Quiet-time Low Energy Ion Spectra Observed on Solar Orbiter During Solar Minimum


Context. The Solar Orbiter spacecraft cruised in the inner heliosphere during Feb. 2020 – Jan. 2021, moving between ~0.5-1.0 au radial distance. The Energetic Particle Detector (EPD) suite operated continuously during this period. Aims. The Suprathermal Ion Spectrograph (SIS) and High Energy Telescope (HET) observations made during intervals in between transient intensity increases were used to determine the low energy ion spectra and composition during quiet times. Methods. Energetic particle spectra and major ion components, including 3He, were measured over the range ~0.1->100 MeV/nucleon. The radial dependence of 4.4 MeV/nucleon 4He and O was measured. A short interval of extremely low intensities ("super-quiet") was also studied. Results. Spectra measured during the quiet period showed transitions, including galactic cosmic rays (GCRs; > 50 MeV/nucleon), anomalous cosmic rays (ACRs; a few to ~50 MeV/nucleon), and a steeply rising "turn-up" spectrum below a few MeV/nucleon whose composition resembled impulsive, 3He-rich solar energetic particle (SEP) events. The radial dependence had large uncertainties but was consistent with a small gradient. During the super-quiet interval, the higher energy components remained similar to the quiet period, while the approximately flat low energy 4He spectrum extended downward, reaching ~300 keV/nucleon before transitioning to a steeply rising spectrum.