Spatial and Temporal Variations of 2 – 10 MeV nuc−1 He/H in Gradual Solar Energetic Particle Events

In a previous work (Kahler and Brown, 2021; henceforth KB21) we found that 4 – 53 MeV nuc−1 He/H ratios measured at peak intensities of 43 gradual solar energetic-particle (SEP) events were significantly correlated with solar-wind (SW) He/H ratios and with SEP event peak intensities. Here, we extend that work by examining He/H ratios from onsets through peaks of 12 large 1.8 – 10.0 MeV nuc−1 SEP events observed with the Low Energy Telescope (LET) on both STEREO A and B (STA/B) spacecraft when their longitudinal separation angle was < 90∘ in 2013 – 2014. We discuss the challenges of choosing matching five 1-hour 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−1. The roles of rigidity-dependent transport or of anomalous He production in large variations of He/H are assessed with 6.0 – 10.0 MeV nuc−1 O/He. The STA/B SW data do not include He/H, but we confirm the 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.