Open AccessObservation of decay phases of solar energetic particle events at 1 and 5 AU from the Sun
Author(s) -
Dalla S.,
Balogh A.,
Heber B.,
Lopate C.,
McKibben R. B.
Publication year - 2002
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2001ja009155
Subject(s) - physics , heliosphere , interplanetary spaceflight , solar energetic particles , astrophysics , solar wind , proton , particle (ecology) , particle acceleration , interplanetary medium , coronal mass ejection , astronomy , nuclear physics , plasma , geology , oceanography
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.