  We analyse the stability of a magnetized medium consisting of a neutral fluid
and a fluid of charged particles, coupled to each other through a drag force
and exposed to differential body forces (for example, as the result of
radiation forces on one phase). We consider a uniform equilibrium and simple
model input physics, but do not arbitrarily restrict the relative orientations
of the magnetic field, slip velocity and wave vector of the disturbance. We
find several instabilities and classify these in terms of wave resonances. We
briefly apply our results to the structure of SiO maser regions appearing in
the winds from late-type stars.
