The large longitudinal spread of solar energetic particles during the January 17, 2010 solar event

TL;DR

This study analyzes multi-spacecraft data of the January 17, 2010 solar energetic particle event, revealing an extensive longitudinal spread likely caused by strong perpendicular diffusion in the interplanetary medium.

Contribution

It compares 1D and 3D particle transport models to explain the wide spread, highlighting the importance of perpendicular diffusion in interplanetary space.

Findings

Nearly 360-degree longitudinal spread observed at 1AU.

3D model indicates high perpendicular to parallel diffusion ratio.

Strong perpendicular transport explains wide particle distribution without long solar injection.

Abstract

We investigate multi-spacecraft observations of the January 17, 2010 solar energetic particle event. Energetic electrons and protons have been observed over a remarkable large longitudinal range at the two STEREO spacecraft and SOHO suggesting a longitudinal spread of nearly 360 degrees at 1AU. The flaring active region, which was on the backside of the Sun as seen from Earth, was separated by more than 100 degrees in longitude from the magnetic footpoints of each of the three spacecraft. The event is characterized by strongly delayed energetic particle onsets with respect to the flare and only small or no anisotropies in the intensity measurements at all three locations. The presence of a coronal shock is evidenced by the observation of a type II radio burst from the Earth and STEREO B. In order to describe the observations in terms of particle transport in the interplanetary medium, including perpendicular diffusion, a 1D model describing the propagation along a magnetic field line (model 1) (Dr\"oge, 2003) and the 3D propagation model (model 2) by (Dr\"oge et al., 2010) including perpendicular diffusion in the interplanetary medium have been applied, respectively. While both models are capable of reproducing the observations, model 1 requires injection functions at the Sun of several hours. Model 2, which includes lateral transport in the solar wind, reveals high values for the ratio of perpendicular to parallel diffusion. Because we do not find evidence for unusual long injection functions at the Sun we favor a scenario with strong perpendicular transport in the interplanetary medium as explanation for the observations.