## 4 Discussion on the Corresponding Processes in Ionosphere
### The Comparison of Plasma Parameters at Different Altitudes
The PAP on CSES recorded the ion-related parameters, including ion composition, density, temperature, and drift velocity. However, since the last half of the month in March 2018, the number of ions reduced largely and the hydrogen and helium ions cannot be distinguished, possibly due to the contamination from the unknown materials. So the absolute measurement from PAP is questionable, but the relative measurement may be taken as a reference due to its stable observing condition and results. By comparing the ion drift velocity along the revisited orbits at UT 06:00 of #3117-0 and #2813-0 on 26 August and 6 August, respectively, it is found that at the equatorial area, the ions were accelerated in a northeast direction, but there was almost no change in vertical drift velocity of ions, which illustrates that a vertical electric field was overlapped on the equatorial ionosphere on 26 August relative to the normal state on 6 August, and it caused the horizontal movement of ions due to \(E\times B\) effect. Different observations demonstrated that the strong electric field was only excited at the main phase of this magnetic storm and influenced the distribution of Ne at the equatorial area to enhance the double peaks. During the recovery phase, the double peak effect was maintained at the peak region of F2 layer to the altitude of Swarm-A at about 450 km but not to the height of CSES of 507 km. Figure 9: The 2 h-averaged TEC curves at different latitudes onboard Swarm A and B during 24–31 August in 2018 ((**a**,**b**) represents Swarm A and (**c**,**d**) represents Swarm B at local day and nighttime). [PERSON] et al. [45] used the multi observations to study the ionospheric responses during the recovery phase of this storm, and they found that the high speed of solar wind from 27-30 August was effective in changing the aurora and convection patterns, and the high solar wind velocity contribute to enhance the daytime eastward electric fields in the low-latitude ionosphere and thus affect the ionospheric fountain structure at the equatorial region. To verify this feature, the linear correlation (r) was calculated between solar wind speed, pressure, and the TEC as listed in Table 3. Negative correlation was exhibited in speed/TEC either in local daytime or nighttime from SWARM A and B, while mostly positive correlations are found in P/TEC (Pressure/TEC). During the whole magnetic storm event time period of 24-31 August, the absolute correlations of speed/TEC in nightside were larger than those in dayside, which illustrates the enhanced action from solar wind speed on the TEC in the nightside. For the recovery stage during 27-31 August, the negative correlation exceeded \(-0.5\), being larger than that in P/TEC, which demonstrates that solar wind speed may play an important role at low and middle latitudes in the nightside of LT21, especially during the recovery phase of this storm. The pressure contributed a lot during the whole event time of 24-31 August, and the correlation decreased during the recovery phase of 27-31 August and smaller than that in r of speed/TEC. Compared with our results in Figure 2, the electric field at the three extremely low frequency bands under 1125 Hz penetrated into the middle and low latitudes not only in the main phase but also continued until even 29 August during the whole recovery phase, where the high solar wind speed also maintained larger than 400 km/s in this time period. So the penetration of the electric field was confirmed by the observations at ULF and ELF frequency bands of CSES, and the effects on the double peaks of Ne at the equatorial area in the daytime, as shown in Figure 6, could be well explained by the simulation of [PERSON] et al. [45]. [PERSON] et al. [46] discussed the solar origin of this magnetic storm and its response in the ionosphere at medium and low latitudes, and they also illustrated the prompt penetration electric field, contributing to equatorial anomaly crests within \(\pm\)15\({}^{\circ}\) of magnetic latitudes.