On the parametrization of the energetic-particle pitch-angle diffusion coefficient

Context: Solar energetic particle (SEP) events are one of the key ingredients of the near-Earth radiation environment. Pitch-angle scattering by fluctuations imposed on the large-scale magnetic field is assumed to be the basic physical process behind diffusive propagation of SEPs in the heliosphere. Various pitch-angle diffusion models have been suggested to parametrize the wave-particle interactions, based on the original results of the classical quasi-linear theory of particle scattering and improved new approaches. Aims: We investigate under which circumstances the different functional forms of the pitch-angle diffusion coefficient can lead to equivalent results. In particular, we use two forms that are commonly used in two types of numerical methods to solve the particle transport equation, i.e., finite difference methods and Monte Carlo simulations. Methods: We estimate the corresponding values of the parameters of the two scattering models by performing a least-square fitting of the functional form of one of the scattering-frequency models to the other. We also perform Monte Carlo simulations of near-relativistic solar electrons to investigate the similarity of the models in terms of observables at 1 AU. Results: Our study shows that the two forms of pitch-angle scattering frequency lead to nearly equivalent results for electron transport from the Sun to 1 AU. We give the equivalent scattering parameters of the two models as curves that can be easily used when comparing the results of the two models. Conclusions: By providing the equivalent parametrizations of two commonly used scattering models, we provide key information on how to relate the results from the two parametrizations to each other and to the theory of particle transport