A Theory Exploring the Effect of Intermittent Slab Turbulence on Cosmic-ray Transport in Turbulence Dominated by a 2D Component
Dr. Jakobus A. le Roux
Department of Physics and CSPAR
17 September 2010
218 Shelby Center
3:00 (Refreshements at 2:30)
In previous work on perpendicular diffusion of cosmic rays, a promising theory known as the nonlinear guiding center (NLGC) theory was developed for cosmic rays diffusing along meandering magnetic field lines [Matthaeus et al., 2003]. This theory was the first that showed promise in reproducing diffusion derived from cosmic ray trajectory calculations in prescribed static, random linear wave, 3D MHD turbulence consisting of a dominant 2D turbulence component and a minor slab turbulence component. Such a turbulence model is considered to be a good first order approximation of nearly incompressible low frequency Alfvenic turbulence found in quiet solar wind conditions near Earth. However, in NLGC theory it is assumed turbulence is uniform in the sense that it sustains a single characteristic pitch-angle scattering time as cosmic rays interact gyroresonantly with uniform small-scale slab turbulence and diffusive along the mostly 2D meandering magnetic field. In reality, the solar wind plasma medium is nonuniform. Intermittent energetic particle beam formation is expected to result in stochastic wave growth so that intermittent patches of intense small-scale slab turbulence are generated. Cosmic ray scattering times are reduced and particle trapping occur in these patches as a consequence. To address this possibility, a generalized version of NLGC theory was developed that specifies a distribution of cosmic ray scattering times associated with intermittent slab turbulence. Whereas original NLGC theory based on uniform turbulence predicted perpendicular cosmic-ray diffusion to be normal, generalized NLGC theory assuming intermittent slab turbulence suggests that cosmic ray diffusion will be nondiffusive (sub- or super-diffusive) during intermediate times. The deviation from normal diffusion depends on sensitively on the distribution of scattering times (the nature of intermittent slab turbulence) and the slope of the energy-containing range of the power spectrum of 2D turbulence (characteristics of the meandering 2D magnetic field).
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