Kinetic description of quasi-stationary axisymmetric collisionless accretion disk plasmas with arbitrary magnetic field configurations

TL;DR

This paper develops a kinetic framework for describing collisionless, axisymmetric accretion disk plasmas with complex magnetic fields, enabling analysis of plasma behavior, flow velocities, temperature anisotropy, and magnetic field generation in astrophysical contexts.

Contribution

It introduces a generalized bi-Maxwellian distribution solution within a gyrokinetic Vlasov framework for collisionless accretion disk plasmas with open magnetic surfaces, accounting for kinetic effects and plasma trapping.

Findings

Derived a reduced gyrokinetic Vlasov equation for accretion disk plasmas.

Demonstrated the existence of kinetic solutions with temperature anisotropy and flow velocities.

Discussed the potential for a kinetic dynamo effect in magnetic field generation.

Abstract

A kinetic treatment is developed for collisionless magnetized plasmas occurring in high-temperature, low-density astrophysical accretion disks, such as are thought to be present in some radiatively-inefficient accretion flows onto black holes. Quasi-stationary configurations are investigated, within the framework of a Vlasov-Maxwell description. The plasma is taken to be axisymmetric and subject to the action of slowly time-varying gravitational and electromagnetic fields. The magnetic field is assumed to be characterized by a family of locally nested but open magnetic surfaces. The slow collisionless dynamics of these plasmas is investigated, yielding a reduced gyrokinetic Vlasov equation for the kinetic distribution function. For doing this, an asymptotic quasi-stationary solution is first determined, represented by a generalized bi-Maxwellian distribution expressed in terms of the relevant adiabatic invariants. The existence of the solution is shown to depend on having suitable kinetic constraints and conditions leading to particle trapping phenomena. With this solution one can treat temperature anisotropy, toroidal and poloidal flow velocities and finite Larmor-radius effects. An asymptotic expansion for the distribution function permits analytic evaluation of all of the relevant fluid fields. Basic theoretical features of the solution and their astrophysical implications are discussed. As an application, the possibility of describing the dynamics of slowly time-varying accretion flows and the self-generation of magnetic field by means of a \textquotedblleft kinetic dynamo effect\textquotedblright\ is discussed. Both effects are shown to be related to intrinsically-kinetic physical mechanisms.