Pressure-anisotropy-induced nonlinearities in the kinetic magnetorotational instability

TL;DR

This paper investigates how pressure anisotropy influences the nonlinear development of the magnetorotational instability in collisionless plasmas, finding that the transition to turbulence resembles that in collisional plasmas, with implications for future simulations.

Contribution

It demonstrates that pressure anisotropy nonlinearities and parasitic instabilities behave similarly in collisionless and collisional regimes, clarifying MRI saturation mechanisms in low-collisionality plasmas.

Findings

Pressure anisotropy is regulated by mirror instability.

Large amplitude MRI modes evolve similarly in collisionless and collisional plasmas.

Route to turbulence in collisionless plasma resembles that in collisional plasma.

Abstract

In collisionless and weakly collisional plasmas, such as hot accretion flows onto compact objects, the magnetorotational instability (MRI) can differ significantly from the standard (collisional) MRI. In particular, pressure anisotropy with respect to the local magnetic-field direction can both change the linear MRI dispersion relation and cause nonlinear modifications to the mode structure and growth rate, even when the field and flow perturbations are small. This work studies these pressure-anisotropy-induced nonlinearities in the weakly nonlinear, high-ion-beta regime, before the MRI saturates into strong turbulence. Our goal is to better understand how the saturation of the MRI in a low collisionality plasma might differ from that in the collisional regime. We focus on two key effects: (i) the direct impact of self-induced pressure-anisotropy nonlinearities on the evolution of an MRI mode, and (ii) the influence of pressure anisotropy on the "parasitic instabilities" that are suspected to cause the mode to break up into turbulence. Our main conclusions are: (i) The mirror instability regulates the pressure anisotropy in such a way that the linear MRI in a collisionless plasma is an approximate nonlinear solution once the mode amplitude becomes larger than the background field (just as in MHD). This implies that differences between the collisionless and collisional MRI become unimportant at large amplitudes. (ii) The break up of large amplitude MRI modes into turbulence via parasitic instabilities is similar in collisionless and collisional plasmas. Together, these conclusions suggest that the route to magnetorotational turbulence in a collisionless plasma may well be similar to that in a collisional plasma, as suggested by recent kinetic simulations. As a supplement to these findings, we offer guidance for the design of future kinetic simulations of magnetorotational turbulence.