Perpendicular Diffusion of Solar Energetic Particles: Model Results and Implications for Electrons

TL;DR

This study uses a 2D transport model with perpendicular diffusion to simulate solar energetic electron propagation, comparing results with observations to understand the role of perpendicular diffusion in SEP longitudinal transport.

Contribution

It introduces a model with transport coefficients derived from first principles, emphasizing the sensitivity to perpendicular diffusion and its impact on SEP transport.

Findings

Perpendicular diffusion significantly influences SEP longitudinal distribution.

Model results are highly sensitive to the magnitude of the perpendicular diffusion coefficient.

Perpendicular diffusion is most effective early in SEP events when intensity gradients are large.

Abstract

The processes responsible for the effective longitudinal transport of solar energetic particles (SEPs) are still not completely understood. We address this issue by simulating SEP electron propagation using a spatially 2D transport model that includes perpendicular diffusion. By implementing, as far as possible, the most reasonable estimates of the transport (diffusion) coefficients, we compare our results, in a qualitative manner, to recent observations {at energies of 55 -- 105 keV}, focusing on the longitudinal distribution of the peak intensity, the maximum anisotropy and the onset time. By using transport coefficients which are derived from first principles, we limit the number of free parameters in the model to: (i) the probability of SEPs following diffusing magnetic field lines, quantified by $a \in [0,1]$, and (ii) the broadness of the Gaussian injection function. It is found that the model solutions are extremely sensitive to the magnitude of the {perpendicular} diffusion coefficient and relatively insensitive to the form of the injection function as long as a reasonable value of $a=0.2$ is used. We illustrate the effects of perpendicular diffusion on the model solutions and discuss the viability of this process as a dominant mechanism by which SEPs are transported in longitude. Lastly, we try to quantity the effectiveness of perpendicular diffusion as an interplay between the magnitude of the relevant diffusion coefficient and the SEP intensity gradient driving the diffusion process. It follows that perpendicular diffusion is extremely effective early in a SEP event when large intensity gradients are present, while the effectiveness quickly decreases with time thereafter.