On the formation and properties of fluid shocks and collisionless shock waves in astrophysical plasmas

Abstract

When two plasmas collide, their interaction can be mediated by collisionless plasma instabilities or binary collisions between particles of each shell. By comparing the maximum growth rate of the collisionless instabilities with the collision frequency between particles of the shells, we determine the critical density separating the collisionless formation from the collisional formation of the resulting shock waves. This critical density is also the density beyond which the shock downstream is field free, as plasma instabilities do not have time to develop electromagnetic patterns. We further determine the conditions on the shells initial density and velocity for the downstream to be collisional. If these quantities fulfil the determined conditions, the collisionality of the downstream also prevents the shock from accelerating particles or generating strong magnetic fields. We compare the speed of sound with the relative speed of collision between the two shells, thus determining the portion of the parameter space where strong shock formation is possible for both classical and degenerate plasmas. Finally, we discuss the observational consequences in several astrophysical settings.

Plasmas in Gamma-Ray Bursts: Particle Acceleration, Magnetic Fields, Radiative Processes and Environments

“…The most widely discussed mechanism by which (weakly magnetized) magnetic fields can be generated is the Weibel instability [e.g., [80][81][82][83][84][85][86][87][88][89][90]. In this model, small fluctuation in the magnetic fields charge separation opposite charges in the background plasma.…”

PIC simulation methods for cosmic radiation and plasma instabilities

“…Collisionless shocks are not sharp jumps, but have a finite width. Strictly parallel nonrelativistic shocks can also deviate from the MHD picture, in which the magnetic field is irrelevant for the shock profile [46]. The processes through which the incoming plasma flow is decelerated and practically isotropized are collective interactions with self-excited plasma waves, whose nature and operation depends on the orientation of the large-scale magnetic field [340].…”

“…Below a well-defined critical density, collisions do not mediate the shock, and the shock formation builds on plasma instabilities [45]. Collisionless shocks and the associated electromagnetic fields and accelerated particles self-consistently evolve, driven by the anisotropy of the particle distribution function that is inherent to relativistic shocks.…”