Viscous shear instability in weakly magnetized, dilute plasmas

When the ion mean free path much exceeds the Larmor radius in a plasma, the viscous stress tensor is altered dramatically and depends only on quantities measured along the field lines. This regime corresponds to typical interstellar medium conditions in galaxies and protogalaxies, even if the magnetic field is extremely weak, with a negligible Lorentz force on all scales of interest. In this work, the only role of the magnetic field is to channel angular momentum transport along its lines of force. We show that differential rotation in such a gas is highly unstable, with a maximum growth rate exceeding that of the magnetorotational instability. The regime of interest has been treated previously by plasma kinetic methods. Where there is overlap, our work appears to be in agreement with the kinetic results. The nonlinear outcome of this instability is likely to be a turbulent process, significantly augmenting the magnetorotational instability, and important to the initial phases of the amplification of small galactic magnetic fields.