Spatial and Temporal Variations of 2-10 MeV/nuc He/H in Gradual Solar Energetic Particle Events

Observational connections between SEPs and their seed ion populations are difficult to establish. Kahler and Ling (2019) found a longitude-dependent correlation between 201 E > 10 MeV SEP event peak intensities and contemporaneous 1-AU H and He suprathermal intensities. High correlations r were found for SEP event sources at 0 to W40° and > W83°, but not for well-connected or eastern longitudes. Kahler and Brown (2021, KB21) found that 4-53 MeV/nuc He/H ratios measured at peak intensities in 43 gradual SEP events were significantly correlated with solar wind (SW) He/H ratios and with SEP event peak intensities. Although we are missing SW He/H ratios, we compare He/H ratios from onsets through peak of 12 large 1.8-10.0 MeV/nuc SEP events observed on both STEREO A and B (STA/B) spacecraft with the Low Energy Telescope when their longitudinal separation angle was < 90° in 2013-2014. We select 5 1-hr periods of STA/B SEP profiles to characterize He/H in the LET energy ranges 1.8-3.6, 4.0-6.0, and 6.0-10.0 MeV/nuc. We confirm KB21 results that SEP He/H extends over a range of 0.001 to 0.1, decreasing with increasing energy, and increasing with event peak H intensities. Differences of STA/B log He/H are not correlated with STA/B angular separations. The six cases of SEP events occurring within interplanetary coronal mass ejections (ICMEs) show somewhat higher H peak intensities and He/H than those outside ICMEs. We interpret the low He/H events as predominately SEP3 and high He/H as SEP4 events in the Reames (2020) SEP event classification system.