Impact of corotation on gradual solar energetic particle event intensity profiles

Abstract

Context. The corotation of particle-filled magnetic flux tubes is generally thought to have a minor influence on the time-intensity profiles of gradual solar energetic particle (SEP) events. For this reason, many SEP models solve the focussed transport equation within the corotating frame, thus neglecting corotation effects. Aims. We use simulations to study the effects of corotation on gradual SEP intensity profiles at a range of observer longitudinal positions relative to the solar source. We study how corotation affects the duration and decay time constant of SEP events as well as the variation in the peak intensity with the observer’s position. Methods. We used a 3D full-orbit test particle code with time-extended SEP injection via a shock-like source. In contrast to the case of focussed transport models, the test particle approach enables us to easily switch corotation on and off. While shock acceleration and downstream features are not modelled directly, our methodology allows us to study how corotation and the time-varying observer-shock magnetic connection influence the intensity profiles detected at several observers. Results. We find that corotation has a strong effect on the SEP intensity profiles for a monoenergetic population of 5 MeV protons, having a dominant influence during the decay phase. Simulations that include corotation display dramatically shortened durations for western events, compared to ones where it is not included. When corotation effects are taken into account, for both eastern and western events, the decay time constant is reduced and its dependence on the value of the scattering mean free path becomes negligible. Corotation reduces the SEP peak intensity for western events and enhances it for eastern ones, thus making the east-west asymmetry in peak intensity stronger, compared to the non-corotation case. Modelling SEP intensity profiles without carefully accounting for the effects of corotation leads to artificially extended decay phases during western events, leading to profiles with a similar shape regardless of the observer’s longitudinal position.