Recent studies suggest that when magnetohydrodynamic (MHD) turbulence is excited by stirring a plasma at large scales, the cascade of energy from large to small scales is anisotropic, in the sense ...that small-scale fluctuations satisfy the inequality k_parallel << k_perp, where k_parallel and k_perp are, respectively, the components of a fluctuation's wave vector parallel and perpendicular to the background magnetic field. Such anisotropic fluctuations are very inefficient at scattering cosmic rays. Results based on the quasilinear approximation for scattering of cosmic rays by anisotropic MHD turbulence are presented and explained. The important role played by molecular-cloud magnetic mirrors in confining and isotropizing cosmic rays when scattering is weak is also discussed.
Astrophys.J. 602 (2004) 170-180 We investigate field-line separation in strong MHD turbulence analytically
and with direct numerical simulations. We find that in the
static-magnetic-field ...approximation the thermal conductivity in galaxy clusters
is reduced by a factor of about 5-10 relative to the Spitzer thermal
conductivity of a non-magnetized plasma. We also estimate how the thermal
conductivity would be affected by efficient turbulent resistivity.
We study relativistic particles undergoing surfing acceleration at perpendicular shocks. We assume that particles undergo diffusion in the component of momentum perpendicular to the shock plane due ...to moderate fluctuations in the shock electric and magnetic fields. We show that dN/dE, the number of surfing-accelerated particles per unit energy, attains a power-law form, dN/dE \propto E^{-b}. We calculate b analytically in the limit of weak momentum diffusion, and use Monte Carlo test-particle calculations to evaluate b in the weak, moderate, and strong momentum-diffusion limits.
Phys.Rev.Lett. 92 (2004) 045001 We investigate field-line separation in strong MHD turbulence using direct
numerical simulations. We find that in the static-magnetic-field approximation
the thermal ...conductivity in galaxy clusters is reduced by a factor of about 50
relative to the Spitzer thermal conductivity of a non-magnetized plasma. This
value is too small for heat conduction to balance radiative cooling in
clusters.
Fast particles diffusing along magnetic field lines in a turbulent plasma can diffuse through and then return to the same eddy many times before the eddy is randomized in the turbulent flow. This ...leads to an enhancement of particle acceleration by large-scale compressible turbulence relative to previous estimates in which isotropic particle diffusion is assumed.
We investigate field-line separation in strong MHD turbulence analytically and with direct numerical simulations. We find that in the static-magnetic-field approximation the thermal conductivity in ...galaxy clusters is reduced by a factor of about 5-10 relative to the Spitzer thermal conductivity of a non-magnetized plasma. We also estimate how the thermal conductivity would be affected by efficient turbulent resistivity.
Resonant interactions between ions and Alfvén/ion-cyclotron (A/IC) waves may play an important role in the heating and acceleration of the fast solar wind. Although such interactions have been ...studied extensively for "parallel" waves, whose wave vectors \({\bf k}\) are aligned with the background magnetic field \({\bf B}_0\), much less is known about interactions between ions and oblique A/IC waves, for which the angle \(\theta\) between \({\bf k}\) and \({\bf B}_0\) is nonzero. In this paper, we present new numerical results on resonant cyclotron interactions between protons and oblique A/IC waves in collisionless low-beta plasmas such as the solar corona. We find that if some mechanism generates oblique high-frequency A/IC waves, then these waves initially modify the proton distribution function in such a way that it becomes unstable to parallel waves. Parallel waves are then amplified to the point that they dominate the wave energy at the large parallel wave numbers at which the waves resonate with the particles. Pitch-angle scattering by these waves then causes the plasma to evolve towards a state in which the proton distribution is constant along a particular set of nested "scattering surfaces" in velocity space, whose shapes have been calculated previously. As the distribution function approaches this state, the imaginary part of the frequency of parallel A/IC waves drops continuously towards zero, but oblique waves continue to undergo cyclotron damping while simultaneously causing protons to diffuse across these kinetic shells to higher energies. We conclude that oblique A/IC waves can be more effective at heating protons than parallel A/IC waves, because for oblique waves the plasma does not relax towards a state in which proton damping of oblique A/IC waves ceases.
We investigate field-line separation in strong MHD turbulence using direct numerical simulations. We find that in the static-magnetic-field approximation the thermal conductivity in galaxy clusters ...is reduced by a factor of about 50 relative to the Spitzer thermal conductivity of a non-magnetized plasma. This value is too small for heat conduction to balance radiative cooling in clusters.
Understanding the physical processes in the solar wind and corona which actively contribute to heating, acceleration, and dissipation is a primary objective of NASA's Parker Solar Probe (PSP) ...mission. Observations of coherent electromagnetic waves at ion scales suggests that linear cyclotron resonance and non-linear processes are dynamically relevant in the inner heliosphere. A wavelet-based statistical study of coherent waves in the first perihelion encounter of PSP demonstrates the presence of transverse electromagnetic waves at ion resonant scales which are observed in 30-50\% of radial field intervals. Average wave amplitudes of approximately 4 nT are measured, while the mean duration of wave events is of order 20 seconds; however long duration wave events can exist without interruption on hour-long timescales. Though ion scale waves are preferentially observed during intervals with a radial mean magnetic field, we show that measurement constraints, associated with single spacecraft sampling of quasi-parallel waves superposed with anisotropic turbulence, render the measured quasi-parallel ion-wave spectrum unobservable when the mean magnetic field is oblique to the solar wind flow; these results imply that the occurrence of coherent ion-scale waves is not limited to a radial field configuration. The lack of strong radial scaling of characteristic wave amplitudes and duration suggests that the waves are generated {\em{in-situ}} through plasma instabilities. Additionally, observations of proton distribution functions indicate that temperature anisotropy may drive the observed ion-scale waves.
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