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Title: THE EFFICIENCY OF SECOND-ORDER FERMI ACCELERATION BY WEAKLY COMPRESSIBLE MAGNETOHYDRODYNAMIC TURBULENCE

We investigate the effects of pitch-angle scattering on the efficiency of particle heating and acceleration by MHD turbulence using phenomenological estimates and simulations of non-relativistic test particles interacting with strong, subsonic MHD turbulence. We include an imposed pitch-angle scattering rate, which is meant to approximate the effects of high-frequency plasma waves and/or velocity space instabilities. We focus on plasma parameters similar to those found in the near-Earth solar wind, though most of our results are more broadly applicable. An important control parameter is the size of the particle mean free path λ{sub mfp} relative to the scale of the turbulent fluctuations L. For small scattering rates, particles interact quasi-resonantly with turbulent fluctuations in magnetic field strength. Scattering increases the long-term efficiency of this resonant heating by factors of a few times 10, but the distribution function does not develop a significant non-thermal power-law tail. For higher scattering rates, the interaction between particles and turbulent fluctuations becomes non-resonant, governed by particles heating and cooling adiabatically as they encounter turbulent density fluctuations. Rapid pitch-angle scattering can produce a power-law tail in the proton distribution function, but this requires fine-tuning of parameters. Moreover, in the near-Earth solar wind, a significant power-law tail cannotmore » develop by this mechanism because the particle acceleration timescales are longer than the adiabatic cooling timescale set by the expansion of the solar wind. Our results thus imply that MHD-scale turbulent fluctuations are unlikely to be the origin of the v {sup –5} tail in the proton distribution function observed in the solar wind.« less
Authors:
 [1] ; ;  [2] ;  [3]
  1. Physics Department, University of California, Berkeley, CA 94720 (United States)
  2. Astronomy Department and Theoretical Astrophysics Center, University of California, Berkeley, CA 94720 (United States)
  3. Space Science Center and Department of Physics, University of New Hampshire, Durham, NH 03824 (United States)
Publication Date:
OSTI Identifier:
22270584
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 777; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; APPROXIMATIONS; ASTRONOMY; ASTROPHYSICS; COMPUTERIZED SIMULATION; DISTRIBUTION FUNCTIONS; FLUCTUATIONS; INCLINATION; MAGNETIC FIELDS; MAGNETOHYDRODYNAMICS; MEAN FREE PATH; PLASMA WAVES; PROTONS; SOLAR WIND; TURBULENCE