The role of interplanetary shocks in the longitude distribution of solar energetic particles

A study of solar proton events with well‐identified sources has been carried out using data from Goddard particle experiments on IMPs 4, 5, 7, and 8 and ISEE 3. The experiments cover the energy range from about 1 to 300 MeV. The 235 events of our study represent approximately 70% of all increases above 10 −3 particles cm −2 sr −1 s −1 MeV minus;1 at energies > 20 MeV detected in a 19.7‐year period commencing mid‐May 1967. It is shown that intensity‐time profiles of solar proton events display an organization with respect to heliolongitude. Whilst it has been known for many years that the profile of a proton event depends on the longitude of the solar event relative to the observer, we suggest that the major controlling agent is the existence of an interplanetary (IP) shock. Furthermore, we explain the change in shape as a function of heliolongitude within the framework of a recently derived model for the large‐scale structure of IP shocks. In particular, the long delay to maximum intensity for far eastern events (a property previously ascribed to coronal processes) and the overall extended duration can be accounted for by IP shock acceleration and continued magnetic connection to the shock even after it has propagated beyond 1 AU.