4.3 Article

Fluctuation dynamo in a weakly collisional plasma

Journal

JOURNAL OF PLASMA PHYSICS
Volume 86, Issue 5, Pages -

Publisher

CAMBRIDGE UNIV PRESS
DOI: 10.1017/S0022377820000860

Keywords

astrophysical plasmas; plasma nonlinear phenomena; plasma instabilities

Funding

  1. US DOE [DE-AC02-09-CH1 1466]
  2. Alfred P. Sloan Research Fellowship in Physics
  3. Rutherford Discovery Fellowship [RDF-U001804]
  4. Marsden Fund [UOO1727]
  5. UK STFC [ST/N000919/1]
  6. UK EPSRC [EP/M022331/1, EP/R034737/1]
  7. National Science Foundation
  8. NSF [PHY-1748958]
  9. EPSRC [EP/R034737/1] Funding Source: UKRI

Ask authors/readers for more resources

The turbulent amplification of cosmic magnetic fields depends upon the material properties of the host plasma. In many hot, dilute astrophysical systems, such as the intracluster medium (ICM) of galaxy clusters, the rarity of particle-particle collisions allows departures from local thermodynamic equilibrium. These departures - pressure anisotropies - exert anisotropic viscous stresses on the plasma motions that inhibit their ability to stretch magnetic-field lines. We present an extensive numerical study of the fluctuation dynamo in a weakly collisional plasma using magnetohydrodynamic (MHD) equations endowed with a field-parallel viscous (Braginskii) stress. When the stress is limited to values consistent with a pressure anisotropy regulated by firehose and mirror instabilities, the Braginskii-MHD dynamo largely resembles its MHD counterpart, particularly when the magnetic field is dynamically weak. If instead the parallel viscous stress is left unabated - a situation relevant to recent kinetic simulations of the fluctuation dynamo and, we argue, to the early stages of the dynamo in a magnetized ICM - the dynamo changes its character, amplifying the magnetic field while exhibiting many characteristics reminiscent of the saturated state of the large-Prandtl-number (P-m greater than or similar to 1) MHD dynamo. We construct an analytic model for the Braginskii-MHD dynamo in this regime, which successfully matches simulated dynamo growth rates and magnetic-energy spectra. A prediction of this model, confirmed by our numerical simulations, is that a Braginskii-MHD plasma without pressure-anisotropy limiters will not support a dynamo if the ratio of perpendicular and parallel viscosities is too small. This ratio reflects the relative allowed rates of field-line stretching and mixing, the latter of which promotes resistive dissipation of the magnetic field. In all cases that do exhibit a viable dynamo, the generated magnetic field is organized into folds that persist into the saturated state and bias the chaotic flow to acquire a scale-dependent spectral anisotropy.

Authors

I am an author on this paper
Click your name to claim this paper and add it to your profile.

Reviews

Primary Rating

4.3
Not enough ratings

Secondary Ratings

Novelty
-
Significance
-
Scientific rigor
-
Rate this paper

Recommended

No Data Available
No Data Available