Authors: Aatiya Ali (Georgia State University), Viacheslav Sadykov (Georgia State University)

Solar energetic particle (SEP) events, particularly solar proton events (SPEs), are critical to monitor due to their potential to disrupt technology and affect operations on Earth and in space. Building on prior work that compiled a catalog of SPEs using NOAA GOES proton flux data, focusing on events where protons with energies ≥10 MeV exceed 10 particle flux units (pfu), we expand our analysis beyond Earth’s magnetosphere by comparing proton flux measurements from SOHO-EPHIN at the Lagrange point 1 (L1), representative of the cislunar environment, with those from the geostationary orbit (GEO) during solar cycles 23 & 24. Of the 83 SPEs concurrently observed at both L1 and GEO, 17 exhibited potential flux contamination in GOES’ low-energy channels. Despite this, peak fluxes and fluences were largely consistent between the two locations, with EPHIN frequently detecting higher particle counts. EPHIN also typically recorded earlier onset, peak flux, and end times, although events at GEO tended to last longer. For potentially-contaminated events, GOES data showed significant variability in peak flux and fluence, likely due to sensor over-saturation during periods of high fluxes of energetic protons (≥50 MeV). We also analyzed GOES-to-EPHIN flux ratios to identify any consistent trends as SPEs propagated from L1 to GEO, but found no correlation between these ratios and local magnetic pressure at GEO or the position of the GOES spacecraft. This study emphasizes the importance of considering spatial and instrument-specific effects in SEP measurements, providing valuable insights for SPE forecasting in environments beyond Earth’s magnetosphere, and aiding future missions like Artemis and operations in near-Earth and cislunar space.