GOES observations of solar protons during ground level enhancements

Abstract

Since 1974, the U.S. National Oceanic and Atmospheric Administration (NOAA) has observed solar proton fluxes from the Geostationary Operational Environmental Satellites (GOES). These observations frequently have served as measurements of the primary component of ground level enhancements (GLEs). Until March 2020, when GOES-14 and -15 were turned off, solar proton measurements were made by the Energetic Particle Sensor (EPS) and the High-Energy Proton and Alpha Detector (HEPAD). EPS had poor energy resolution above 100 MeV, and NOAA derived a >100 MeV integral flux from the EPS channels to support alerts issued by the Space Weather Forecast Office. HEPAD provided some energy resolution in the 330-700 MeV range and a >700 MeV integral channel. Starting with GOES-16, a new instrument called the Solar and Galactic Proton Sensor (SGPS) has replaced EPS and HEPAD. SGPS uses three solid-state telescopes to observe solar proton fluxes between 1 and 500 MeV with a >500 MeV integral channel. The >100 MeV integral flux is now derived from SGPS observations and includes the >500 MeV flux in its derivation. In this paper, we describe the older EPS and HEPAD observations and the new SGPS solar proton observations. We also compare methods for detecting solar proton event onsets currently used with GOES and neutron monitor observations and recommend some innovations.