## 1 Introduction  
The radiation belts, also known as Van Allen belts, are regions of the Earth's magnetosphere where energetic charged particles are subject to long-term magnetic trapping. The outer belt is mostly populated by electrons with hundreds of keV to MeV energies, while the inner belt mostly consists of an intense radiation of energetic protons (from MeV up to a few GeV), electrons/positrons (up to \(\sim\)8 MeV), and a minor component of ions [1,2]. Proton populations with energies above a few tens of MeV originate from the \(\beta\)-decay of free neutrons produced in the interaction between galactic cosmic-rays and the Earth's atmosphere in a mechanism called _Cosmic Ray Albedo Neutron Decay_ (CRAND) [3,4]. Since the discovery of the Van Allen radiation belts, after the launch of the first Explorer satellites in 1958 and the Pioneer in 1959 [5,6], the scientific community has been considerably involved in modeling this space radiation environment. All these efforts were mostly aimed to meet the practical need of better understanding the significant radiation hazard to spacecraft and human crews. Several studies reported a direct association between the dynamic radiation environment and system or subsystem performances [7,8]. To address and solve these problems, more accurate, comprehensive, and up-to-date space radiation environment models have been developed by the National Reconnaissance Office (NRO) and the Air Force Research Laboratory (AFRL); the new AE9/AP9/SPM set of models for high-energy electrons, protons, and space plasma, respectively, is derived from 45 datasets obtained from sensors on board various satellites. These datasets have been processed to create maps of the particle fluxes along with estimates of uncertainties from both imperfect measurements and space weather variability [9]. A detailed comparison between the older AE8/AP8 and the newer AE9/AP9 models is reported in [10].  
Gradual deterioration of spacecraft systems and components--and their overall performances--with accumulated dose is a fact, and various failures, due to phenomena associated with Single Event Effects (SEEs) or electrostatic discharge, are particularly common.  
The first empirical models of the radiation belts, developed in the 1960s and 1970s by NASA, tried to describe and represent the radiation environment and their early versions, namely, AE8 and AP8 [11], were widely employed in spite of their limitations, especially at low altitudes [11,12,13]. Despite being successful in describing the radiation environment, even AE9/AP9/SPM are partly incomplete, and often their predictions are not based on a statistically sufficient sample of direct measurements [10]. For this reason, it is of key importance to test them and, above all, to provide new and reliable datasets from in-flight instruments to improve their output and accuracy. Among the scientific payloads on board the China Seismo-Electromagnetic Satellite (CSES-01), in Low-Earth Orbit since February 2018, the High-Energy Particle Detector (HEPD) has gone through an intense period of testing and calibration, and it is able to measure \(>\)3 MeV electrons and \(>\)35 MeV protons with high efficiency. With an overall expected mission duration of \(>\)5 years, and together with other similar missions planned in the coming years, measurements from HEPD could enable the testing and validation of the aforementioned models. In this work, we have used orbital information from CSES-01 and the AP9 model to estimate the flux of trapped protons over the South Atlantic Anomaly (SAA) during a short testing period, i.e., January 1, 2021, in order to assess the radiation level at CSES orbit in view of a comparison to experimental HEPD data in a forthcoming publication. A brief description of the South Atlantic Anomaly is given in Section 2, while some details on the CSES mission and the HEPD payload are given in Section 3. The analysis is described in Section 4, results are presented in Section 5, and, finally, a brief discussion is presented in Section 6.