Physics – Plasma Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsh52a..07p&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #SH52A-07
Physics
Plasma Physics
[7500] Solar Physics, Astrophysics, And Astronomy, [7807] Space Plasma Physics / Charged Particle Motion And Acceleration, [7863] Space Plasma Physics / Turbulence, [7867] Space Plasma Physics / Wave/Particle Interactions
Scientific paper
We study the heating of charged test particles in three-dimensional numerical simulations of weakly compressible magnetohydrodynamic (MHD) turbulence ``Alfvénic turbulence''); these results are relevant to particle heating and acceleration in the solar wind, solar flares, accretion disks onto black holes, and other astrophysics and heliospheric environments. The physics of particle heating depends on whether the gyrofrequency of a particle, Ω0, is comparable to the frequency of a turbulent fluctuation, ω, that is resolved on the computational domain. Particles with Ω0 ≈ ω undergo strong perpendicular heating (relative to the local magnetic field) and pitch angle scattering. By contrast, particles with Ω0 » ω undergo strong parallel heating. Simulations with a finite resistivity produce additional parallel heating due to parallel electric fields in small-scale current sheets. Many of our results are consistent with linear theory predictions for the particle heating produced by the Alfvén and slow magnetosonic waves that make up Alfvénic turbulence. However, in contrast to linear theory predictions, energy exchange is not dominated by discrete resonances between particles and waves; instead, the resonances are substantially ``broadened.'' We discuss the implications of our results for solar and astrophysics problems, in particular the thermodynamics of the near-Earth solar wind. This requires an extrapolation of our results to higher numerical resolution, because the dynamic range that can be simulated is far less than the true dynamic range between the proton cyclotron frequency and the outer-scale frequency of MHD turbulence. We conclude that Alfvénic turbulence produces significant parallel heating via the interaction between particles and magnetic field compressions (``slow waves''). However, on scales above the proton Larmor radius Alfvénic turbulence does not produce significant perpendicular heating of protons or minor ions, consistent with linear theory. Instead, the Alfvén wave energy cascades to perpendicular scales below the proton Larmor radius, initiating a kinetic Alfvén wave cascade. Standard deviation of the parallel (solid line) and perpendicular (dashed line) velocities as a function of the particles' gyrofrequency. The strong diffusion for low Ω0 particles is consistent with linear theory predictions for cyclotron resonance. High Ω0 particles are consistent with Landau resonance due to μ▽‖B forces.
Lehe Remi
Parrish Ian J.
Quataert Eliot
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