Spiral waves and instability in magnetized astrophysical disks

It is shown that an ionized disk in a vertical magnetic field (e.g., the dipole field of the central object for an accretion or planetary disk) is subject to spiral instabilities of low azimuthal wavenumber, driven by differential rotation. The amplification is due to the Swing mechanism already known for the spiral instability of galactic disks, with self-gravity replaced by magnetic forces. In realistic cases the growth time is a few tens of rotation periods. Such instabilities, or waves driven by irregularities in the ambient magnetic field, might be important in particular for accretion disks, and to explain the spokes in Saturn's rings.