Magnetic field line curvature induced pitch angle diffusion in the radiation belts

Details Magnetic field line curvature induced pitch angle diffusion in the radiation belts Magnetic field line curvature induced pitch angle diffusion in the radiation belts

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsm42c0614y&link_type=abstract

American Geophysical Union, Fall Meeting 2003, abstract #SM42C-0614

Physics

2720 Energetic Particles, Trapped, 2730 Magnetosphere: Inner, 2753 Numerical Modeling

Scientific paper

We derive a diffusion equation to model the pitch angle dynamics of a trapped charged particle population strongly influenced by magnetic field line curvature. The equation is based on a recently published, empirical model for pitch angle scattering of single particles. In order to examine the predictions of this equation in the inner magnetosphere we define a diffusion time. Often the purely exponential decay time of the lowest eigenfunction is used to characterize the decay of a population. However, because our equation is singular for 90o equatorial pitch angles there is no physically meaningful lowest order eigenfunction. We define the diffusion time as the time it takes for a distribution to decay to half its initial density. Because this depends on the initial distribution we examine the decay of three distributions that vary in the degree to which they are peaked about the 90o equatorial pitch angle. We calculate diffusion times for these distributions as a function of Dst, season, and UT (season and UT being proxies for the angle between the dipole axis and the solar wind) using Tsyganenko 01 magnetic field model. We show that the diffusion is strongly dependent on both the initial distribution and Dst, and examine the weaker dependence on the direction of the dipole axis relative to the solar wind. Using the results of these calculations we estimate thresholds in parameter space for which the particle populations will decay in 10 hours for electrons, protons, and O+8 ions. To demonstrate the sharpness of these thresholds we also estimate the threshold for decay in 1000 hours for protons.

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