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

TL;DR

The study tackles how solar proton events in the 10-50 MeV range manifest differently at L1 (SOHO/EPHIN) and GEO (GOES) during Solar Cycles 23–24. It builds a catalog of 83 SPEs observed concurrently, classifies them into S1–S4, and compares onset, peak, and end times as well as peak fluxes and fluences between the two vantage points. The results show EPHIN typically detects earlier onsets and timings for weaker events (S1–S3), while GOES data are prone to contamination that inflates counts for strong events (S4), leading to larger GOES peak fluxes and fluences. Correlations with geomagnetic indices are generally weak, suggesting minimal near-Earth modulation of ≥10 MeV protons at GEO; these findings illustrate the importance of accounting for magnetospheric and instrumental effects when forecasting SPEs for cislunar missions and emphasize the need to correct GOES flux contamination for reliable space-weather assessments.

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 SOHO/EPHIN at Lagrange Point 1 (L1) with those from NOAA/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 to NOAA's solar radiation storm scales). EPHIN detected earlier onsets and longer durations across all categories, along with earlier peaks and ends for S1-S3, while GOES recorded slightly earlier peak and end times for S4. S1 median timing offsets (EPHIN relative to GOES) were -20 +/- 50 min (onsets), -1.00 +/- 1.42 hr (peaks), and -1.08 +/- 2.21 hr (ends), with similar trends for S2-S3 and near-simultaneity for S4 (peaks ~ -0.17 +/- 1.62 hr; ends ~ +0.04 +/- 3.33 hr). Flux comparisons show that EPHIN measurements modestly exceed GOES for S1 (median ratios ~1.11 for peaks and ~1.06 for fluence) and are lower than GOES for stronger events (peaks ~0.97 +/- 0.29, 0.84 +/- 0.21; fluence ~0.84 +/- 0.16, 0.75 +/- 0.16 for S2-S3). The EPHIN-to-GOES peak flux and fluence ratios reach 0.16 +/- 0.03 and 0.29 +/- 0.07, respectively, for S4 events, originating from contamination of lower-energy GOES channels. Correlation analyses show no significant flux dependence on geomagnetic indices, field strength, or spacecraft position, suggesting minimal near-Earth modulation of >=10 MeV proton access at GEO. 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.

Figures (10)

• Figure 1: List of 83 SPEs observed concurrently by EPHIN and GOES during SC 23 and the majority of SC 24. The first column provides the NOAA-like S-scale classification for each event based on its peak flux, while the last column specifies the GOES instrument designated as the primary detector at the time of observation. Reported peak fluxes are given in pfu, and fluences in pfu$\cdot$s.
• Figure 2: Fig. \ref{['sample-list']} continued.
• Figure 3: Merging 3 GOES-detected SPEs into one as they all occur during the same (singular) event detected by EPHIN.
• Figure 4: Flux profile of the SPE shown in Fig. \ref{['tab:overlap']}, after applying the adjustments described therein. The vertical lines mark the event intervals identified by each instrument, while the red-shaded regions highlight periods when GOES fluxes momentarily drop below 10 pfu but rise back above this threshold within 10 minutes.
• Figure 5: Timing offsets in SPE onset (top), peak flux (middle), and end times (bottom) as observed by GOES and EPHIN during SCs 23 & 24. Each bar corresponds to a single event, with the x-axis enumerating events in chronological order. Bar colors denote the NOAA S-scale intensity, while hatching indicates which instrument recorded the earlier time. Shaded backgrounds distinguish SC 23 and SC 24. Instances where bars are absent (e.g., onset for event #3) indicate no measurable offset, i.e., both instruments detected the property simultaneously ($\Delta = 0$ hrs).