GOES Observations of Solar Protons during Ground Level Enhancements

Juan Rodriguez and Brian Kress, University of Colorado CIRES, Boulder, Colorado, USA NOAA National Centers for Environmental Information, Boulder, Colorado, USA

Since 1974, the U.S. National Oceanic and Atmospheric Administration (NOAA) has made observations of solar proton fluxes on the GOES satellites. These observations frequently have been combined with neutron monitor observations to estimate the energy spectrum during ground level enhancements. 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. Although reliable flux calibrations have been difficult to achieve, an enhancement above cosmic ray backgrounds in the >700 MeV HEPAD channel was considered a signature of a GLE in the primary fluxes. The series of HEPADs on GOES-6 and 8-15 observed all GLEs between GLE 39 (February 1984) and GLE 72 (September 2017). Starting with GOES-16, a new instrument called the Solar and Galactic Proton Sensor (SGPS) has replaced EPS and HEPAD. SGPS uses a three-telescope design to observe solar proton fluxes between 1 and 500 MeV with a >500 MeV integral channel. The SGPS >500 MeV channel addresses a measurement requirement of the U.S. Federal Aviation Administration. In this paper, we will describe the new SGPS solar proton observations and compare them to the older EPS and HEPAD observations. We will also compare methods for detecting solar proton event onsets used with GOES and neutron monitor observations.