The Effect of Coherent Structures on Stochastic Acceleration in MHD Turbulence

• Published in: The Astrophysical journal 2006-01, Vol.637 (1), p.322-332
• Main Authors: Arzner, Kaspar, Knaepen, Bernard, Carati, Daniele, Denewet, Nicolas, Vlahos, Loukas
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/498341

We investigate the influence of coherent structures on particle acceleration in the strongly turbulent solar corona. By randomizing the Fourier phases of a pseudospectral simulation of isotropic magnetohydrodynamic (MHD) turbulence (Re 6 300) and tracing collisionless test protons in both the exact-MHD and phase-randomized fields, it is found that the phase correlations enhance the acceleration efficiency during the first adiabatic stage of the acceleration process. The underlying physical mechanism is identified as the dynamical MHD alignment of the magnetic field with the electric current, which favors parallel (resistive) electric fields responsible for initial injection. Conversely, the alignment of the magnetic field with the bulk velocity weakens the acceleration by convective electric fields - super(u) X super(b) at a nonadiabatic stage of the acceleration process. We point out that nonphysical parallel electric fields in random-phase turbulence proxies lead to artificial acceleration and that the dynamical MHD alignment can be taken into account on the level of the joint two-point function of the magnetic and electric fields and is therefore amenable to Fokker-Planck descriptions of stochastic acceleration.