Observation of decay phases of solar energetic particle events at 1 and 5 AU from the Sun

Abstract

The slow decay phase of gradual solar energetic particle (SEP) events has been interpreted as an indication of particle reservoirs being established in the inner heliosphere. The same phenomenon is sometimes termed spectral invariance and explained in terms of a magnetic bottle effect, whereby a barrier impedes particle escape. In alternative to the above picture, decay-phase SEPs have been ascribed to (1) continuous acceleration at an interplanetary shock front, (2) interplanetary scattering, or (3) leakage from the solar atmosphere over several days. In this paper we investigate two large gradual SEP events characterized by comparable signatures at 1 AU from the Sun. We use measurements at 1 AU made by the ACE and IMP8 spacecraft, and at 5.2 AU by the Ulysses spacecraft. At 5.2 AU, the ∼MeV proton intensities during the decay phase of the two events are found to have strikingly different profiles, showing in one case a long-duration smooth decay and in the other a depletion in particle intensity. We discuss how the four above mentioned models on the origin of decay-phase SEPs would interpret the observations.