Theory of one-dimensional Vlasov-Maxwell equilibria: with applications to collisionless current sheets and flux tubes

TL;DR

This paper develops a Hermite polynomial-based method to construct and analyze one-dimensional Vlasov-Maxwell equilibria, including new models for low plasma beta and asymmetric current sheets, with applications to collisionless plasma structures.

Contribution

It introduces a Hermite expansion approach for solving the inverse problem in Vlasov-Maxwell equilibria and constructs new models for force-free Harris sheets and asymmetric current sheets.

Findings

Constructed new Vlasov-Maxwell equilibria for low plasma beta.

Proven convergence of Hermite expansions for all plasma beta values.

Developed models suitable for particle-in-cell simulations.

Abstract

We study the theory of Vlasov-Maxwell equilibria in one spatial dimension, as well as its application to current sheet and flux tube models. The 'inverse problem' is that of determining a Vlasov-Maxwell equilibrium distribution function self-consistent with a given magnetic field. We develop the theory of inversion using expansions in Hermite polynomials of the canonical momenta. Sufficient conditions for the convergence of a Hermite expansion are found, given a pressure tensor. For large classes of DFs, we prove that non-negativity of the distribution function is contingent on the magnetisation of the plasma, and make conjectures for all classes. The inverse problem is considered for nonlinear 'force-free Harris sheets'. By applying the Hermite method, we construct new models that can describe sub-unity values of the plasma beta $(\beta_{pl})$ for the first time. Whilst analytical convergence is proven for all $\beta_{pl}$, numerical convergence is attained for $\beta_{pl}= 0.85$, and then $\beta_{pl} = 0.05$ after a 're-gauging' process. We consider the properties that a pressure tensor must satisfy to be consistent with 'asymmetric Harris sheets', and construct new examples. It is possible to analytically solve the inverse problem in some cases, others must be tackled numerically. We present new exact Vlasov-Maxwell equilibria for asymmetric current sheets, which can be written as a sum of shifted Maxwellian distributions, ideal for implementations in particle-in-cell simulations. We study the correspondence between the microscopic and macroscopic descriptions of equilibrium in cylindrical geometry, and then attempt to find Vlasov-Maxwell equilibria for the nonlinear force-free 'Gold-Hoyle' model. However, it is necessary to include a background field, which can be arbitrarily weak if desired. The equilibrium can be electrically non-neutral, depending on the bulk flows.