Studying the mirror acceleration via kinetic simulations of relativistic plasma turbulence

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.