A detailed survey of the parallel mean free path of solar energetic particle protons and electrons

TL;DR

This paper compares observed and theoretical parallel mean free paths of solar energetic particles, revealing good agreement for protons and moderate agreement for electrons, and highlights the influence of turbulence variability.

Contribution

It provides a comprehensive survey of SEP pMFPs, combining observational data with theoretical models to understand particle transport in the solar environment.

Findings

Good match between observed and theoretical pMFPs for protons.

Inter-event variations explained by turbulence fluctuations.

Moderate agreement for electron pMFPs sensitive to turbulence dissipation.

Abstract

In this work, more than a dozen solar energetic particle (SEP) events are identified where the source region is magnetically well-connected to at least one spacecraft at 1~au. The observed intensity-time profiles, for all available proton and electron energy channels, are compared to results computed using a numerical 1D SEP transport model in order to derive the parallel mean free paths (pMFPs) as a function of energy (or rigidity) at 1~au. These inversion results are then compared to theoretical estimates of the pMFP, using observed turbulence quantities with observationally-motivated variations as input. For protons, a very good comparison between inversion and theoretical results is obtained. It is shown that the observed inter-event variations in the inversion pMFP values can be explained by natural variations in the background turbulence values. For electrons, there is relatively good agreement with pMFPs derived assuming the damping model of dynamical turbulence, although the theoretical values are extremely sensitive to the details of the turbulence dissipation range which themselves display a high level of variation.