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This theoretical study examines how ions are heated in collisionless astrophysical plasmas through their interaction with turbulent magnetic fluctuations. Using quasi-linear theory, the researchers demonstrate that the degree of turbulence imbalance determines whether ion heating occurs primarily through stochastic mechanisms or cyclotron-resonant processes, with the heating preferentially occurring perpendicular to the magnetic field. The derived heating rate formula applies across different turbulence conditions and recovers previous empirical results while explaining the suppression of heating at low turbulent amplitudes due to magnetic moment conservation.
Why it matters
Understanding ion heating mechanisms is crucial for modeling the evolution of collisionless plasmas found throughout the universe, including in the solar wind, accretion disks, and galaxy clusters. The analytical predictions provided can be directly tested against plasma simulations and space-based observations, potentially improving our ability to predict plasma behavior in extreme astrophysical environments.
arXiv:2512.03472v2 Announce Type: replace
Abstract: In collisionless astrophysical plasmas, turbulence mediates the partitioning of free energy among cascade channels and its dissipation into ion and electron heat. The resulting ion heating is often anisotropic, with ions observed to be preferentially heated perpendicular to the local magnetic field; understanding the mechanisms responsible for this heating is a key step in understanding the evolution of such plasmas. In this paper, we use the framework of quasi-linear theory to compute analytically the heating rates of ions interacting with turbulent, large-scale Alfv’enic fluctuations. We show how the imbalance of the turbulence (the difference in energies between Alfv’enic fluctuations travelling parallel and antiparallel to the magnetic field) modifies the spatiotemporal spectrum of these fluctuations, allowing the heating mechanism to smoothly transition between stochastic heating in balanced turbulence and cyclotron-resonant heating in imbalanced turbulence. The resultant heating rate is found to have a general form regardless of the level of imbalance, exhibiting a suppression related to the conservation of the ions’ magnetic moment at small turbulent amplitudes and recovering previous empirical results in a formal calculation. The results of this work help to consolidate our qualitative understanding of ion heating within astrophysical plasmas, as well as yielding specific quantitative predictions to analyse simulations and observations.
Source: Quasi-linear theory of perpendicular ion heating by critically balanced turbulence