Thermal disequilibration of ions and electrons by collisionless plasma turbulence

Yohei Kawazura; Michael Barnes; Alexander A. Schekochihin

doi:10.1073/pnas.1812491116

paper

Thermal disequilibration of ions and electrons by collisionless plasma turbulence

arXiv:1807.07702 · doi:10.1073/pnas.1812491116

Abstract

Does overall thermal equilibrium exist between ions and electrons in a weakly collisional, magnetised, 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 discs around black holes. In the context of discs, 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 Alfvénic 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, $β_{i}$ : it ranges from $\sim 0.05$ at $β_{i} = 0.1$ to at least $30$ for $β_{i} ≳ 10$ . 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 Alfvénic turbulence will tend towards a nonequilibrium state in which one of the species is significantly hotter than the other, viz., hotter ions at high $β_{i}$ , hotter electrons at low $β_{i}$ . Spectra of electromagnetic fields and the ion distribution function in 5D phase space exhibit an interesting new magnetically dominated regime at high $β_{i}$ and a tendency for the ion heating to be mediated by nonlinear phase mixing ("entropy cascade") when $β_{i} ≲ 1$ and by linear phase mixing (Landau damping) when $β_{i} ≫ 1$

Thermal disequilibration of ions and electrons by collisionless plasma turbulence

Abstract

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