SEP events and multi-spacecraft observations: Constraints on theory

Fifteen solar energetic particle (SEP) events have been analyzed using proton flux data recorded by the Helios 1, Helios 2, and IMP 8 spacecraft in the energy range ∼4–40 MeV during 1974–1982. For each of the events at least two of the spacecraft have their nominal magnetic footpoint within 20° in heliocentric longitude from each other. The SEP events are sub-grouped as a function of their heliocentric longitudinal separation and heliocentric radial distance from the SEP associated solar flare and several case studies are presented in this paper. Main results concerning their usage in estimating the SEP radial dependence are given. Moreover, we investigate the behavior of the third not connected spacecraft in order to study the dependence of the proton flux as a function of flare location. It is found that the contribution of the longitudinal gradient in determining variations in the SEP proton flux is particularly relevant for spacecraft having their magnetic connection footpoint separated from the flare between 30° and 50°.