# Spreading protons in the heliosphere: a note on cross-field diffusion effects

## TL;DR

This study investigates how cross-field diffusion influences the transport and dispersion of solar energetic protons in the heliosphere, especially near high-speed solar wind streams and CIRs, using a particle transport model.

## Contribution

It introduces a detailed particle transport model incorporating cross-field diffusion effects in a realistic solar wind environment, highlighting its impact on SEP spread and intensity.

## Key findings

- Cross-field diffusion disperses SEPs efficiently, reducing their in-situ detectability.
- Rarefaction regions in solar wind further decrease SEP intensities.
- Strong diffusion causes merging of particle populations at shock waves.

## Abstract

We study how a high-speed solar wind stream embedded in a slow solar wind affects the transport and energy changes of solar energetic protons in interplanetary space, assuming different levels of cross-field diffusion. This is done using a particle transport model that computes directional particle intensities and first order parallel anisotropies in a background solar wind generated by the magnetohydrodynamic model EUHFORIA. In particular, we consider a mono-energetic 4 MeV proton injection over an extended region located at a heliographic radial distance of 0.1 AU. By using different values for the perpendicular proton mean free path, we study how cross-field diffusion may affect the energetic particle spread and intensity profiles near a high-speed solar wind stream and a corotating interaction region (CIR). We find that both a strong cross-field diffusion and a solar wind rarefaction region are capable of dispersing SEPs efficiently, producing overall low particle intensities which can in some cases prevent the SEPs from being detected in-situ, since their intensity may drop below the detected pre-event intensity levels. We also discuss how accelerated particle populations form on the reverse and forward shock waves, separated by the stream interface inside the CIR. Under strong levels of cross-field diffusion, particles cross the SI and hence both accelerated particle populations merge together.