Finite Larmor Radius Effects on the Magnetorotational Instability

The linear dispersion relation for the magnetorotational instability (MRI) is derived including finite Larmor radius (FLR) effects. In particular, the Braginskii form of the ion gyroviscosity, which represents the first-order FLR corrections to the two-fluid equations, is retained. It is shown that FLR effects are the most important effects in the limit of weak magnetic fields, and are much more important than the Hall effect when beta >> 1, where beta is the ratio of the ion thermal pressure to the magnetic pressure. FLR effects may completely stabilize even MRI modes having wavelengths much greater than the ion Larmor radius. Some implications for astrophysical accretion disks are discussed.