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Register a new userConstraints on ion vs. electron heating by plasma turbulence at low beta
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Added by
Alexander Schekochihin
Publication date
2018
fields
Physics
and research's language is
English
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It is shown that in low-beta, weakly collisional plasmas, such as the solar corona, some instances of the solar wind, the aurora, inner regions of accretion discs, their coronae, and some laboratory plasmas, Alfvenic fluctuations produce no ion heating within the gyrokinetic approximation, i.e., as long as their amplitudes (at the Larmor scale) are small and their frequencies stay below the ion Larmor frequency (even as their spatial scales can be above or below the ion Larmor scale). Thus, all low-frequency ion heating in such plasmas is due to compressive fluctuations (slow modes). Because these fluctuations energetically decouple from the Alfvenic ones already in the inertial range, the above conclusion means that the energy partition between ions and electrons in low-beta plasmas is decided at the outer scale, where turbulence is launched, and can be determined from magnetohydrodynamic (MHD) models of the relevant astrophysical systems. Any additional ion heating must come from non-gyrokinetic mechanisms such as cyclotron heating or the stochastic heating owing to distortions of ions Larmor orbits. An exception to these conclusions occurs in the Hall limit, i.e., when the ratio of the ion to electron temperatures is as low as the ion beta (equivalently, the electron beta is order unity). In this regime, slow modes couple to Alfvenic ones well above the Larmor scale (viz., at the ion inertial or ion sound scale), so the Alfvenic and compressive cascades join and then separate again into two cascades of fluctuations that linearly resemble kinetic Alfven and ion cyclotron waves, with the former heating electrons and the latter ions. The two cascades are shown to decouple, scalings for them are derived, and it is argued physically that the two species will be heated by them at approximately equal rates.
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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 $Q_rmi/Q_rme$ as a function of the compressive-to-Alfvenic driving power ratio $P_compr/P_AW$, of the ratio of ion thermal pressure to magnetic pressure $beta_rmi$, and of the ratio of ion-to-electron background temperatures $T_rmi/T_rme$. It is shown that $Q_rmi/Q_rme$ is an increasing function of $P_compr/P_AW$. When the compressive driving is sufficiently large, $Q_rmi/Q_rme$ approaches $simeq P_compr/P_AW$. 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 Alfven waves is absent for both low and high $beta_rmi$, 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 $beta_rmi$ 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*.
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