### Abstract

The heating, acceleration, and pitch-angle scattering of charged particles by magnetohydrodynamic (MHD) turbulence are important in a wide range of astrophysical environments, including the solar wind, accreting black holes, and galaxy clusters. We simulate the interaction of high-gyrofrequency test particles with fully dynamical simulations of subsonic MHD turbulence, focusing on the parameter regime with β ∼ 1, where β is the ratio of gas to magnetic pressure. We use the simulation results to calibrate analytical expressions for test particle velocity-space diffusion coefficients and provide simple fits that can be used in other work. The test particle velocity diffusion in our simulations is due to a combination of two processes: interactions between particles and magnetic compressions in the turbulence (as in linear transit-time damping; TTD) and what we refer to as Fermi Type-B (FTB) interactions, in which charged particles moving on field lines may be thought of as beads sliding along moving wires. We show that test particle heating rates are consistent with a TTD resonance that is broadened according to a decorrelation prescription that is Gaussian in time (but inconsistent with Lorentzian broadening due to an exponential decorrelation function, a prescription widely used in the literature). TTD dominates the heating for v_{s} ≫ v_{A} (e.g., electrons), where v_{s} is the thermal speed of species s and v_{A} is the Alfvén speed, while FTB dominates for v_{s} ≪ v_{A} (e.g., minor ions). Proton heating rates for β ∼ 1 are comparable to the turbulent cascade rate. Finally, we show that velocity diffusion of collisionless, large gyrofrequency particles due to large-scale MHD turbulence does not produce a power-law distribution function.

Original language | English (US) |
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Article number | 78 |

Journal | Astrophysical Journal |

Volume | 758 |

Issue number | 2 |

DOIs | |

State | Published - Oct 20 2012 |

### All Science Journal Classification (ASJC) codes

- Astronomy and Astrophysics
- Space and Planetary Science

### Keywords

- acceleration of particles
- magnetohydrodynamics (MHD)
- plasmas
- solar wind
- turbulence

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## Cite this

*Astrophysical Journal*,

*758*(2), [78]. https://doi.org/10.1088/0004-637X/758/2/78