Variations of Peak He/H Ratios in Solar Energetic (E > 4 MeV) Particle Events and Comparisons with Solar Wind He/H Ratios

Abstract

We determine solar energetic particle (SEP) event He/H peak intensity ratios A He observed in the four energy channels of the SOHO EPHIN detector covering 4–53 MeV nuc −1 . Those SEP A He values range over two orders of magnitude for 43 large western hemisphere SEP events through the period 1997–2017. A He of each SEP event are compared with average solar wind (SW) A He values measured for 8 hr after the SEP event onsets with the solar wind experiment Faraday Cup instrument on Wind. A He in the 4–8 and 8–21 MeV nuc −1 range are significantly correlated with SW A He (CC = 0.45 and 0.41), but less so in the 21–41 and 41–53 MeV nuc −1 range, where uncertainties of the A He values are higher. Median SEP A He values decline slightly with increasing energy and are ≤0.5 their associated SW values. Both median SEP and SW A He decline by a factor of ∼2 from the 27 events of cycle 23 to the 16 of cycle 24. Those results suggest a connection between the SW and the seed population of SEP events. An unexpected result is that SEP A He increases with event peak intensities in our 43 events as He peak intensities increase faster than those of H in larger events.