Relation of Decline Characteristics of 2-4.6 MeV Protons in SEP Events to Solar Wind Parameters
AIR FORCE RESEARCH LAB HANSCOM AFB MA SPACE VEHICLES DIRECTORATE
Pagination or Media Count:
The shape of the particle flux decline in solar energetic particle SEP events is of particular importance in understanding the propagation of energetic particles in the interplanetary medium. Power-law time profiles indicate the dominance of diffusive propagation, whereas exponential-law decline emphasizes convection transport and adiabatic deceleration. Values obtained theoretically for the decay time in the latter case are reasonably close to the fitted slopes in nearly half of all events when the solar wind speed stays nearly constant. Dependencies of characteristic decay time tau and spectral index gamma on environmental plasma parameters are considered. Parts of exponential-law declines when solar wind speed a decreases with time, b is constant, and c increases with time through the interval are analyzed separately. Both average values and dispersions of size distributions of tau for these three groups markedly differ in accordance with theoretical expectations.