Electron and ion heating by kinetic turbulence in strongly magnetized, trans-relativistic plasma*

Turbulence is responsible for heating collisionless plasmas in high-energy astrophysical systems such as black-hole accretion flows, where electrons attain relativistic energies. However, the electron-to-ion heating ratio remains uncertain from a theoretical perspective. Models based on linear Landau damping predict that in the gyrokinetic limit, Alfvenic turbulence would preferentially heat electrons when the plasma beta is low (i.e., magnetic pressure dominates thermal pressure). We test this prediction in the semirelativistic regime (sub-relativistic ions, ultra-relativistic electrons) by performing particle-in-cell (PIC) simulations of strong turbulence in low-beta plasma with a strong background magnetic field (relative to large-scale fluctuations). We confirm that electrons are preferentially heated, but also find that the ion heating is greatly enhanced compared to the model predictions, due to additional ion heating channels and relativistic effects (which reduce the kinetic scale separation between electrons and ions). We analyze the heating mechanisms by studying the phase-space structure of the particle distributions. These results have implications for modeling emission from astrophysical sources, including supermassive black holes observed by the Event Horizon Telescope.