Turnover in Gleissberg Cycle Dependence of Inner Zone Proton Flux

Abstract The NOAA POES satellites orbit through the South Atlantic Anomaly (SAA) allowing access to the trapped inner belt high energy proton population. A previous study found a long‐term increase in proton flux averaged over the 11‐year solar cycle oscillation corresponding to solar activity consistent with the Centennial Gleissberg Cycle (CGC). F10.7 flux maxima have been decreasing over the ∼40‐year period of 1980–2021, correlating with an average increase in the varying proton population. Bregou et al. ’s (2022, https://doi.org/10.1029/2022sw003072 ) long‐term study of the peak flux over the SAA in NOAA‐15 shows an increase in proton flux from 1998 until 2021. The observed flux increase is correlated with both the ∼11‐year solar cycle and the overall decreasing F10.7 flux over the period studied. This long‐term decrease in F10.7 flux and increase in proton flux is concluded to be the manifestation of the CGC minimum and accompanying decrease in solar Extreme Ultraviolet irradiance (EUV). We extend Bregou et al., ’s (2022, https://doi.org/10.1029/2022sw003072 ) study to 2024 and observe a rapid increase in F10.7 flux at the beginning of Solar Cycle 25, a proxy for EUV, leading to increased proton loss to the expanding atmosphere and a steep decrease in the inner zone proton flux from 2022 to 2024 in NOAA‐15 and NOAA‐19 measurements. A model calculation of the inner zone proton flux generally agrees with the long‐term trend in flux magnitude.