On the relationship between heavy-ion composition variability in gradual SEP events and the associated IMF source regions

We investigated the relationship between heavy-ion composition variability in gradual solar energetic particle (SEP) events and the source regions of the associated interplanetary magnetic field (IMF) along which these SEPs are transported. In particular, we found that the SEP elemental composition has a positive correlation with the unsigned photospheric magnetic field strength around the IMF footpoints. Furthermore, the IMF source region can be generally categorized into two classes, depending on whether the associated solar wind, and correspondingly the footpoints of the open magnetic field along which the SEPs are transported, are from a coronal hole (CH) near an active region (an 'AR field-source') or from a CH without an AR in its close proximity (a 'CH field-source'). This study is based on 24 events observed in years 1998 and 2003-2006. We present in this paper the results for Fe/O as an example, and show that the observed SEP Fe/O ratio at 3-30 MeV/nucleon is preferentially enhanced in the case of AR field-source. By contrast, the distributions of Fe/O for the in-situ solar-wind thermal particles in these time intervals are essentially indistinguishable between the two field-sources. Our results suggest that suprathermal ions are the preferred seed particles for shock acceleration, and the composition of these suprathermals depends on the magnetic field where they are generated, with suprathermals associated with active regions, i.e. stronger photospheric magnetic field, having larger heavy-ion enhancements than suprathermals associated with coronal holes with weak magnetic field. These results open a new window in understanding the origin and production of the suprathermal seed particles for SEPs.