An introductory guide to fluid models with anisotropic temperatures. Part 1. CGL description and collisionless fluid hierarchy
Abstract
We present a detailed guide to advanced collisionless fluid models that incorporate kinetic effects into the fluid framework, and that are much closer to the collisionless kinetic description than traditional magnetohydrodynamics. Such fluid models are directly applicable to modelling the turbulent evolution of a vast array of astrophysical plasmas, such as the solar corona and the solar wind, the interstellar medium, as well as accretion disks and galaxy clusters. The text can be viewed as a detailed guide to Landau fluid models and it is divided into two parts. Part 1 is dedicated to fluid models that are obtained by closing the fluid hierarchy with simple (nonLandau fluid) closures. Part 2 is dedicated to Landau fluid closures. Here in Part 1, we discuss the fluid model of ChewGoldbergerLow (CGL) in great detail, together with fluid models that contain dispersive effects introduced by the Hall term and by the finite Larmor radius corrections to the pressure tensor. We consider dispersive effects introduced by the nongyrotropic heat flux vectors. We investigate the parallel and oblique firehose instability, and show that the nongyrotropic heat flux strongly influences the maximum growth rate of these instabilities. Furthermore, we discuss fluid models that contain evolution equations for the gyrotropic heat flux fluctuations and that are closed at the fourthmoment level by prescribing a specific form for the distribution function. For the biMaxwellian distribution, such a closure is known as the `normal' closure. We also discuss a fluid closure for the bikappa distribution. Finally, by considering onedimensional Maxwellian fluid closures at higherorder moments, we show that such fluid models are always unstable. The last possible non Landau fluid closure is therefore the `normal' closure, and beyond the fourthorder moment, Landau fluid closures are required.
 Publication:

Journal of Plasma Physics
 Pub Date:
 December 2019
 DOI:
 10.1017/S0022377819000801
 arXiv:
 arXiv:1901.09354
 Bibcode:
 2019JPlPh..85f2002H
 Keywords:

 astrophysical plasmas;
 space plasma physics;
 plasma waves;
 Physics  Plasma Physics
 EPrint:
 Improved version, accepted to JPP Lecture Notes. Some parts were shortened and some parts were expanded. The text now contains Conclusions