Thermal disequilibration of ions and electrons by collisionless plasma turbulence

Does overall thermal equilibrium exist between ions and electrons in a weakly collisional, magnetized, turbulent plasma? And, if not, how is thermal energy partitioned between ions and electrons? This is a fundamental question in plasma physics, the answer to which is also crucial for predicting the properties of far-distant astronomical objects such as accretion disks around black holes. In the context of disks, this question was posed nearly two decades ago and has since generated a sizeable literature. Here we provide the answer for the case in which energy is injected into the plasma via Alfvenic turbulence: Collisionless turbulent heating typically acts to disequilibrate the ion and electron temperatures. Numerical simulations using a hybrid fluid-gyrokinetic model indicate that the ion-electron heating-rate ratio is an increasing function of the thermal-to-magnetic energy ratio, beta(i): It ranges from similar to 0.05 at beta(i) = 0.1 to at least 30 for beta(i) greater than or similar to 10. This energy partition is approximately insensitive to the ion-to-electron temperature ratio T-i/T-e. Thus, in the absence of other equilibrating mechanisms, a collisionless plasma system heated via Alfvenic turbulence will tend toward a nonequilibrium state in which one of the species is significantly hotter than the other, i.e., hotter ions at high beta(i) and hotter electrons at low beta(i). Spectra of electromagnetic fields and the ion distribution function in 5D phase space exhibit an interesting new magnetically dominated regime at high beta(i) and a tendency for the ion heating to be mediated by nonlinear phase mixing (entropy cascade) when beta(i) less than or similar to 1 and by linear phase mixing (Landau damping) when beta(i) >> 1.