## 4 Discussion
It has to be noted that there were three silicon units that got saturated (see the arrow in Figure 9d); such saturation phenomena only occur under geomagnetically disturbed conditions, and data from the saturated silicon units were eliminated in the statistical analysis from Figures 6 and 7. In contrast to the quiet condition presented by Figure 8c, the slot region in Figure 9c is invisible; such disappearance can be ascribed to large scale precipitation extending to very low \(L\) shell region (\(L\)-2). In other words, the slot region was refilled by a large  
Figure 9: Same as Figure 8 but under disturbed space weather conditions in the nightside ionosphere recorded by orbit No. 0031290 on 27 August 2018.  
quantity of energetic particles that were probably accelerated or scattered through the ELF/VLF wave-particle interaction process in the outer radiation belt. The slot region refilling phenomena during this storm time was also investigated by [PERSON] et al. [26] based on a conjugated observations between the CSES and RBSP (Radiation Belt Storm Probes), and they found that the ELF/VLF whistler-mode chorus waves in the radiation belt could efficiently accelerate and diffuse the relativistic electrons at the extremely low \(L\) shell area in the ionosphere. We further computed equatorial pitch angles by using the field-line tracing method based on the IGRF (International Geomagnetic Reference Field) model and the total magnetic field intensity recorded by the HPM onboard the CSES (see details in the work of [PERSON] et al. [20]). Figures 10a and 11a show the observed local pitch angles, Figures 10b and 11b the computed equatorial pitch angles for the half orbits of Figures 8 and 9. It can be seen that for geomagnetic latitudes higher than \(\sim\)40\({}^{\circ}\), the computed equatorial pitch angles for the local observed particles were overwhelmingly lower than the equatorial loss cone (denoted by the thick black dashed lines in Figures 10b and 11b), indicating these particles in the high latitude ionosphere as basically lost. However, at lower geomagnetic latitudes roughly within \(\pm\)40\({}^{\circ}\), we can see that certain particles were larger than the equatorial loss cone, indicating that they were most likely trapped instead of precipitation into this orbit space. It can be said that the CSES provided a good coverage of the energetic particle precipitations in the high latitude ionosphere. After investigating a large amount of half orbit data (with every day of 62 half orbits) from 20 August to 4 September 2018, some basic features of energetic electron fluxes could be recovered: (1) under the quiet condition, the CSES could well depict the outer/inner radiation belt and the slot region (as Figure 8) well, whereas under disturbed conditions, such a region boundary was invisible due to large scale precipitation extending to very low \(L\) shells; (2) the regions poleward from geomagnetic latitudes 50\({}^{\circ}\) corresponded to the highest electron precipitation without any distinction between solar quiet or active conditions, and the regions below geomagnetic latitudes 30\({}^{\circ}\) generally got precipitations during the storm time and occasionally at quiet time; and (3) the ELF/VLF waves recorded  
Figure 10: The comparison between the observed local pitch angels (**a**) and the computed equatorial pitch angles (**b**) for orbit No. 0030970 on 24 August 2018. The thick dashed line in (**b**) displays the equatorial loss cone (see text). T1–T9 with different colors in the legend denote the results from the nine silicon-slice units of HEPP-L onboard the CSES. by the CSES mainly included structure-less VLF whistler waves, structured quasi-periodic emissions, and structure-less ELF hiss waves. On the contrary, no whistler-mode chorus waves were found during this storm.