Prof Steven Balbus FRS, FInstP

Abstract:

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.

Abstract:

In this paper and a companion work, we report on the first global numerical simulations of self-gravitating magnetized tori, subject in particular to the influence of the magnetorotational instability (MRI). In this work, Paper I, we restrict our calculations to the study of the axisymmetric evolution of such tori. Our goals are twofold: (1) to investigate how self-gravity influences the global structure and evolution of the disks and (2) to determine whether turbulent density inhomogeneities can be enhanced by self-gravity in this regime. As in non-self-gravitating models, the linear growth of the MRI is followed by a turbulent phase, during which angular momentum is transported outward. As a result, self-gravitating tori quickly develop a dual structure composed of an inner thin Keplerian disk fed by a thicker self-gravitating disk, whose rotation profile is close to a Mestel disk. Our results show that the effects of self-gravity enhance density fluctuations much less than they smooth the disk, giving it more coherence. We discuss the expected changes that will occur in three-dimensional simulations, the results of which are presented in a companion paper.