  We have studied non-stationary and non-axisymmetric perturbations of a
magnetohydrodynamic accretion onto a rotating (Kerr) black hole. Assuming that
the magnetic field dominates the plasma accretion, we find that the accretion
suffers a large radial acceleration resulting from the Lorentz force, and
becomes highly variable compared with the electromagnetic field there. In fact,
we further find an interesting perturbed structure of the plasma velocity with
a large peak in some narrow region located slightly inside of the
fast-magnetosonic surface. This is due to the concentrated propagation of the
fluid disturbances in the form of fast-magnetosonic waves along the separatrix
surface. If the fast-magnetosonic speed is smaller in the polar regions than in
the equatorial regions, the critical surface has a prolate shape for radial
poloidal field lines. In this case, only the waves that propagate towards the
equator can escape from the super-fast-magnetosonic region and collimate
polewards as they propagate outwards in the sub-fast-magnetosonic regions. We
further discuss the capabilities of such collimated waves in accelerating
particles due to cyclotron resonance in an electron-positron plasma.
