Cosmic-ray propagation in simulations of cross-helical plasma turbulence
b'Turbulence is a ubiquitous phenomenon in astrophysical plasmas. Most of these systems exhibit a property called cross helicity, a non-zero correlation between velocity fluctuations and magnetic-field fluctuations. In the presence of a magnetic mean-field, such as in the solar wind or in the interstellar medium, cross helicity is equivalent to an imbalance between Alfven waves co- and counter-propagating with respect to the mean-field direction. Although this imbalance can have a dramatic influence on the heating and scattering rate of charged particles which propagate through the plasma, it is often neglected in computational studies of turbulent particle transport.\\n\\nIn an effort to remedy this situation, we present numerical simulations of magnetohydrodynamic turbulence in which we can control the energy and the cross helicity of the system, without injecting kinetic or magnetic helicity as an unwanted side effect. Varying the strength of a magnetic guide-field allows us to determine the degree of anisotropy that the system assumes as a steady-state configuration. Detailed analysis proves that these simulations conform to theoretical models of realistic turbulence.\\n\\nThe diffusion of cosmic-ray particles in turbulent plasmas is often calculated using quasilinear theory and a simplified description of the electromagnetic-field spectra. By computing the trajectories of test-particles in dynamically evolving turbulence simulations with non-zero cross helicity, we study whether such quasilinear predictions of the heating rate of charged particles are valid under realistic conditions. Theory and numerical results agree well for particles propagating at the Alfven velocity, unless resistive effects play a dominant role.\\n\\nFurthermore, strongly anisotropic field configurations are used to compare quasilinear pitch-angle diffusion coefficients with measurements of test-particle scattering after one gyroperiod. In particular, we focus on the scaling of the scattering rate with cross helicity. We observe excellent agreement in simulations of both balanced and imbalanced turbulence and explain the role of the magnetic moment, an approximate invariant of charged-particle motion, for pitch-angle scattering on timescales of several gyroperiods.'