The model dependence of solar energetic particle mean free paths under weak scattering

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(-1), by solving the focused transport equation using the Markov stochastic process theory. With different functions of the pitch angle diffusion coefficient D-mu mu, 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.