The impact of corotation on gradual solar energetic particle event intensity profiles

Abstract

Context. 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 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 and the variation of peak intensity with observer position.
Methods. We use a 3D full-orbit test particle code with time-extended SEP injection via a shock-like source. Unlike with focussed transport models, the test particle approach enables us to switch corotation on and off easily. While shock acceleration is not modelled directly, our methodology allows us to study how corotation and the time-varying observer-shock magnetic connection influence intensity profiles detected at several observers.
Results. We find that corotation strongly affects SEP intensity profiles, for a monoenergetic population of 5 MeV protons, being a dominant influence during the decay phase. Simulations including corotation display dramatically shortened durations for western events, compared to those which do not include it. 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 no-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 observer longitudinal position.