## 2 Data and Methods  
### CSES-01 Mission and HEPD Detector  
The CSES-01 [19] is the first item of a multi-satellite constellation under construction by several missions scheduled for the next few years. The satellite was designed for the observation of variations in particle fluxes, plasma parameters, and the electromagnetic field and waves, induced by both natural and anthropogenic sources in the near-Earth space. One major goal of this Chinese-Italian space mission is to investigate possible correlations between the above-mentioned perturbations and the occurrence of high-magnitude earthquakes. Other fundamental targets are the study of space weather phenomena [18,20] and cosmic ray propagation [21]. The CSES-01 relies on the Chinese three-axis stabilized CAST2000 platform, and it is flying in a Sun-synchronous polar orbit at a \(\sim\)507 km altitude with a 97\({}^{\circ}\) inclination and a five-day revisit time. Nine scientific payloads are present on board the satellite [22, 23, 24, 25, 26, 27, 28, 29, 30], among which is the HEPD particle detector, which was designed and built by the Italian Limadou Collaboration. A schematic representation of the apparatus is reported in Figure 1. The HEPD is made up of a silicon tracking system; a trigger system that includes one plastic scintillator layer segmented into six paddles; a range calorimeter comprising a tower of 16 plastic scintillator planes, a matrix of 3 \(\times\) 3 LYSO (lutetium-yttrium oxy-orthosilicate) scintillator crystals, and an anti-coincidence (VETO) system equipped with 5 plastic scintillator planes, out of which 4 are placed at the lateral sides of the apparatus and 1 at the bottom (see [30; 31]). Thanks to this set of subdetectors, the HEPD is optimized to detect electrons in the energy range between 3 and 100 MeV and protons between 30 and 250 MeV, as well as light nuclei. In addition, the apparatus can detect different particle populations (solar, trapped, galactic, etc.) according to the satellite position (defined by the McIlwain L-shell parameter) and detected energy. The transmission of a dedicated command allows setting one of the eight predefined trigger mask configurations [32], which are the result of different logic combinations of counters from the various subdetectors. Hence, the different trigger masks define the aperture and the energy acceptance of the instrument. The trigger condition, labeled as T, corresponds to an above-threshold signal only in the trigger plane, and it is associated with the lowest energy threshold. By requiring a deeper penetration of the particle inside the detector (i.e., using the trigger plane counters and a set of tower planes in \"AND\" configuration, such as T & P1, T & P1 & P2, and so on), the geometric factor of the HEPD decreases, and consequently, the energy threshold for triggering increases. In July 2018 (late commissioning phase), the HEPD was configured with a trigger condition, labeled as T & P1 & P2, which corresponds to event acquisition and processing only when the released signals in the trigger plane and the first two calorimeter planes (P1, P2) are above predefined thresholds. However, for each of the predefined masks, even when not selected for the online acquisition, a rate meter independently provides the corresponding trigger  
Figure 1: Schematic of the HEPD detector. All mechanical structures (as well as the lateral VETO plane located in the front) have been removed from the figure for visualization purposes. counting rate (1 s resolution). In this paper, we used the rate meters of three trigger masks (T, T & P1, and T & P1 & P2), corresponding to the integral number of particles per second above different energy thresholds. Due to adjustments in attitude and additional scheduled maneuvers, the CSES-01 payloads are usually switched off at latitudes below \(-65^{\circ}\) and above +65\({}^{\circ}\). However, the HEPD can benefit from its large field of view (\(\pm 60^{\circ}\)) and geometrical acceptance to collect particles at large L-shells, though for a short time per day.