Collisional behaviors of astrophysical 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.

20 pages, 4 figures, to appear in the Lab Astrophysics Special Issue of "Journal of Plasma Physics"

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• Simulation study of the formation of a non-relativistic pair shock
• Nuclear Fusion in Laser-Driven Counter-Streaming Collisionless Plasmas
• Simulations of Galaxy Cluster Collisions with a Dark Plasma Component
• Kinetic inhibition of MHD-shocks in the vicinity of a parallel magnetic field
• On the cosmological abundance of magnetic monopoles
• Collisionless shocks in self-interacting dark matter
• The interplay of the collisionless nonlinear thin-shell instability with the ion acoustic instability