Article

# The Model Dependence of Solar Energetic Particle Mean Free Paths under Weak Scattering

(Impact Factor: 5.99). 03/2005; 627(1):562-566. DOI: 10.1086/430136

ABSTRACT

The mean free path is widely used to measure the level of solar energetic particles' diffusive transport. We model a solar energetic particle event observed by Wind STEP at 0.31–0.62 MeV nucleon À1 , by solving the focused transport equation using the Markov stochastic process theory. With different functions of the pitch angle diffusion coefficient D , we obtain different parallel mean free paths for the same event. We show that the different values of the mean free path are due to the high anisotropy of the solar energetic particles. This makes it problematic to use just the mean free path to describe the strength of the solar energetic particle scattering, because the mean free path is only defined for a nearly isotropic distribution. Instead, a more complete function of pitch angle diffusion coefficient is needed.

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Journal of Geophysical Research Atmospheres 08/2006; 111:A08101. DOI:10.1029/2005JA011512 · 3.43 Impact Factor
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ABSTRACT: A new code for solving radiation belt diffusion equations has been developed and applied to the 2-D bounce-averaged energy pitch angle quasi-linear diffusion equation. The code uses Monte Carlo methods to solve Ito stochastic differential equations (SDEs) which are mathematically equivalent to radiation belt diffusion equations. We show that our SDE code solves the diffusion equation with off-diagonal diffusion coefficients in contrast to standard finite difference codes which are generally unstable when off-diagonal diffusion coefficients are included. Our results are in excellent agreement with previous results. We have also investigated effects of assuming purely parallel propagating electromagnetic waves when calculating the diffusion coefficients and find that this assumption leads to errors of more than an order of magnitude in flux at some equatorial pitch angles for the specific chorus wave model we use. Further work is needed to investigate the sensitivity of our results to the wave model parameters. Generalization of the method to 3-D is straight-forward, thus making this model a very promising new way to investigate the relative roles of pitch angle, energy, and radial diffusion in radiation belt dynamics.
07/2008; 113(A7):10. DOI:10.1029/2007JA012985