## 3 Observations
As seen in Figure 2c,d, before the storm initial phase that began at epoch time \(t_{0}-16\) h (14:00 UT on 25 August 2018) and indicated by a slight increase of Dst index, the ionosphere remained in a relatively quiet electromagnetic environment (denoted by the blue areas in Figure 2c,d from epoch time \(t=-30\) h to \(-16\) h). Meanwhile, along with the consistent southern reversal of IMF \(Bz\) and the corresponding decrease of the Dst index, the wave activities started to grow up (\(\sim\)\(t=-9\) h) and reached ELF/VLF maximum emissions around the end of the main phase and the beginning of the recovery phase (\(t=-2\) to 20 h). We can see a close correlation between the ELF/VLF wave activities, the IMF \(Bz\), and the solar wind (velocity and pressure); when IMF \(Bz\) and solar wind were stable at times when IMF \(Bz\) was close to 0 nT, the wave activities were weak (e.g., 20 h \(<t<\) 28 h and 60 h \(<t<\) 72 h), while when the solar wind conditions were strongly fluctuating, strong ELF/VLF emissions were observed (e.g., 2 h \(<t<\) 20 h). Figure 1: The variation of solar wind conditions and geomagnetic Dst and Kp indices from 20 August to 4 September 2018. From top to bottom: (**a**) the \(z\) component of the interplanetary magnetic field in the geocentric solar magnetospheric (GSM) coordinate system (interplanetary magnetic field (IMF \(Bz\)), (**b**) solar wind velocity, (**c**) solar dynamic pressure, (**d**) geomagnetic activity Kp index, and (**e**) Dst index. Figure 2e shows that the energetic electron populations at 1 MeV \(<E<\) 3 MeV were also smaller before the initial and early main phases but gradually increased to a higher level during the near-end main phase and remained to be enhanced during the whole recovery phase. The energetic electron populations were stronger at \(L\) shell between \(\sim\)2 and 3, where it was expected to find the inner radiation belt [26]. Figure 2 shows that both  
Figure 3: Same as Figure 2 but for the nightside ionosphere. Figure 2: The variation of ionospheric extremely/very low frequency (ELF/VLF) wave intensity and energetic electron fluxes during the geomagnetic storm that occurred from 25 to 29 August 2018. **(a)** IMF \(Bz\) in the GSM coordinates, **(b)** Dst index, **(c,d)** the power spectral density (PSD) values of the magnetic and the electric field at frequency range from 1 to 2 kHz, and **(e)** the energetic electron fluxes at energy level from 1 to 3 MeV. Data were integrated by the superposed epoch method [9], and the vertical solid line denotes the reference time \(t_{0}\) that is the time of Dst minimum on 06:00 to 07:00 UT on 26 August 2018. The y axis in (**c**–**e**) indicates the \(L\) shell from 1 to 7. The white blank regions denote data gaps. waves and fluxes clearly reflected the remarkable temporal variability of the boundary of the inner radiation belt (i.e., filling and fluctuations of the magnetosphere slot region). Figure 3 shows the results for the nightside ionosphere, where the ELF/VLF wave activities and electron fluxes also showed a close correlation of the different storm phases, though with a relatively weaker intensity than the dayside. It is interesting that in the nightside ionosphere, the waves at 1 kHz \(<\)\(f\) \(<\) 2 kHz were predominantly enhanced during the main phase in contrast to the ones at dayside; this feature is worth further exploration after more storm cases are accumulated. To further depict the temporal and spatial distribution of the wave activities of all the ELF/VLF frequency bands, we divided the electromagnetic field measurements into seven frequency bands: 0.2-1 kHz, 1-3 kHz, 3-6 kHz, 6-9 kHz, 9-12 kHz, 12-15 kHz, and 15-20 kHz.