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 to 2023)

TL;DR

This study delivers a comprehensive, multi-spacecraft catalog of solar energetic proton events observed by the STEREO High Energy Telescopes and near-Earth missions from 2006 to 2023, capturing STEREO A’s first Sun-relative orbit. It identifies roughly $450$ SEP events and about $1000$ observations, showing that far-side solar activity contributes substantially to Earth’s SEP population and that SEP properties correlate with CME speeds and flare intensities while spreading broadly in longitude. Proton measurements are cross-calibrated with SOHO instruments, enabling reliable cross-mission comparisons, and the analysis reveals a Rieger-like periodicity in SEP occurrence during Cycle 25’s rising phase. These results enhance understanding of SEP transport in the inner heliosphere and provide a valuable dataset for space weather prediction and solar-eruption–SEP linkage studies.

Abstract

The twin STEREO A and B spacecraft were launched in October 2006 into heliocentric orbits at around 1 AU, advancing ahead of or lagging behind Earth, respectively, at around 22 deg./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 paper 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's 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 approximately 25 MeV protons observed by the STEREO HETs and/or instruments on spacecraft near Earth in Richardson et al. (2014) is updated to include around 450 SEP events and a total of around 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.

Figures (29)

• Figure 1: Summary of STEREO A (panel 2) and B (panel 4) 14-24 MeV proton intensities and SOHO ERNE 17-22 MeV proton intensities (panel 3) from STEREO launch in October 2006 to December 2023, just beyond the end of the first solar orbit (relative to Earth) of STEREO A in August 2023. Contact with STEREO B was lost on October 1 2014, when the STEREO spacecraft were both $\sim180^\circ$ heliolongitude from Earth. The top panel shows the monthly sunspot number, extending from the late declining phase of Solar Cycle 23 to the ascending phase of Solar Cycle 25. The bottom two panels show the number of individual $\sim25$ MeV proton events per month and per year cataloged in Tables 1 to \ref{['tab11']}. Note that these SEP rates are influenced by the loss of STEREO B observations and STEREO A approaching Earth in Cycle 25 (cf., Figure \ref{['fig:stloc']}).
• Figure 2: Locations of STEREO A (1, red) and B (3, blue) relative to Earth (2, green) on January 1 of 2010, 2016, 2019 and 2023. Contact was lost with STEREO B in October 2014. Nominal Parker spiral magnetic field lines (assuming a solar wind speed of 400 km/s) passing each location are also shown. Figures produced using Solar-MACH giesler2023.
• Figure 3: Comparison of GOES $>10$ and $>30$ MeV proton fluxes in (s cm$^2$ sr)$^{-1}$ (= proton flux unit, pfu) and 7.8-25 MeV and 25-40.9 MeV proton intensities (in (MeV s cm$^2$ sr)$^{-1}$) from SOHO/EPHIN for an interval in June-July, 2012, illustrating how the high background in the GOES fluxes obscures all but the largest SEP events that are observed by EPHIN. Using the standard/operational SEP event definition of $>10$ pfu at $>10$ MeV further limits the number of "SEP events", and these are unrepresentative of the large dynamic range of SEP event intensities evident in the EPHIN observations. From Richardson2023.
• Figure 4: Comparison of hourly-averaged proton intensities observed by SOHO/EPHIN (7.8-25 MeV), SOHO/ERNE (14-17 MeV, 21-28 MeV and 32-40 MeV) and STEREO A HET (13.6-15.1, 20.8-23.8 MeV and 33.4-35.8 MeV) during a sequence of SEP events between July 23 and August 15, 2023 when STEREO A moved from $1.9^\circ$ east of Earth to $0.3^\circ$ west. The intensities are consistent in channels of similar energy ranges.
• Figure 5: The top panels shows the correlation of SOHO/ERNE and STEREO A HET hourly-averaged proton intensities ((MeV s cm$^2$ sr)$^{-1}$) in two energy ranges during July 19-September 3, 2023 when STEREO A moved from $2.3^\circ$ east of Earth to $2.0^\circ$ west. The red lines indicate equal intensities. The bottom panels show essentially identical correlations in December 2006 near the start of the STEREO mission. The energy channels for SOHO/ERNE (STEREO A HET) are 13--17 (13.6--15.1) MeV in the left panels and 21--28 (20.8--23.8) MeV in the right panels. The "quantization" in the HET data in the right panels is due to low particle counts.