## 3 Data Analysis  
### The Electromagnetic Disturbances on CSES Satellite  
In order to detect the three components in electric field, four sensors (a-d) are installed at different directions outside the CSES platform with boom lengths of 5 m, where the detailed position of four sensors (a-d) in the satellite coordinate can be found in the paper of [32]. The waveform data are recorded totally in ULF (ultra low frequency) and ELF (extremely low frequency) bands, but only in burst mode in VLF (very low frequency) band. The spectrum has been calculated and averaged onboard CSES in VLF and HF (high frequency) bands under the survey mode, so the electric field spectra between each two sensors, such as Eab, Ecd, and Ead, have been stored and downloaded directly to the ground [32], where the spectra in VLF and HF bands cannot be transferred to satellite or geographical coordinates again on ground. Figure 2 exhibited the power density values in Ead at six frequency bands selected randomly from VLF spectra from 25 to 29 August 2018, including 225 Hz\(\pm\), 725 Hz\(\pm\), 1125 Hz\(\pm\), 5000 Hz\(\pm\),  
Figure 1: The geomagnetic indices curves from 24 to 31 August in 2018 (from top to bottom are respectively flow speed, interplanetary magnetic field (IMF) Bz (black line) and By (red line), flow pressure, interplanetary electric field (IEF) Ey, SYM-H index, and ASY-H index; all these parameters are published at the website of [[https://omniweb.gsfc.nasa.gov/form/omni_min.html](https://omniweb.gsfc.nasa.gov/form/omni_min.html)]([https://omniweb.gsfc.nasa.gov/form/omni_min.html](https://omniweb.gsfc.nasa.gov/form/omni_min.html)), accessed on 8 August 2022). 7500 Hz\(\pm\) and 13,500 Hz \(\pm\) 100 Hz. It can be seen that since the last few hours of 25 August, the electric field signals were enhanced significantly at all chosen frequency points and reached the maximum on 26 and 27 August. On 28 and 29 August, they became weakened but still continued. In the top three panels of Figure 2, the strong disturbed signals not only distributed at high latitudes beyond 50\({}^{\circ}\) but also extended to the low latitudinal region at 20-40\({}^{\circ}\), including to the equatorial region of \(\pm\)20\({}^{\circ}\) on 26 August (Figure 2a-c). Compared to the electric field strength at three frequency points shown in Figure 2a-c, the one at 725 Hz showed the maximal values and larger covering area than that at 225 Hz (Figure 2a), especially on 26 August at the equatorial area. Among the three bottom panels in Figure 2d-f, the electric field had the similar evolutional trend with those at lower frequency points in Figure 2a-c, but their strength reduced at least one order in magnitude. During the main phase of this magnetic storm on 25-26 August, the enhanced electric field signals were mainly located in three regions, mid latitude of 40-60\({}^{\circ}\) in the northern and southern hemispheres, respectively, and the equatorial area. In addition, Figure 2 illustrates the further interaction between the electric field and high electron density in the equatorial region in local daytime during this geomagnetic storm, obviously at higher frequency points in VLF band relative to their normal weak background (Figure 2d-f). These disturbances were not the direct electromagnetic waves induced by the geomagnetic storm but from the secondary effect due to the enhancement of Ne in this region  
In local nighttime, four frequency points in VLF electric field were selected from CSES and exhibited in Figure 3. The similar variational trend was observed from 25 August until 29 during the whole magnetic storm event. At 225 Hz (Figure 3a), the regular peak structures can be found at latitude of 40-65\({}^{\circ}\) each day, typically in the southern hemisphere with strong electric field perturbations.