Change of a Weibel-type to an Alfv\'enic shock in pair plasma by upstream waves

TL;DR

This study uses particle-in-cell simulations to show how upstream waves can transform a Weibel-type shock into an Alfvénic shock in pair plasma, revealing the dynamic evolution of shock structures.

Contribution

It demonstrates the transition mechanism from a Weibel-type to an Alfvénic shock driven by upstream wave interactions in pair plasma.

Findings

Shock changes from Weibel-type to Alfvénic due to upstream wave growth.

Upstream waves are driven by escaping downstream particles.

The resulting Alfvénic shock has a Mach number of about 6.

Abstract

We examine with particle-in-cell (PIC) simulations how a parallel shock in pair plasma reacts to upstream waves, which are driven by escaping downstream particles. Initially, the shock is sustained in the two-dimensional simulation by a magnetic filamentation (beam-Weibel) instability. Escaping particles drive an electrostatic beam instability upstream. Modifications of the upstream plasma by these waves hardly affect the shock. In time, a decreasing density and increasing temperature of the escaping particles quench the beam instability. A larger thermal energy along than perpendicular to the magnetic field destabilizes the pair-Alfv\'en mode. In the rest frame of the upstream plasma, the group velocity of the growing pair-Alfv\'en waves is below that of the shock and the latter catches up with the waves. Accumulating pair-Alfv\'en waves gradually change the shock in the two-dimensional simulation from a Weibel-type shock into an Alfv\'enic shock with a Mach number that is about 6 for our initial conditions.