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

TL;DR

This study compares 10-50 MeV solar proton event measurements at L1 and GEO, revealing timing offsets, flux differences, and minimal geomagnetic influence, aiding space weather forecasting and mission planning.

Contribution

It provides a detailed comparison of proton flux measurements at L1 and GEO, highlighting systematic differences and offering insights for improved space weather prediction.

Findings

EPHIN detects earlier onsets and longer durations than GOES.

Timing offsets vary by event strength, with S1-S3 showing earlier EPHIN peaks.

Flux ratios indicate measurement differences, especially for stronger events.

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.