The decay phase of solar energetic particle events

Abstract

The shape of the particle flux profile in the decay phase of solar energetic particle (SEP) events is of particular importance in understanding the propagation of energetic particles in the interplanetary medium. The majority of few-MeV proton events (>90%) exhibit exponential law declines, suggesting the dominant role of convection transport and adiabatic deceleration. In this case, the characteristic decay time τ should depend on the spectral index, the solar wind speed, and the distance from the Sun. More than 600 decays observed in the fluxes of 0.5-48 MeV protons by IMP 8 in 1973-2001 were selected and analyzed in detail. We consider the dependence of τ on environmental plasma parameters. The observed decay times, τ _obs, are compared with those considering convection transport and adiabatic deceleration, τ _theor, and fair agreement is found (within 25%) in nearly 50% of all declines with constant values of V. Although high-energy proton profiles at various radial distances (Helios, IMP 8, Ulysses) are surprisingly identical, MeV protons in the same events decay more slowly, with increasing r, in agreement with the model. The solar cycle variation and the distribution of the τ values as a function of energy are also examined. The dependence on the heliographic longitude suggests that events with significantly different τ _obs and τ _theor can be explained by the variation of magnetic connection between the observer and a flare site. The observational results are compared with numerical simulations in the frame of a simple particle propagation model involving scattering, adiabatic cooling, and a propagating shock.