Magnetorotational instability (MRI or Velikhov-Chandrasekhar instability or Balbus-Hawley instability) is an instability in a body of fluid, e.g., a disk, that occurs in a conducting fluid within a magnetic field when the angular velocity decreases with radial distance. In such conditions, small disturbances receive positive feedback, resulting in the instability. (Disk simulations need sufficiently high resolution to model the initial small disturbance.) The result can be turbulence, termed magnetoturbulence or magnetorotational turbulence. It is thought to occur in accretion disks and to be a contributor to Type I planetary migration. The criteria for magnetorotational instability in a Keplerian disk in an axial magnetic field is:
dΩ² ———— > 0 dlnR
Modeling MRI is more difficult if the disk is not fully ionized.