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.