Solar Energetic Proton Events Observed by the High Energy Telescopes on the STEREO Spacecraft or at the Earth During the First Solar Orbit of STEREO A (2006 – 2023)

Abstract The twin Solar TErrestrial Relations Observatory (STEREO) A and B spacecraft were launched in October 2006 into heliocentric orbits at 1 ∼ 1 AU, advancing ahead of or lagging behind Earth, respectively, at 22^ ∼ 22 ∘ /year. The spacecraft provide in situ observations of the solar wind and energetic particle populations, as well as remote sensing observations of solar activity and the corona. In particular, the High Energy Telescopes (HETs) on the STEREO spacecraft observe 0. 7 – 4 MeV electrons and 13 – 100 MeV protons. This article summarizes observations of solar energetic particle (SEP) events made by the STEREO HETs from the beginning of the mission through Solar Cycle 24 to December 2023, approaching the maximum of Solar Cycle 25 and encompassing STEREO A first full orbit of the Sun relative to Earth, completed in August 2023; contact with STEREO B was lost in October 2014. Specifically, the catalog of SEP events including \, 25 MeV ∼ 25 MeV protons observed by the STEREO HETs and/or instruments on spacecraft near Earth in Richardson et al. (Solar Phys. 289, 3059, 2014) is updated to include 450 ∼ 450 SEP events and a total of 1000 ∼ 1000 separate observations of these events from the various spacecraft locations. These extensive observations can provide unique insight into the propagation of energetic protons in the inner heliosphere and how the properties of the particle events are related to those of the associated solar eruptions. In particular, we examine the association of coronal mass ejections (CMEs) and SEP events with all 397 M and X-class solar X-ray flares in the period June 2010 – January 2014 and demonstrate that, for these events, the occurrence of a CME accompanying a flare is required for the detection of a 25 MeV ∼ 25 MeV proton event. On the other hand, many flares accompanied by CMEs are not followed by detected SEP events. The longitudinal width and intensity of the associated SEP events generally increase with the CME speed and the flare intensity. We also note evidence for a 150 ∼ 150 day “Rieger-like” periodicity in the SEP occurrence rate in 2020 – 2023 during the rising phase of Solar Cycle 25.