Magneto-thermal Instability in Dilute Magnetized Plasmas
type: CreativeWork
description: In dilute, magnetised plasmas exchanges of heat across magnetic field lines are strongly suppressed, as charged particles rapidly gyrate around magnetic field lines but only infrequently scatter due to Coulomb collisions. The anisotropic heat conduction, in the presence of a large-scale temperature gradient, destabilises otherwise stable entropy-stratified flows (according to the classic Schwarzschild criterion) via the magneto-thermal instability (MTI) and generates turbulence. The MTI is relevant in the outskirts of galaxy clusters, where it may supply a source of observed turbulence in the intra-cluster medium (ICM), and in the plasma of hot accretion flows around black holes and neutron stars, where the MTI can interact with the magneto-rotational instability (MRI) to produce complex dynamical cycles by attacking the temperature gradients produced by the MRI. In the first part of the thesis, we take a fresh look at the problem of the MTI in the absence of shear and rotation, and construct a general theory that explains the MTI saturation mechanism and provides scalings and estimates for the turbulent levels. We simulate MTI turbulence with a Boussinesq code, SNOOPY, and find that in two dimensions the saturation mechanism involves an inverse cascade carrying kinetic energy from the short MTI injection scales to larger scales, where it is arrested by the stable entropy stratification; in three dimensions, on the other hand, most energy is dissipated at the same scale as its injection, and turbulent eddies are vertically elongated at or below the thermal conduction length, but relatively isotropic on larger scales. Similar to 2D, however, the saturated turbulent energy levels and the integral scale follow clear power-laws that depend on the thermal diffusivity, temperature gradient, and buoyancy frequency. We then show that our scaling laws are consistent with extant observations of ICM turbulence if the thermal conductivity is reduced by a factor of about ten fro…
abstract: In dilute, magnetised plasmas exchanges of heat across magnetic field lines are strongly suppressed, as charged particles rapidly gyrate around magnetic field lines but only infrequently scatter due to Coulomb collisions. The anisotropic heat conduction, in the presence of a large-scale temperature gradient, destabilises otherwise stable entropy-stratified flows (according to the classic Schwarzschild criterion) via the magneto-thermal instability (MTI) and generates turbulence. The MTI is relevant in the outskirts of galaxy clusters, where it may supply a source of observed turbulence in the intra-cluster medium (ICM), and in the plasma of hot accretion flows around black holes and neutron stars, where the MTI can interact with the magneto-rotational instability (MRI) to produce complex dynamical cycles by attacking the temperature gradients produced by the MRI. In the first part of the thesis, we take a fresh look at the problem of the MTI in the absence of shear and rotation, and construct a general theory that explains the MTI saturation mechanism and provides scalings and estimates for the turbulent levels. We simulate MTI turbulence with a Boussinesq code, SNOOPY, and find that in two dimensions the saturation mechanism involves an inverse cascade carrying kinetic energy from the short MTI injection scales to larger scales, where it is arrested by the stable entropy stratification; in three dimensions, on the other hand, most energy is dissipated at the same scale as its injection, and turbulent eddies are vertically elongated at or below the thermal conduction length, but relatively isotropic on larger scales. Similar to 2D, however, the saturated turbulent energy levels and the integral scale follow clear power-laws that depend on the thermal diffusivity, temperature gradient, and buoyancy frequency. We then show that our scaling laws are consistent with extant observations of ICM turbulence if the thermal conductivity is reduced by a factor of about ten fro…
author: Perrone, Lorenzo Maria
language: eng
keywords: astrophysics, instability, magnetic dynamo, magnetic fields, plasma physics, turbulence
license: https://www.rioxx.net/licenses/all-rights-reserved/
date published: 2022-08-01
date modified: 2026-07-22T22:24:01Z
provider: Cambridge University
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: University of Cambridge

PropertyValue
property ID: DOI
value: 10.17863/CAM.90358