remainder of the ray path. Consequently, these waves should be observed with basically linear polarization at all frequenci es and all latitudes in the early morning local time sector. Wave grow th below Omega(He+) is severely limited owing to the narrow bandwidth for instability and the small resonant path lengths. In the afternoon sector, where plasma densities can exceed 10(7) m(-3), intense convect ive amplification is possible both above and below Omega(He+). Waves b elow Omega(He+) are not subject to reflection when the O+ concentratio n is small and therefore should be observed with left-hand polarizatio n near the equator and essentially linear polarization at higher latit udes. Since the He+ concentration is usually large in the afternoon se ctor, guided mode waves above Omega(He+) reflect to form a background distribution with basically linear polarization. We suggest that the s trong left-hand polarized emissions observed by AMPTE in the afternoon sector near the equator are probably due to strongly growing low grou p velocity waves at frequencies just below Omega(He+), and on the ungu ided mode above Omega(He+).