New results on protons inside the South Atlantic Anomaly, at energies between 40 and 250 MeV in the period 2018–2020, from the CSES-01 satellite mission

The High-Energy Particle Detector (HEPD) on board the China Seismo-Electromagnetic Satellite (CSES-01) was launched in February 2018, with a foreseen mission lifetime of over 5 years. It is providing crucial new insight in the physical dynamics of the radiation belts in the Earth’s magnetosphere, in particular in the South Atlantic Anomaly (SAA). In this work, proton data from HEPD in the 40 MeV–250 MeV energy range, collected inside the SAA during the period between August 2018 and December 2020, are presented and compared with the up-to-date AP9 model by NASA. These are the first results on SAA protons at Low-Earth Orbit during the minimum activity phase between the 24th and the 25th solar cycles below 250 MeV. They enable an extensive testing and validation of current theoretical and empirical models aimed at predictions of temporal changes in this critical region of space. HEPD is advancing the observations collected by the PAMELA space experiment and NASA Van Allen Probe during the last 15 years through the 23rd and 24th solar cycles.

Figure 1

Vertical ($\theta <15°$) proton electronics ADC signal on plane ${\mathrm{P}}_1$ as a function of the total energy deposited in the calorimeter inside (top panel) and outside the SAA (bottom panel). The absence of any electron population inside the Anomaly—which would have been confined in the area around 500 ADC—is evident.

Figure 2

South Atlantic Anomaly proton fluxes as a function of energy (top panels), local pitch angle (middle panels) and L-shell (Earth radii, bottom panels) obtained by HEPD (black squares) between August 2018 and December 2020, and compared with predictions from the AP9 model at 95% C.L. (red dashed line).

Figure 3

Time profiles (1-day time binning) of omnidirectional protons inside the SAA measured by HEPD from August 2018 to December 2020, normalized to the month of August 2018. The three panels refer to three different energy ranges; low (40 MeV–68 MeV, left panel), medium (68 MeV–133 MeV, middle panel) and high (133 MeV–200 MeV, right panel). The gap in proton data between May 2019 and June 2019 refers to a period of instrument malfunctioning.

Figure 4

Geographical maps of omnidirectional proton fluxes (August 2018–December 2020) as a function of latitude and longitude for a low-energy bin (40 MeV–45 MeV, upper panel) and for a higher one (200 MeV–230 MeV, bottom panel). In both panels, the isolines of the reconstructed magnetic field are also shown for clarity.