Effect of flow-aligned external magnetic fields on mushroom instability

TL;DR

This study explores how flow-aligned external magnetic fields influence mushroom instabilities in relativistic jets, combining theoretical dispersion analysis and PIC simulations to reveal suppression effects and stability characteristics.

Contribution

The paper provides the first combined theoretical and simulation analysis of magnetic field effects on mushroom instabilities in magnetized relativistic jets.

Findings

External magnetic fields suppress MI growth.

MIs are more robust than ESKHIs against magnetic suppression.

Simulations confirm analytical predictions and show saturation structures.

Abstract

Mushroom instability (MI) is a shear instability considered responsible for generating and amplifying magnetic fields in relativistic jets. While astrophysical jets are usually magnetized, how MI acts in magnetized jets remains poorly understood. In this paper, we investigate the effect of a flow-aligned external magnetic field on MI, with both theoretical analyses and particle-in-cell (PIC) simulations. In the limit of a cold and collisionless plasma, we derive a generalized dispersion relation for linear growth rates of the magnetized MIs. Numerical solutions of the dispersion relation reveal that the external magnetic field always suppresses the growth of MI, though MIs are much more robust against the external magnetic field than electron-scale Kelvin-Helmholtz instabilities (ESKHIs). Analyses are also extended to instabilities with an arbitrary wavevector in the shear interface plane, where coupling effect is observed for sub-relativistic scenarios. Two-dimensional PIC simulations of single-mode MIs reach a good agreement with our analytical predictions, and we observe formation of a quasi-steady saturation structure in magnetized runs. In simulations with finite temperatures, we observe the competition and cooperation between MIs and a diffusion-induced DC magnetic field.