# Kinetic inhibition of MHD-shocks in the vicinity of a parallel magnetic field

## TL;DR

This paper demonstrates that a strong flow-aligned magnetic field can inhibit shock formation in collisionless pair plasmas, challenging traditional MHD predictions and revealing new plasma behavior.

## Contribution

It shows that magnetic fields aligned with plasma flow can suppress shock formation, a violation of classical MHD theory, supported by micro-physics analysis and particle-in-cell simulations.

## Key findings

- Magnetic field suppresses isotropization in plasma
- Shock formation is inhibited by strong flow-aligned fields
- Results extend beyond strictly parallel magnetic configurations

## Abstract

According to magnetohydrodynamics (MHD), the encounter of two collisional magnetized plasmas at high velocity gives rise to shock waves. Investigations conducted so far have found that the same conclusion still holds in the case of collisionless plasmas. For the case of a flow-aligned field, MHD stipulates that the field and the fluid are disconnected, so that the shock produced is independent of the field. We present a violation of this MHD prediction when considering the encounter of two cold pair plasmas along a flow-aligned magnetic field. As the guiding magnetic field grows, isotropization is progressively suppressed, resulting in a strong influence of the field on the resulting structure. A micro-physics analysis allows to understand the mechanisms at work. Particle-in-cell simulations also support our conclusions and show that the results are not restricted to a strictly parallel field.