## 4 Discussion  
We present two half-orbit observations from the CSES as examples to discuss the exact electromagnetic environment in the ionosphere under geomagnetic quiet (Figure 8) and disturbed conditions (Figure 9). Figure 8 shows some typical half-orbit data of the CSES under a quiet space weather condition recorded by orbit No. 0030970 on 24 August 2018 (denoted by the vertical line Figure 1). The ELF/VLF waves are presented by the PSD values of the magnetic and electric fields (Figure 8a,b), respectively. The overlapping dashed curves represent the local proton cyclotron frequency \(f_{\mathrm{cp}}\), which was computed by the total magnetic field values provided by the HPM onboard the CSES. From the energy spectrum of energetic electrons at 0.1 MeV \(<\) E \(<\) 3 MeV (Figure 8c), the outer/inner radiation belt is clearly visible (denoted by black arrows). In addition, we can see from Figure 8 that even under quiet space weather conditions at geomagnetic latitudes between -40\({}^{\circ}\) and -70\({}^{\circ}\), there was a much higher level of ELF/VLF wave activity and energetic electron fluxes than the lower latitudes. Since the CSES is switched off at latitudes over -65\({}^{\circ}\), the region above 70\({}^{\circ}\) cannot be observed by it. Note that the electric field enhancement over the equatorial area (denoted by black rectangle in Figures 8b and 9b) was due to artificial interferences from the EFD. The variation of energy spectrum with respect to the local pitch angle distributions was further examined, as shown in Figure 8d, by using data from the nine silicon-slice units of HEPP-L onboard the CSES. Distinctly, at high latitudes over 50\({}^{\circ}\), where ELF hiss  
Figure 8: The ELF/VLF wave activities and energetic electron fluxes on the dayside ionosphere during magnetic quiet time recorded by No. 0030970 on 24 August 2018. From top to bottom: (**a,b**) power spectral density values of the magnetic and electric fields, (**c**) the energetic fluxes at energy band from 0.1 to 3.0 MeV, and (**d**) the energetic electron fluxes distributions along the pitch angles. Data are displayed as a function of universal time (UT), magnetic local time (MLT), geomagnetic latitude (mlat), and \(L\) shell, respectively. waves appeared below 900 Hz, the energetic fluxes overwhelmingly got distributed at pitch angles from \(\sim\)65\({}^{\circ}\) to 120\({}^{\circ}\). The region within geomagnetic latitudes \(\pm\)30\({}^{\circ}\) saw that the energetic electron fluxes dramatically declined, but the wave activities simultaneously enhanced at a frequency higher than \(f_{\rm cp}\). Figure 9 shows data from another half orbit that were recorded by orbit No. 0030970 during geomagnetic storm time on 27 August 2018 (recovery phase, denoted by the vertical solid line in Figure 1). Compared to the quiet condition in Figure 8, the CSES witnessed much stronger ELF/VLF activities and energetic electron fluxes enhancement under the active storm condition. Three typical electromagnetic waves could especially be identified, as follows:  
(1) The ionospheric his waves that mainly appeared at frequencies around/below \(f_{\rm cp}\) at geomagnetic latitudes from 20\({}^{\circ}\) (relatively weak) to 65\({}^{\circ}\) (predominantly) or at frequencies above \(f_{\rm cp}\) to 800 Hz over the equatorial area with 20\({}^{\circ}\), showing structure-less wave spectral property [22]; (2) the non-structured VLF waves at frequencies from \(\sim\)1000 to 1800 Hz at high latitudes over 55\({}^{\circ}\); and (3) the structured quasi-periodic structures (QP waves) at latitudes from \(\sim\)35\({}^{\circ}\) to 50\({}^{\circ}\)[20]. For energetic electron fluxes, the CSES saw a direct enhancement of \(E<\) 0.9 MeV along the entire orbit trace, as well as increases of particles 0.1 MeV \(<E<\) 3 MeV at high latitudes around \(\sim\)55\({}^{\circ}\) to 60\({}^{\circ}\).