SPRG Seminars
February 22, 2011:
" Time-dependent perpendicular transport of charged particles in a turbulent magnetic field beyond quasi-linear theory "
Federico Fraschetti, University of Arizona
We present an analytic derivation of the temporal dependence of the perpendicular transport coefficient of charged particles in magnetostatic turbulence, for times smaller than the time needed for charged particles to travel the turbulence correlation length. This time window is left unexplored in most transport models, e.g., the so-called quasi-linear theory for particle transport in magnetic turbulence. In our analysis all magnetic scales are taken to be much larger than the particle gyroradius, so that perpendicular transport is assumed to be dominated by the guiding center motion. In contrast with previous transport models based on guiding center motion, 1) parallel or perpendicular diffusion of the charged particle from the average magnetic field is not assumed, 2) the particle motion is not assumed to be tied to the magnetic field lines, but the evolution of the average square distance from the field lines depends upon the turbulence geometry.
Particle drift from the local magnetic field lines and magnetic field lines random walk are evaluated separately for slab and 3D isotropic turbulence. Contributions of wavelength scales shorter and longer than the turbulence coherence length are compared. In contrast to slab case, particles in 3D isotropic turbulence unexpectedly diffuse from local magnetic field lines; this result questions the common assumption that particle magnetization is independent on turbulence geometry. Extensions of this model will allow for a study of solar wind or astrophysical blast waves anisotropies.