Abstract
The partition of irreversible heating between ions and electrons in compressively driven (but subsonic) collisionless turbulence is investigated by means of nonlinear hybrid gyrokinetic simulations. We derive a prescription for the ion-to-electron heating ratio as a function of the compressive-to-Alfvénic driving power ratio , of the ratio of ion thermal pressure to magnetic pressure , and of the ratio of ion-to-electron background temperatures . It is shown that is an increasing function of . When the compressive driving is sufficiently large, approaches . This indicates that, in turbulence with large compressive fluctuations, the partition of heating is decided at the injection scales, rather than at kinetic scales. Analysis of phase-space spectra shows that the energy transfer from inertial-range compressive fluctuations to sub-Larmor-scale kinetic Alfvén waves is absent for both low and high , meaning that the compressive driving is directly connected to the ion-entropy fluctuations, which are converted into ion thermal energy. This result suggests that preferential electron heating is a very special case requiring low and no, or weak, compressive driving. Our heating prescription has wide-ranging applications, including to the solar wind and to hot accretion disks such as M87 and Sgr A*.
- Received 10 April 2020
- Revised 25 September 2020
- Accepted 22 October 2020
DOI:https://doi.org/10.1103/PhysRevX.10.041050
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
A collisionless plasma is quite different from a cafe latte. In a latte, the temperature of the coffee and milk quickly become the same. In a plasma, the two main components—ions and electrons—can maintain different temperatures. Exploring this energy partition is crucial for understanding various astrophysical phenomena, such as the solar wind and accretion disks. For example, knowing how to determine the ion-to-electron temperature ratio is indispensable for interpreting images taken by the Event Horizon Telescope of accretion disks around supermassive black holes. To that end, we carry out numerical simulations of microscale plasma turbulence and calculate the irreversible heating that occurs as a result of the dissipation of this turbulence.
Previous studies investigated only a particular type of turbulence consisting solely of Alfvénic perturbations—small incompressible distortions of magnetic field lines. We set up a more general, and much more realistic, situation, where the turbulence contains both compressive and Alfvénic perturbations simultaneously. Our results show that ions are preferentially heated when compressive driving is large or when plasma pressure dominates magnetic pressure. Preferential electron heating can happen only in fairly special cases.
Our results have broad application because they are valid for any collisionless plasma system in which the gyrokinetic approximation (low-frequency, magnetized fluctuations) is valid. For example, such models can be used in global magnetohydrodynamic simulations of accretion disks. Our results can also be tested via in situ spacecraft measurements in the Sun’s heliosphere.