Collisional behaviors of astrophysical collisionless plasmas

TL;DR

This paper reviews how collisionless astrophysical plasmas exhibit fluid-like behaviors such as shock formation and Maxwellian distributions, despite lacking binary collisions, by exploring the underlying processes and conditions involved.

Contribution

It provides a comprehensive analysis of the mechanisms enabling collisionless plasmas to mimic collisional fluid behaviors and discusses the applicability of fluid models to such plasmas.

Findings

Collisionless shocks can form similarly to fluid shocks under certain conditions.

Rankine-Hugoniot relations are approximately fulfilled in collisionless shocks.

Maxwellian distributions are common in PIC simulations of collisionless plasmas.

Abstract

In collisional fluids, a number of key processes rely on the frequency of binary collisions. Collisions seem necessary to generate a shock wave when two fluids collide fast enough, to fulfill the Rankine-Hugoniot relations, to establish an equation of state or a Maxwellian distribution. Yet, these seemingly collisional features are routinely either observed or assumed, in relation with collision\emph{less} astrophysical plasmas. This article will review our current answers to the following questions: How do colliding collisionless plasmas end-up generating a shock as if they were fluids? To which extent are the Rankine-Hugoniot relations fulfilled in this case? Do collisionless shocks propagate like fluid ones? Can we use an equation of state to describe collisionless plasmas, like MHD codes for astrophysics do? Why are Maxwellian distributions ubiquitous in Particle-In-Cell simulations of collisionless shocks? Time and length scales defining the border between the collisional and the collisionless behavior will be given when relevant. In general, when the time and length scales involved in the collisionless processes responsible for the fluid-like behavior may be neglected, the system may be treated like a fluid.