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) 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 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.