The Effect of Coherent Structures on Stochastic Acceleration in MHD Turbulence

We investigate the influence of coherent structures on particle accelerationin the strongly turbulent solar corona. By randomizing the Fourier phases of apseudo-spectral simulation of isotropic MHD turbulence (Re $\sim 300$), andtracing collisionless test protons in both the exact-MHD and phase-randomizedfields, it is found that the phase correlations enhance the accelerationefficiency during the first adiabatic stage of the acceleration process. Theunderlying physical mechanism is identified as the dynamical MHD alignment ofthe magnetic field with the electric current, which favours parallel(resistive) electric fields responsible for initial injection. Conversely, thealignment of the magnetic field with the bulk velocity weakens the accelerationby convective electric fields $- \bfu \times \bfb$ at a non-adiabatic stage ofthe acceleration process. We point out that non-physical parallel electricfields in random-phase turbulence proxies lead to artificial acceleration, andthat the dynamical MHD alignment can be taken into account on the level of thejoint two-point function of the magnetic and electric fields, and is thereforeamenable to Fokker-Planck descriptions of stochastic acceleration.Comment: accepted for publication in Ap