Studying the mirror acceleration via kinetic simulations of relativistic plasma turbulence

TL;DR

This paper uses 3D PIC simulations to investigate mirror acceleration as an efficient relativistic particle energization mechanism in turbulence-compressed magnetic fields, revealing significant energy gains and anisotropic pitch angle distributions.

Contribution

It extends the study of mirror acceleration to relativistic turbulence using detailed particle tracking in 3D PIC simulations, highlighting its efficiency and effects on particle distribution.

Findings

Particles gain significant energy during mirror interactions within one gyro-orbit.

Momentum gain is mainly perpendicular to the magnetic field and correlates with magnetic field strengthening.

Particle pitch angles become more anisotropic at higher energies, favoring large angles.

Abstract

Efficient relativistic turbulent acceleration of particles is indicated by recent astrophysical observations. The Type II mechanism with acceleration due to the temporal variations of magnetic field strengths remains underexplored. The mirror acceleration has recently been proposed as an efficient Type II mechanism for particle energization in turbulence-compressed magnetic fields. We perform a 3D particle-in-cell (PIC) simulation of pair plasma to extend its study to relativistic turbulence. By tracking individual particles, we see that the particles interacting with transverse magnetic mirrors can have a significant energy gain during one mirror interaction and within one gyro-orbit. As expected for the mirror acceleration, we statistically find that the momentum gain is preferentially in the direction perpendicular to the local magnetic field and positively correlated with the local magnetic field strengthening. As a result, the particle pitch angle distribution becomes increasingly anisotropic toward higher energies, with a concentration at large pitch angles. The mirror acceleration facilitates a spatial confinement of particles by stochastically increasing their pitch angles, which further enhances the mirror acceleration.