Magnetorotational Turbulence and Dynamo in a Collisionless Plasma

TL;DR

This paper presents the first 3D kinetic simulation of magnetorotational turbulence and dynamo in a collisionless plasma, revealing how turbulence, angular momentum transport, and particle acceleration occur in such environments.

Contribution

It introduces a novel kinetic simulation approach to study MRI-driven turbulence and dynamo in collisionless plasmas, highlighting the roles of pressure anisotropy and kinetic instabilities.

Findings

Self-sustained turbulence and angular momentum transport observed.

Ion acceleration characterized by a kappa distribution.

Energy spectra indicate Alfvénic cascades at different scales.

Abstract

We present results from the first 3D kinetic numerical simulation of magnetorotational turbulence and dynamo, using the local shearing-box model of a collisionless accretion disc. The kinetic magnetorotational instability grows from a subthermal magnetic field having zero net flux over the computational domain to generate self-sustained turbulence and outward angular-momentum transport. Significant Maxwell and Reynolds stresses are accompanied by comparable viscous stresses produced by field-aligned ion pressure anisotropy, which is regulated primarily by the mirror and ion-cyclotron instabilities through particle trapping and pitch-angle scattering. The latter endow the plasma with an effective viscosity that is biased with respect to the magnetic-field direction and spatio-temporally variable. Energy spectra suggest an Alfv\'en-wave cascade at large scales and a kinetic-Alfv\'en-wave cascade at small scales, with strong small-scale density fluctuations and weak non-axisymmetric density waves. Ions undergo non-thermal particle acceleration, their distribution accurately described by a kappa distribution. These results have implications for the properties of low-collisionality accretion flows, such as that near the black hole at the Galactic center.