Fluctuation Dynamo in Collisionless and Weakly Collisional Magnetized Plasmas

TL;DR

This paper investigates how kinetic instabilities influence magnetic field amplification in weakly collisional plasmas, revealing two dynamo regimes and proposing microphysical closures for fluid simulations.

Contribution

It demonstrates the role of kinetic ion-Larmor scale instabilities in enabling the fluctuation dynamo in collisionless plasmas and introduces new microphysical closures bridging different regimes.

Findings

Kinetic instabilities reduce pressure anisotropy, enabling dynamo action.

Two regimes of fluctuation dynamo identified based on pressure regulation.

Collisionless plasmas exhibit a cascade to sub-Larmor scales with faster dynamo growth.

Abstract

In weakly collisional astrophysical plasmas, such as the intracluster medium of galaxy clusters, the amplification of cosmic magnetic fields by chaotic fluid motions is hampered by the adiabatic production of magnetic-field-aligned pressure anisotropy. This anisotropy drives a viscous stress parallel to the field that inhibits the plasma's ability to stretch magnetic-field lines. I demonstrate through the use of kinetic simulations that in high-$\beta$ plasmas, kinetic ion-Larmor scale instabilities sever the adiabatic link between the thermal and magnetic pressures, reducing this viscous stress and thereby allowing the dynamo to operate. Two distinct regimes of the fluctuation dynamo in a magnetized plasma are identified: one in which these instabilities efficiently regulate the pressure anisotropy and one in which this regulation is imperfect. I elucidate the role of these kinetic instabilities on the plasma viscosity and determine how the fields and flows self-organize to allow the dynamo to operate in the face of parallel viscous stresses. In the case of efficient pressure-anisotropy regulation, the plasma dynamo closely resembles its more traditional Pm ~ 1 MHD counterpart. When the regulation is imperfect, the dynamo exhibits characteristics remarkably similar to those found in the saturated state of the MHD dynamo. A novel set of microphysical closures for fluid simulations that bridges these two regimes are constructed, exhibiting explosive magnetic-field growth caused by a field-strength-dependent viscosity. The dynamos in both collisionless and weakly collisional plasmas are then closely compared to each other, revealing substantial differences in how sub-parallel viscous motions behave. The former (collisionless) scenario experiences a cascade of stretching motions to sub-Larmor scales that lead to increasingly fast dynamo as the magnetic Reynolds number is increased.