Beyond Maxwell-Boltzmann: Transport in Quasiequilibrium Plasmas

TL;DR

This paper develops a theoretical framework for understanding transport phenomena in space plasmas with non-Maxwellian, superstatistical distributions, revealing enhanced transport coefficients due to suprathermal particles.

Contribution

It introduces macroscopic relations for superstatistical plasmas, derives transport coefficients, and quantifies their enhancement across main superstatistics classes.

Findings

Transport coefficients are systematically increased in quasiequilibrium plasmas.

Superstatistical effects align with observed electron distributions in the solar wind.

Enhanced transport is linked to energetic particles in non-Maxwellian tails.

Abstract

Space plasmas are generally characterized by non-Maxwellian distributions with suprathermal populations, as routinely revealed by in situ observations. Such departures from standard Maxwellian distributions can be understood as signatures of quasiequilibrium states, in which the distribution of the medium can be expressed as a continuous superposition of Maxwellian distributions, namely through superstatistics. Here, we construct macroscopic relations linking fluxes to their associated driving forces in such plasmas, where superstatistical effects enter the picture through the transport coefficients. After comparing the resulting superstatistical distributions with observed electron distributions in the solar wind, we turn to the kinetic response of quasiequilibrium plasmas and derive the corresponding transport coefficients, including the electric and thermal conductivities, the mobility, and the diffusion coefficient. We further extend the analysis to viscous plasmas and compute the shear and bulk viscosity coefficients. Overall, quasiequilibrium effects are found to systematically enhance the transport coefficients relative to their Maxwellian values. We quantify this enhancement for the three main universality classes of superstatistics, which are the most commonly encountered in experimental and observational situations, and interpret it as a consequence of the increased population of energetic particles in the non-Maxwellian tails.