Comparative Analysis of 10 ‐ 50 MeV Solar Proton Events at Lagrange Point 1 and the Geostationary Orbit

Abstract Solar proton events (SPEs) pose radiation hazards, disrupt technology, and impact operations on Earth and in space, making continuous monitoring essential. We compare 10–50 MeV proton flux measurements from Solar and Heliospheric Observatory/EPHIN at Lagrange Point‐1 (L1) with those from NOAA/Geostationary Operational Environmental Satellite (GOES) in geostationary orbit (GEO) during Solar Cycle 23 and most of Cycle 24. We identify 83 10 pfu SPEs observed at both locations and classify them into S1–S4 categories (comparable but not identical to NOAA's solar radiation storm scales). EPHIN detected earlier onsets for S1–S3 events, longer durations across all categories, and earlier peaks and ends for S1–S3; while GOES recorded slightly earlier peak/end times for S4. Median timing offsets (EPHIN relative to GOES) were min (S1 onsets), hr (S1 peaks), hr (S1 ends), with similar trends for S2–S3 and near‐simultaneity for S4 (peaks hr; ends hr). Flux comparisons show that EPHIN measurements modestly exceed GOES for S1 (median ratios for peaks and for fluence) and are lower than GOES for stronger events (peaks , ; fluence , for S2, S3). The EPHIN‐to‐GOES peak flux and fluence ratios reach and , correspondingly, for S4 events, originating from strong contamination of lower‐energy GOES channels. Correlation analyses show no significant EPHIN‐to‐GOES flux dependence on geomagnetic indices, local magnetic field strength, or spacecraft position, suggesting minimal near‐Earth modulation of 10 MeV proton access at GEO by near‐Earth conditions. These results highlight systematic differences in how SPEs manifest at L1 versus GEO and offer practical guidance for forecasting beyond Earth's magnetosphere, supporting mission planning for near‐Earth and cislunar exploration, including Artemis.