Magnetohydrodynamic Turbulence in Heliospheric Physics: From Coronal and Solar Wind Heating to Kinetic Effects

We have recently studied the Parker field line tangling problem for coronal heating comprehensively via longtime high-resolution simulations of the dynamics of a coronal loop in Cartesian geometry within the framework of reduced magnetohydrodynamics (RMHD). Although the turbulent cascade prevents the magnetic field lines from becoming strongly entangled, current sheets are continuously formed and dissipated. Current sheets are the result of the nonlinear cascade that transfer energy from the scale of convective motions down to the dissipative scales, where it is finally converted to heat and/or particle acceleration. We have derived scaling laws for this process which depend only on the ratio between the photospheric eddy turnover time and the loop Alfvén wave crossing time.
Along open field lines in the solar corona, nonlinear couplings are driven by reflection of Alfvén waves off the solar wind density gradients. I will also discuss simulations of this process using a simplified shell-model, starting from the coronal base up to 17 solar radii, well beyond the Alfvénnic critical point. Turbulent dissipation is found to account for at least half of the heating required to sustain the background imposed solar wind and its shape is found to be determined by the reflection-determined turbulent heating below 1.5 solar radii. Therefore reflection and reflection-driven turbulence are shown to play a key role in the acceleration of the fast solar wind and origin of the turbulent spectrum found at 0.3 AU in the heliosphere.
At smaller scales, Alfvénnic turbulence folds into wave-particle interactions. If time permits I will discuss interactions occurring through ion-cyclotron resonance and nonlinear trapping due to the growth of parametric instabilities. Cyclotron interactions control the evolution of the temperature anisotropy providing a perpendicular heating which contrasts the adiabatic cooling caused by the expansion of the solar wind. Ion-acoustic modes driven by parametric effects produce a velocity beam in the particle distribution function, and the resulting proton distribution functions are in reasonable agreement with Helios data.