Instability - RWI Presentation
If the inner edge of the disk ddoes not reflect waves, another form of global modes can exist if the radial profiles of density, magnetic field, rotation frequency, etc. create an extremum of a certain quantity in the disk (we will not describe this quantity here, but this is similar to the constraint for the hydrodynamical Kelvin-Helmholtz instabiliy). This can be created by an extremum of density or magnetic field, but it also occurs naturally, because of the relativistic rotation curve, just outside the Marginally Stable Orbit.
Rossby waves can be trapped in this extremum and also establish a standing pattern. The resulting Rossby-Wave Instability was studied in hydrodynamical disks by Lovelace and his coworkers. We find it much more unstable in magnetized disks, and have used this to explain, in particular, the quasi-periodicity observed during the flares of Saggitarius A*, the black hole at the center of the Galaxy.
