## 4 Materials and Methods  
The AE9/AP9/SPM set of models (version V1.50.001-release date December 2017) was downloaded from the Virtual Distributed Laboratory (VDL) website of the Air Force Research Laboratory ([[https://www.vdl.afrl.af.mil/programs/ae9](https://www.vdl.afrl.af.mil/programs/ae9) ap9/]([https://www.vdl.afrl.af.mil/programs/ae9](https://www.vdl.afrl.af.mil/programs/ae9) ap9/), accessed on 1 February 2021). Element Set (ELSET) data--including Two-Line Elements (TLE) for the CSES satellite on 1 January 2021--have been retrieved from the Space-Track website ([[https://www.space-track.org/](https://www.space-track.org/)]([https://www.space-track.org/](https://www.space-track.org/)), accessed on 1 February 2021) and inserted in the code to generate the ephemeris of the satellite (at a 5 s resolution). The Simplified General Perturbations (_SGP4_) (the SGP4 propagator considers secular and periodic variations due the oblateness of the Earth, gravitational effects from Sun and Moon, and orbital drag and decay.) propagator has been preferred to the default _Kepler_ with _J2_ perturbation effects, for cross-checking purposes. Indeed, these orbital results have been further compared to the ones obtained using 2-min broadcast information downloaded from the satellite itself, to verify the correctness of the procedure. This cross-check includes the following:  
* TLEs propagation using a chain of custom programs (including various SGP4 routines) to comply with the technique employed by the AE9/AP9 models;
* IGRF-13 [20] model-based routines have been applied to the calculated trajectory to reconstruct the intensity of the magnetic field at a 1 second resolution, together with [PERSON]'s L parameter in dipolar approximation [44]; these will be useful benchmarks in the future comparison to HEPD data. After these steps, two sets of geographical/geomagnetic coordinates have been obtained: one calculated by NASA routines and another extracted by Limadou external routines used for trajectory propagation and magnetic field reconstruction since launch. A comparison between such datasets has been performed to assure the best possible agreement over the chosen testing period--January 1, 2021. The resulting discrepancies are \(<\)0.08% for LATs/LONs, \(<\)0.13% for altitude, and \(<\)0.21% for magnetic field intensity. Small discrepancies are probably due to the models using the last TLE entry for orbit propagation, unlike external code picking the TLE closest in time. After ephemeris generation, an omnidirectional differential spectrum of trapped protons as a function of kinetic energy was created using AP9. This spectrum, averaged over all orbits of a single day, is shown in Figure 1; the blue arrow represents the HEPD low-energy thresholds for protons (30 MeV). Inside the inner radiation belt, trapped electron populations present a sharp threshold at \(\sim\)8 MeV, thus electrons of higher energy are virtually nonexistent. This means that in our future analysis, trapped protons will not be affected by any low-energy electrons contamination inside the SAA, consequently improving HEPD sensitivity to protons measurements. The AP9 model could also be very useful to help define a fiducial area on the Earth's surface (longitude vs. latitude) that may be applied to the future HEPD data analysis of trapped particles. However, the geographical extension of the inner belt (and consequently of the SAA) is largely dependent on energy, as shown in Figure 2. The surface contours for the \(>\)1, \(>\)10, and \(>\)100 MeV trapped protons are depicted as red, blue, and green curves, respectively, in the panel. These contours highlight how the low-energy trapped proton component is distributed in the southern regions of the SAA-even superimposing to the outer belt, while higher energy populations are more clearly enclosed in the classical boundaries of the inner belt, i.e., in the area above Brazil and the Atlantic Ocean.