A drift-diffusion model is used to simulate the low-altitude electron distribution, accounting for azimuthal drift, pitch angle diffusion, and atmospheric backscattering effects during a rapid electron dropout event on August 21st, 2013, at L=4.5. Additional external loss effects are introduced during times when the low-altitude electron distribution cannot be reproduced by diffusion alone. The model utilizes low-altitude electron count rate data from five POES/MetOp satellites to quantify pitch angle diffusion rates. Low-altitude data provides critical constraint on the model because it includes the drift loss cone region where the electron distribution in longitude is highly dependent on the balance between azimuthal drift and pitch angle diffusion. Furthermore, a newly derived angular response function for the detectors onboard POES/MetOp is employed to accurately incorporate the bounce loss cone measurements, which have been previously contaminated by electrons from outside the nominal field-of-view. While constrained by low-altitude data, the model also shows reasonable agreement with high-altitude data. Pitch angle diffusion rates during the event are quantified and are faster at lower energies. Precipitation is determined to account for all of the total loss observed for 350 keV electrons, 76% for 600 keV and 45% for 900 keV. Predictions made in the MeV range are deemed unreliable as the integral energy channels E3 and P6 fail to provide the necessary constraint at relativistic energies.

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.

Solar Energetic Protons (SEPs) have been shown to contribute significantly to the inner zone trapped proton population for energies < 100 MeV and L > 1.3 (Selesnick et al., 2007). The Relativistic Electron Proton Telescope (REPT) on the Van Allen Probes launched 30 August 2012 observed a double-peaked (in L) inner zone population throughout the 7-year lifetime of the mission. It has been proposed that a strong SEP event accompanied by a CME-shock in early March 2012 provided the SEP source for the higher L trapped proton population, which then diffused radially inward to be observed by REPT at L ~ 2. Here, we follow trajectories of SEP protons launched isotropically from a sphere at 7 Re in 15s cadence fields from an LFM-RCM global MHD simulation driven by measured upstream solar wind parameters. The timescale of the interplanetary shock arrival is captured, launching a magnetosonic impulse propagating azimuthally along the dawn and dusk flanks inside the magnetosphere, shown previously to produce SEP trapping. The MHD-test particle simulation uses GOES proton energy spectra to weight the initial radial profile required for the radial diffusion calculation over the following two years. GOES proton measurements also provide a dynamic outer boundary condition for radial diffusion. A direct comparison with REPT measurements 20 months following the trapping event in March 2012 provides good agreement with this novel combination of short-term and long-term evolution of the newly trapped protons.

Inner zone proton flux from 1980 to mid-2021is examined using NOAA POES satellite data, indicating a long-term increase corresponding to a one hundred year minimum in solar activity consistent with the Centennial Gleissberg Cycle. Variation of inner belt protons is correlated with decreasing F10.7 maxima over the 40-year period, serving as proxy for solar EUV input to Earth’s atmosphere. Extending an earlier study (Qin et al., 2014) of > 70 MeV protons from 1980 – 2021 using the South Atlantic Anomaly (SAA) peak flux, and at fixed L = 1.3, a comparison is made between the > 35, > 70 and > 140 MeV energy channels on POES. All three energies show an increase in proton flux over the period 1998 – 2021 using a single spacecraft. The observed flux increase is correlated with decreasing F10.7 over the longer 40-year time interval, as with the ~11-year solar cycle. A phase lag during Solar Cycle 24 (January 2010 – June 2021) between the F10.7 minimum and proton flux maximum was determined to be ~500 days, the same at all energies studied. A model calculation of the inner zone proton flux is found to generally confirm the long-term trend examined both in absolute magnitude and phase lag. It is concluded that this long-term trend is a manifestation of the concurrent Gleissberg cycle minimum and accompanying decrease in solar EUV. Reduced EUV at solar maximum (F10.7proxy) reduces proton loss to the atmosphere following solar maximum, thus explaining the long-term flux increase observed.