Cosmic-ray propagation in simulations of cross-helical plasma turbulence

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. In 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. The 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. Furthermore, 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.