Characteristics of Solar Energetic Ions as a Function of Longitude

Since the 2006 launch of STEREO, multi-spacecraft studies have yielded several surprising results regarding the spread of solar energetic particles (SEPs) within the inner heliosphere. We have investigated the role of energy and ridigity, using ACE and STEREO 10 MeV n−1 oxygen data to identify 41 large SEP events observed by two or three spacecraft. We calculated fluence spectra from ∼0.1 to >10 MeV n−1 for H, He, O, and Fe for each event at the observing spacecraft (including SOHO and GOES). The particle fluences at 0.3, 1, and 10 MeV n−1 were examined as a function of the distance between the associated solar flare longitude and the spacecraft magnetic footpoints at the Sun to determine the longitudinal spread of particles and study how the distribution centers and widths depend on energy and charge-to-mass (Q/M) for the first time. On average, the three-spacecraft event distributions were centered at 22 4° west of the flare site and were 43 1° wide, though there was substantial variability, while the fit to the aggregate of the two-spacecraft event fluences yielded significantly wider distributions at 0.3 and 1 MeV n−1. The widths derived from both the three- and two-spacecraft events show an energy dependence with distributions narrowing with increasing energy, consistent with lower energy ions experiencing more field line co-rotation, or being accelerated over a larger portion of the CME-driven shock or for longer times as the shock expands. Surprisingly, no clear evidence was found for a Q/M dependence to the widths or centers suggesting that rigidity-related processes are not the dominant means of spreading particles in longitude.