Physics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsh11c1111z&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #SH11C-1111
Physics
7807 Charged Particle Motion And Acceleration, 7839 Nonlinear Phenomena, 7859 Transport Processes, 7863 Turbulence
Scientific paper
The problem of perpendicular diffusion by a particle in a turbulent plasma is a problem of enduring interest, and one that has yet to be fully solved. Analytic models do not agree with either observations or numerical simulations. Recently, a nonlinear theory was developed by Matthaeus et al. [2003] which, for the first time, appears to be consistent with numerical simulations in both the high- and low-energy particle regimes. Their approach is to assume that perpendicular transport is governed by the velocity of gyrocenters that follow magnetic field lines. Qin et al. [2002a,b] showed using numerical simulations that perpendicular diffusion could occur only in the presence of a transverse complex magnetic field. Flux surfaces with high transverse complexity are characterized by the rapid separation of nearby magnetic field lines and are therefore important to perpendicular diffusion. In particular, it appears that the combination of slab and 2D turbulence (a ``two-component'' model) is necessary to produce transverse complexity, and that slab turbulence alone, for example, is insufficient. The nonlinear theory is expressed through the solution of an integral equation. While the numerical solution of the integral equation in the appropriate parameter regime shows excellent agreement with numerical simulations for both high- and low-energy particles, the physical content is difficult to evaluate, nor is it evident how κ xx scales with parameters such as the energy density in magnetic fluctuations, mean field strength, particle gyroradius, MHD turbulence correlation length scales, parallel diffusion coefficient, etc. Furthermore, the integral equation formulation is not readily amenable to inclusion in models and numerical codes that require the perpendicular diffusion coefficient explicitly, such as heliospheric cosmic ray modulation models. We therefore introduce an explicit model for turbulence in the solar wind and solve the integral equation approximately for κ xx. We show that our approximate solution agrees very well with the numerical solution of the fully nonlinear integral equation. The approximate solution reveals the dependence of κ xx on the characteristics of the turbulent magnetofluid and particle energy (through the particle gyroradius). We conclude by using the perpendicular diffusion coefficient to evaluate 1) the particle acceleration timescale for diffusive shock acceleration at perpendicular shocks, and 2) the diffusion coefficient for cosmic ray modulation throughout the heliosphere.
Florinski Vladimir
le Roux Jakobus A.
Li Gaojun
Matthaeus William H.
Webb Gary M.
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