Calculation of Drift and Diffusion Coefficients for Cosmic Rays inside the Heliospheric Termination Shock

Details Calculation of Drift and Diffusion Coefficients for Cosmic Rays inside the Heliospheric Termination Shock Calculation of Drift and Diffusion Coefficients for Cosmic Rays inside the Heliospheric Termination Shock

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsh51b1682b&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #SH51B-1682

Physics

[2104] Interplanetary Physics / Cosmic Rays, [2149] Interplanetary Physics / Mhd Waves And Turbulence, [2164] Interplanetary Physics / Solar Wind Plasma

Scientific paper

Any realistic two- or three-dimensional numerical model for the modulation of cosmic rays requires a diffusion tensor as input. Such a tensor consists of coefficients for diffusion parallel and perpendicular to the mean magnetic field, and for drifts. In turn, all of these coefficients depend upon the assumed turbulence spectra, and how they evolve as function of position throughout the heliosphere. For the case of diffusion a number of theoretical expressions exist that has been tested against direct numerical simulations, and from good to reasonable agreement has been found. What remains is to use turbulence spectra that are in agreement with observations in the high-wavenumber regime and are at least in agreement with theoretical constraints in the small-wavenumber regime, coupled with models for the spatial evolution of turbulence quantities such as the magnetic variance and relevant correlation scale. However, in the case of the drift coefficient, we are not aware of any theory that has been shown to give even reasonable agreement with the few published direct numerical simulations. For the drift coefficient then, one option is to parameterize direct numerical simulations in terms of the relevant turbulence quantities. What again remains before implementation in a numerical modulation model is the choice of turbulence spectra and a model for their spatial evolution, i.e. the same requirements as for the diffusion coefficients. We present a diffusion tensor that takes into account the above and show resulting sample solutions from a three-dimensional numerical modulation model. We emphasize that some properties of turbulence quantities can be obtained indirectly by fitting cosmic-ray observations and illustrate this for the case of the turbulence ultrascale for which no observations currently exist. We acknowledge funding by the South African National Research Foundation.

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