An accretion-ejection instability in magnetized disks

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