Suprathermal Ion Backgrounds of Solar Energetic Particle Events

Gradual solar energetic particle (SEP) events result from the acceleration of low-energy suprathermal seed particles to high (E > 10 MeV nuc(-1)) energies by shocks driven by coronal mass ejections (CMEs). Several studies suggest connections between suprathermal particles measured in situ at 1 au and the seed particles accelerated near the Sun to E > 10 MeV. We correlate E > 10 MeV SEP event peak intensities (Ip) with suprathermal H and He intensities at each of two energies averaged over four periods around the SEP onsets in the Geostationary Operational Environmental Satellite spacecraft during the period of 1998-2016. The 201 SEP events are sorted into four groups by their associated solar source longitudes and are further separated between events in transient CMEs and in normal solar wind (SW). The mean Ip in CME SW that is larger than in normal SW that was found earlier is confirmed. The suprathermals significantly correlate (correlation coefficients CCs approximate to 0.4-0.6) with the SEP Ip from the 0 degrees to W40 degrees range, but only weakly (CCs approximate to 0.0-0.3) with SEP events from the well-connected W41 degrees to W83 degrees range. Eastern hemisphere SEP intensities are uncorrelated (CC similar or equal to 0.0) with suprathermals in normal SW but are well correlated (CC approximate to 0.4-0.7) for those in transient CME SW, which we interpret with a simple model involving perpendicular shock acceleration of the suprathermals. The in situ B field magnitude shows no correlation with SEP intensities for any longitude range or timescale. These results confirm that tracking suprathermal intensities at 1 au can be useful in forecasting the Ip of SEP events.