Simulations of Solar Energetic Particles Response to Interplanetary Coronal Mass Ejections in the Gradual Events

Abstract:

The gradual Solar Energetic Particle (SEP) events are usually associated with shock waves driven by Interplanetary Coronal Mass Ejections (ICMEs). Multi-spacecraft observations show that low-energy (<20 MeV) proton intensities may decrease when the spacecraft enters into the ICMEs. Based on a numerical solution of a focused transport equation, we obtained the fluxes of solar energetic particles (SEPs) accelerated by an interplanetary shock in a three-dimensional magnetic field. The shock is treated as a moving source of energetic particles with an assumed particle distribution function, while the Interplanetary Magnetic Field (IMF) disturbances generated by an ICME behind the shock are modeled by a Stokes' stream function with a spherical boundary. We computed the time profiles of particle flux and anisotropy as measured by multi-spacecraft. With perpendicular diffusion, energetic particles can cross magnetic field lines, and they can still be detected after the spacecraft enters into the ICMEs. By comparing our simulations with observations, the particle perpendicular diffusion coefficients can be qualitatively determined, and the radial and latitudinal variations of SEPs response to ICMEs have been studied.