Test Particle Acceleration by Turbulent MHD Fields on Alfvenic Timescales

We study the short time acceleration of charged particles in turbulent fields obtained from direct numerical solutions of the compressible 3D MHD equations. Different test particle simulations are performed, varying the ratio between the typical turbulent timescale and the nominal gyroperiod of the particles. Starting from monoenergetic particles moving at the characteristic plasma Alfven speed, we find consistent acceleration to high energies in times of the order of the turbulent timescale. Extended power laws in the distribution of particle energies are obtained, with different slopes at intermediate and high energies. Linear scaling laws of the maximum and mean energy of the particles with the turbulent time / nominal gyroperiod ratio are observed and mechanisms based on reconnecting fields are proposed to explain them.