Source Region of the Interplanetary Magnetic Field and Variability in Heavy-Ion Composition in Gradual Solar Energetic Particle Events

We report a correlation between the nature of interplanetary magnetic field-line (IMF) source regions and heavy-ion composition in gradual solar energetic particle (SEP) events. This study is based on 25 large gradual SEP events observed in years 1998 and 2003-2006. We find that the IMF source regions, and the underlying magnetic field distribution, have a crucial effect on the SEP elemental composition at ~0.1-10 MeV/nucleon, which depends on whether the footpoint of the open magnetic field along which the SEPs are transported is from near an active region (an 'AR field-source') or from a coronal hole (an 'CH field-source') without an AR in its close proximity. Specifically, the observed SEP Fe/O ratio is preferentially enhanced in the case of the AR field-source, and lower than average Fe/O values are preferentially associated with the CH field source. Events with lower-than-average SEP C/O are also preferentially associated with the AR field-source. By contrast, the distributions of Fe/O and C/O for the in-situ solar-wind thermal particles in these time intervals are essentially indistinguishable, consistent with previous studies. These results tentatively suggest that suprathermal ions are the preferred seed particles for shock acceleration, with suprathermals associated with the AR field-sources having larger heavy-ion enhancements than suprathermals associated with the CH field-sources. To our knowledge, these results are the first successful example of tracking SEP event-to-event compositional variation back to differences in the IMF source regions at the Sun.