Drift mirror instability revisited, 1, Cold electron temperature limit

Linear theory of the drift-mirror instability in high-beta plasma is recons idered referring to basic principles for the two cases of a one- and a two- component ion plasma ill presence of a cold electron background. In both ca ses the cold electrons serve to shortcut the parallel electric field compon ent which imposes the condition of vanishing the field-aligned current. The corresponding loci-frequency dispersion relation is derived in the fluid a pproximation as well as from kinetic theory including nonvanishing gradient s in the density. The free energy of the unstable mode is taken from two so urces: the pressure anisotropy and the spatial inhomogeneity of the plasma. The dispersion relation contains a correction which originates from the in clusion of the bending of the magnetic field that is caused by the reaction of the field to the total pressure force. It is shown that. the mirror for ce substantially reduces the phase velocity which is in favour of instabili ty since this requires phase velocities. less than the drift speed. The dir ection of phase velocity becomes antiparallel to that of the pure density-g radient drift velocity. Even for a mirror-stable plasma an instability aris es which is solely due to inhomogeneity. We analyze the transition to the c lassical mirror instability. Application to ring current (hot ring current ions plus cold plasmaphere ions) and magnetosheath (hot sheath ions only) c onditions is presented and is in agreement with observational indication of the apparent stability of the pure mirror mode in the ring current.