The South Atlantic Anomaly (SAA) is an area where Earth's magnetic field is particularly low. This comes from the fact that the Earth's magnetic axis is tilted with respect to the Earth's rotation axis by an angle of approximately 11° and that the axis of the magnetic idealized dipole is located some 400 km away from the Earth's center (Brekke, 2013). As a consequence, the SAA is also a region where inner radiation belt particles can mirror at lower altitudes increasing the local particle flux.Unlike the geographic poles, Earth's magnetic poles are not fixed and tend to wander over time, and in parallel, the intensity of Earth's magnetic field is varying over time, for example, presently decreasing (Brekke, 2013). Hence the SAA has been drifting and growing in recent years (Amit et al., 2021;Anderson et al., 2018;Stassinopoulos et al., 2015), and those secular trends have an effect on the inner belt, especially as observed at Low Earth Orbit (LEO) (Girgis et al., 2021).In addition to secular drifts, the inner belt particles are affected by solar cycle variation. In fact, the low-altitude geomagnetically trapped population is known to be coupled to the atmospheric density through changes induced by solar activity (Anderson et al., 2018;Bruno et al., 2021b). Protons with energies above a few tens of MeV mostly originate through the Cosmic-Ray Albedo Neutron Decay (CRAND) mechanism. Because the CRAND source comes mostly from low geomagnetic latitudes due to high cutoff rigidities, the solar modulation of the cosmic rays has a relatively small effect on the variability of the trapped protons (Bruno et al., 2021b). The major variations are due to the atmospheric loss processes, including ionization and scattering of neutral and ionized atoms, induced by solar extreme ultraviolet (EUV) radiation heating the Earth's upper atmosphere. In fact, increased EUV during solar maxima leads to higher neutral and ionospheric densities, causing a decrease of proton fluxes concentrated at low altitudes and L shells.One of the particularities of the particle fluxes at low altitudes is that the high-energy trapped proton fluxes are strongly anisotropic, that is, the proton fluxes depend on their arrival direction in the plane perpendicular to the local magnetic field vector as well as on their pitch angle (angle between their velocity vector and

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Abstract. Relativistic radiation belt electron observations from the Energetic Particle Telescope (EPT) onboard the PROBA-V satellite are compared to those performed by the Magnetic Electron Ion Spectrometer (MagEIS) onboard the Van Allen Probes (VAPs) formerly known as the Radiation Belt Storm Probes (RBSP). Despite their very different orbits, both instruments are able to measure fluxes of electrons trapped on a given magnetic shell. In the outer belt, the comparison of high and low altitude fluxes is performed during the first three months of 2014, featuring the most intense storms of the year. In the inner belt, measurements from the two instruments are compared only at conjunction, when the satellites are physically close to each other. Due to the low number of conjunctions, the whole period of mutual operation of both instrument is used (i.e. May 2013–October 2019). The comparisons show that flux variations appear simultaneously on both spacecraft, but the fluxes observed by the EPT are almost always lower than for MagEIS, as expected from their different orbits. In addition, this difference in flux intensity increases with electron energy. During geomagnetic storms, it is also shown that dropout events (i.e. sudden depletion of electrons) in the outer belt are more pronounced at low altitudes than near geomagnetic equator. The effect of the equatorial pitch angle value of electrons is investigated in the outer belt. The results show a good agreement between observations of the two instruments, especially if low pitch angle electrons near the equator are considered.