Perpendicular diffusion: Variations on the NLGC theme

A proper understanding of the processes underlying perpendicular diffusion is of vital importance to any study of cosmic ray modulation. Several theories have been proposed to explain this phenomenon, including the non-linear guiding center (NLGC) theory of Matthaeus et al. (2003), which has been found to yield perpendicular diffusion coefficients in good agreement with numerical simulations. Agreement with simulations, however, was not perfect, and several modifications to this theory have recently been proposed. In this study, expressions for perpendicular mean free paths are derived from several of these modifications, namely the extended NLGC theory of Shalchi (2006), the unified non-linear transport theory of Shalchi (2010), and the random ballistic interpretation of NLGC theory proposed by Ruffolo et al. (2012). These mean free paths are compared with one another, using as inputs for the basic turbulence quantities the results yielded by a two-component turbulence transport model (Oughton et al., 2011). Their effects on cosmic ray intensities, computed using a 3D steady-state cosmic-ray modulation code, will also be discussed.