Solar Orbiter survey of inner heliosphere low energy ions (greater than ∼100 keV/nucleon), 2020–2025

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Abstract
Context. The Solar Orbiter spacecraft cruised in the inner heliosphere from April 2020 to December 2025, moving between ∼0.3–1.0 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) were used to survey the energetic particle population H-Fe from solar energetic particles (SEPs), corotating interaction regions (CIRs), 3 He rich SEPs and other sources. Methods. The particle populations were organized by source type, energy, radial location, and time in order to construct a statistical description of the ion population and characteristics of its sources and transport processes. In addition, “quiet” periods outside transient activity were included to cover the remaining time intervals. Results. The mix of particle population sources below ∼1 MeV/nucleon showed little variation with radial distance from the Sun, and changed from primarily quiet and CIR sources at solar minimum to SEP (gradual and 3 He rich) at maximum. The fluences of O summed over the entire period showed surprisingly little variation with radius once the observing time and 1/r 2 correction was made, indicating that energy changing processes (either acceleration or deceleration) play a small role outside ∼0.3 au. The relative abundances of elements H-Fe did not change with radial distance, suggesting that the particle population is energized primarily within 0.4 au distance, and the spacecraft is observing a population where only transport is playing a role. The composition of the quiet periods, measured with the Fe/O ratio, was observed to increase from CIR-like to SEP-like as solar activity moved from minimum to maximum. This behavior was seen in all radial distance bins and has been previously reported from surveys at 1 au. Spectra collected during the quietest periods of the survey, April 2020–April 2021, revealed low energy steep spectra with H and 4 He slopes and abundances similar to CIRs but at much lower intensity levels, and heavier ions consistent with SEPs, but with low statistical accuracy. No evidence was found for other source populations.
Funding Information
  • Heliophysics Division (NNN06AA01C)
  • Heliophysics Division (80GFSC25CA035)
  • Deutsches Zentrum für Luft- und Raumfahrt (50OT2002)
  • MICIU (MICIU/AEI/10.13039/501100011033)
  • European Space Agency (SOL.ASTR.CON.00004)

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