Studying mirror acceleration via kinetic simulations of relativistic plasma turbulence

Abstract: Efficient turbulent acceleration of particles is indicated by recent astrophysical observations, but its mechanism is not well understood. Mirror acceleration has recently been proposed as an efficient mechanism for particle energization in turbulence-compressed magnetic fields. We employ a 3D particle-in-cell (PIC) simulation of pair plasma in magnetized and relativistic turbulence to study this new mechanism and its acceleration efficiency. By tracking individual particles, we see that reversal of a particle's moving direction and significant energy gain can happen during one mirror interaction and within one gyro-orbit. As expected for mirror acceleration, we statistically find that (1) energy gain is preferentially in the direction perpendicular to the local magnetic field and positively correlated with local magnetic field strengthening, and (2) the particle pitch angle distribution becomes increasingly anisotropic toward higher energies, with a concentration at large pitch angles. Our results demonstrate that the mirror acceleration causes a strong confinement of particles by stochastically increasing their pitch angles. This, in turn, facilitates repeated mirror acceleration with the mirroring condition well satisfied. We conclude that mirror acceleration is a promising mechanism accounting for efficient acceleration in magnetized and turbulent astrophysical plasmas.