## 3 Observations  
Figure 1 shows major parameters describing the solar wind conditions and the geomagnetic activity indices Dst and Kp from 20 August to 4 September 2018. It can be seen that the north-south component of the interplanetary magnetic field (IMF \(Bz\)) in the GSM (geocentric solar magnetospheric) coordinate system suffered a slight northern enhancement of up to \(\sim\)9 nT at -14:00 UT on 25 August 2018 before suddenly turning southward with a steep drop to the minimum value of \(-\)16.8 nT at 05:00 UT on 26 August, followed by a very fluctuating recovery back to an average value of \(\sim\)0 nT. This prolonged southward alignment of the IMF \(Bz\) triggered a dayside magnetic field line reconnection at the magnetopause, directly inducing a strong geomagnetic storm during which the geomagnetic equatorial Dst index dropped to the minimum value of \(-\)174 nT at 06:00 to 07:00 UT on 26 August, followed by a long-term recovery phase lasting until 3 September 2018. The source for this intense geomagnetic storm was identified as the interplanetary coronal mass ejection (ICME) from the Sun that occurred downstream of a filament eruption on 20 August 2018 [43]. In particular, from the point of view of solar wind velocity, this ICME event was pretty interesting, since a fast speed of more than 600 km/s, along with the subsequent consistent slowdown to less than 400 km/s and slight recovery, was reached prior to the onset of the geomagnetic storm before again giving rise to a fast speed (up to \(\sim\)500 km/s) in the early recovery phase and a new marked decrease in the late part of the recovery. The solar wind dynamic pressure, which was a direct indicator of solar wind and ion and electron density, intensely increased during the main and early recovery phases, reached the peak at the early recovery phase, and remained fluctuating around 2 nPa for the most of other times. The effects of this geomagnetic storm on Earth's ionosphere were investigated by [PERSON] et al. [44] based on multi-parameter measurements, including total electron content (TEC), geomagnetic field intensity, and O/N2 ratio data, and their results showed that a positive ionosphere storm (increase of TEC) in the southern hemisphere and a negative storm in the northern one were formed. [PERSON] et al. [5] also reported simultaneous responses from multi-type CSES payloads to this event, thus leading to the assessment of the good performance of the CSES in reacting to the different phases of the storm. Strong ELF/VLF emissions were especially observed, with the simultaneous enhancement of energetic electron fluxes in the South Atlantic Anomaly (SAA), outer/inner radiation belt, and slot region [5]. In the following section, we mainly focus on the temporal and spatial distributions of the storm-time ELF/VLF waves across a broad frequency band from 200 Hz to 20 kHz, as well as the energetic electron fluxes at energies 0.1 MeV \(<E<\) 3.0 MeV by using data from the SCM, EFD, and HEPP-L onboard the CSES. We adopted the superposed epoch analysis method by defining the reference time \(t_{0}\) as the time of Dst minimum (from 06:00 UT on 26 August 2018), as we did in previous work [6]. We mainly computed the average power spectral density (PSD) values of the electric and magnetic field at a frequency range of 1 kHz \(<f<\) 2 kHz, as well as the average energetic electron fluxes data at an energy level of 1 MeV \(<E<\) 3 MeV for each orbit. Then, these data were binned as a function of \(L\) shell in steps 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 (namely, from 00: 00 UT 25 August to 00:00 UT 30 August). The results are presented in Figure 2 (dayside) and Figure 3 (nightside).