Kinetic magnetohydrodynamics and Landau fluid closure in relativity

TL;DR

This paper develops a relativistic fluid model incorporating kinetic effects like pressure anisotropy and heat conduction, crucial for understanding weakly collisional plasmas near black holes.

Contribution

It derives relativistic drift kinetic equations and introduces a novel Landau fluid closure for anisotropic heat flow in relativistic plasmas, without relying on strong collisions.

Findings

Provides a theoretical framework for weakly collisional relativistic plasmas.

Introduces a new analytic Landau fluid closure capturing anisotropic heat flow.

Offers a complementary approach to kinetic simulations for black hole accretion disks.

Abstract

Diffuse accretion flows near a supermassive black hole are fundamentally weakly collisional. In such weakly collisional plasmas, the ion and electron distribution functions can deviate significantly from thermal equilibrium, and particle kinetic effects can influence large-scale fluid motion by driving pressure anisotropy, heat conduction, and plasma instabilities. Modeling these plasma effects in highly relativistic flows could be important for interpreting horizon-scale observations of black hole images. In this paper, we present a theoretical framework for understanding weakly collisional plasmas in general relativity by deriving the relativistic drift kinetic equations from the Vlasov-Maxwell equations. We present the evolution equations for the moments of the gyroaveraged distribution function and introduce a new analytic Landau fluid closure to capture anisotropic heat flow in relativistic plasmas. Unlike standard (collisional) general relativistic magnetohydrodynamics or extended magnetohydrodynamics, our model does not rely on strong collisions to enforce thermal equilibrium and consistently incorporates Landau damping in a fluid closure. The model introduced in this work provides a complementary approach to fully kinetic simulations in understanding weakly collisional effects in low-luminosity relativistic black hole accretion disks.