  We present an instability occurring in the inner part of disks threaded by a
moderately strong vertical (poloidal) magnetic field. Its mechanism is such
that a spiral density wave in the disk, driven by magnetic stresses (rather
than self-gravity as in galactic spirals), becomes unstable by exchanging
angular momentum with a Rossby vortex it generates at its corotation radius.
This angular momentum can then ``leak'' as Alfven waves emitted toward the
corona of the disk thus providing, as an element of the accretion process, an
energetic source for a wind or a jet. As galactic spirals, this instability
forms low azimuthal wavenumber, standing spiral patterns which might provide an
explanation for low-frequency QPOs in low-mass X-ray binaries.
