## 3 Observations
By using the same method from Figure 2, the average PSD values of each band from each orbit were computed and binned as a function of \(L\) shell values in a step of 0.2 \(L\), with a time interval of 1 h in the epoch time period from \(t_{0}-30\) h to \(t_{0}+90\) h. Similar for the energetic electrons, the fluxes from the six energy bands were integrated: 0.1-0.5 MeV, 0.5-1 MeV, 1.0-1.5 MeV, 1.5-2.0 MeV, 2-2.5 MeV, and 2.5-3 MeV. Figures 4 and 5 show the results of ELF/VLF waves in the magnetic field from the day and night side ionospheres, respectively. In Figures 4 and 5, the black vertical dashed lines denote the reference epoch time \(t_{0}\). It can be seen that the arrival of an ICME at the magnetopause location directly led to strong ELF/VLF wave activities in the ionosphere. Specifically, before the ICME hit the magnetopause (\(t<t_{0}-16\) h), the ELF/VLF wave activities at all frequency bands were relatively quiet. At the early stage of main phase (from \(t_{0}-16\) h to \(\sim\)\(t_{0}-4\) h), the ELF/VLF waves were mainly excited at frequencies below 6 kHz. However, from the near-end main phase to the early recovery phase (from \(t_{0}-6\) h to \(\sim\)\(t_{0}+6\) h), the wave activities significantly grew up in the entire frequency range from 0.2 to 20 kHz. During the long recovery phase, the wave enhancements mainly occurred below 6 kHz. These results from the CSES were consistent with our previous statistical analysis for CME-driven storms based on DEMETER observations [9]. Figure 4: The temporal and spatial evolution of the ionospheric ELF/VLF wave intensities at frequency range from 0.2 to 20 kHz at the magnetic field in the dayside ionosphere, revealed by the search-coil magnetometer (SCM) onboard the China Seismo-Electromagnetic Satellite (CSES) during the geomagnetic storm on 26 August 2018. From top to bottom: the PSD values at different frequency bands of (**a**) 15-20 kHz, (**b**) 12-15 kHz, (**c**) 9-12 kHz, (**d**) 6–9 kHz, (**e**) 3–6 kHz, (**f**) 1–3 kHz, and (**g**) 0.2–1 kHz. The overlapping black curves denote the location of plasmapause estimated by the (**h**) Dst index. The vertical dashed line denotes the time of the Dst minimum. Figure 5 shows that the nightside wave activities were weaker than those on the dayside. They also predominantly appeared at frequency below 3 kHz, and the phenomenon of the whole frequency band enhancement only appeared during the hours when the Dst reached its minimum values. The results of the electric field are not presented here, partially because they showed very similar features to those seen in the magnetic field and partially because there were high noises over the equatorial area that originated from instruments or satellite platforms (see the strong horizontal enhancement denoted by black rectangles in Figures 8 and 9 in Discussion Section). We also estimated the location of the plasmapause by using the empirical linear model of [PERSON] [45], which was based on the magnetic indices Kp and Dst values. According to that model, the plasmapause (denoted by the black curves superposed on the plots in Figures 4 and 5) was significantly compressed during the storm time from \(L\) shell values of \(-\)5 to \(-\)3. According to the estimated plasmapause location, it can be said that the strong wave emissions at the frequencies below 6 kHz mainly occurred at a broad \(L\) shell extension (covering the \(L\) shell from \(-\)1 to 7), both inside and outside the plasmapause; meanwhile, waves at frequencies higher than 9 kHz mainly appeared outside the plasmapause (\(L\) shell \(-\)3 to 7), which means that waves in this portion of the spectrum mainly appeared in the radiation belt (more likely in the outer radiation belt). Figures 6 and 7 show the temporal and spatial evolution of energetic electron fluxes at energies from 0.1 to 3 MeV in the day and night side ionospheres, respectively.