The kinetic theory of quasi-stationary collisionless accretion disc plasmas

TL;DR

This paper develops a kinetic theory framework for collisionless accretion disc plasmas, revealing the existence of a self-sustaining kinetic dynamo that generates magnetic fields without turbulence.

Contribution

It formulates the Vlasov-Maxwell kinetic theory for quasi-stationary, collisionless, non-relativistic accretion disc plasmas, including temperature anisotropies and magnetic field self-generation.

Findings

Existence of temperature anisotropies in kinetic solutions

Identification of a kinetic dynamo mechanism

Self-generation of magnetic fields without turbulence

Abstract

Astrophysical plasmas in accretion discs are usually treated in the framework of fluid or MHD approaches but there are some situations where these treatments become inadequate and one needs to revert to the more fundamental underlying kinetic theory. This occurs when the plasma becomes effectively collisionless or weakly-collisional such as, for example, in radiatively inefficient accretion flows onto black holes. In this paper, we lay down the basics of kinetic theory in these contexts. In particular, we formulate the kinetic theory for quasi-stationary collisionless accretion disc plasmas in the framework of a Vlasov-Maxwell description, taking the plasma to be non-relativistic, axisymmetric, gravitationally-bound and subject to electromagnetic fields. Quasi-stationary solutions for the kinetic distribution functions are constructed which are shown to admit temperature anisotropies. The physical implications of the theory are then investigated and the equations of state and angular momentum conservation law are discussed. Analysis of the Ampere equation reveals the existence of a quasi-stationary kinetic dynamo which gives rise to self-generation of poloidal and azimuthal magnetic fields and operates even in the absence of turbulence and/or instability phenomena.