Shearing Box Simulations of the MRI in a Collisionless Plasma

We describe local shearing box simulations of turbulence driven by themagnetorotational instability (MRI) in a collisionless plasma. Collisionlesseffects may be important in radiatively inefficient accretion flows, such asnear the black hole in the Galactic Center. The MHD version of ZEUS is modifiedto evolve an anisotropic pressure tensor. A fluid closure approximation is usedto calculate heat conduction along magnetic field lines. The anisotropicpressure tensor provides a qualitatively new mechanism for transporting angularmomentum in accretion flows (in addition to the Maxwell and Reynolds stresses).We estimate limits on the pressure anisotropy due to pitch angle scattering bykinetic instabilities. Such instabilities provide an effective ``collision''rate in a collisionless plasma and lead to more MHD-like dynamics. We find thatthe MRI leads to efficient growth of the magnetic field in a collisionlessplasma, with saturation amplitudes comparable to those in MHD. In the saturatedstate, the anisotropic stress is comparable to the Maxwell stress, implyingthat the rate of angular momentum transport may be moderately enhanced in acollisionless plasma.Comment: 20 pages, 9 figures, submitted to Ap