  We show that the interaction of an accretion disk with the magnetosphere of a
neutron star can excite resonant shear Alfven waves with Hz-kHz frequencies in
a region of enhanced density gradients. This is the the region where accretion
material flows along the magnetic field lines in the magnetosphere. We argue
that due to the pressure anisotropy produced by the plasma flow, firehose
instabilities are likely to occur. Furthermore, for a dipolar field topology,
we show that a new instability develops due to both magnetic field curvature
and the plasma flow.
