Jacob W. Lynn

University of California, Berkeley, Berkeley, California, United States

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Publications (4)13.47 Total impact

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    ABSTRACT: We use analytic estimates and numerical simulations of test particles interacting with magnetohydrodynamic (MHD) turbulence to show that subsonic MHD turbulence produces efficient second-order Fermi acceleration of relativistic particles. This acceleration is not well-described by standard quasi-linear theory but is a consequence of resonance broadening of wave-particle interactions in MHD turbulence. We provide momentum diffusion coefficients that can be used for astrophysical and heliospheric applications and discuss the implications of our results for accretion flows onto black holes. In particular, we show that particle acceleration by subsonic turbulence in radiatively inefficient accretion flows can produce a non-thermal tail in the electron distribution function that is likely important for modeling and interpreting the emission from low luminosity systems such as Sgr A* and M87.
    03/2014;
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    ABSTRACT: 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 lambda_{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-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 cannot 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^{-5} tail in the proton distribution function observed in the solar wind.
    The Astrophysical Journal 06/2013; · 6.73 Impact Factor
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    ABSTRACT: The heating, acceleration, and pitch-angle scattering of charged particles by 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 beta ~ 1, where beta 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 spiralling around moving wires. We show that test particle heating rates are consistent with a TTD resonance which is broadened according to a decorrelation prescription that is Gaussian in time. 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 Alfven speed, while FTB dominates for v_s << v_A (e.g. minor ions). Proton heating rates for beta ~ 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.
    The Astrophysical Journal 03/2012; 758(2). · 6.73 Impact Factor
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    ABSTRACT: The solar wind exhibits sustained heating after escaping the solar corona, and it is anticipated that the damping of turbulent plasma modes contributes to this heating. To investigate this, we simulate the interaction of charged test particles with fully dynamical simulations of magnetohydrodynamic Alfvenic turbulence, in which particles may be accelerated by the cyclotron resonance with Alfven modes (providing perpendicular acceleration) or transit-time damping with compressive modes via the μ ∇ B force (parallel acceleration). We implement a novel time-correlated driving scheme for the turbulence, which allows us to calculate heating rates over long times for initially thermal distributions of high-gyrofrequency particles, and investigate whether energy gain is primarily thermal or non-thermal. Additionally, we calculate particle diffusion coefficients. Some agreement is found with prior theories of turbulent acceleration, but discrepancies remain. Specifically, we find that: 1) the heating rate of thermal distributions with vth >> vAlfven, where vth is the typical speed of the Maxwellian, is proportional to vth-1, implying that protons in the solar wind are heated much more effectively than electrons by turbulence above the proton gyroscale, and that minor ions are heated more effectively still; 2) thermal distributions heated by transit-time damping (TTD) develop only weak non-thermal tails, which are still exponential rather than power-law; 3) D∥ , the parallel momentum diffusion coefficient, is proportional to v⊥4, consistent with parallel acceleration of particles via TTD, for high-v⊥ particles (surprisingly, for lower v⊥, D∥ approaches a constant); 4) the resonances predicted by quasi-linear theory are observed to be highly broadened.
    AGU Fall Meeting Abstracts. 12/2011;

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  • 2012
    • University of California, Berkeley
      • Department of Physics
      Berkeley, California, United States