## 2 The CSES Satellite and Its Primary Application in EQ Prediction Practice
### Application of CSES Data
Four pictures (one month's data) are usually exhibited once a week; however, an ionospheric anomaly is valid only within the next two weeks once an anomaly is presented, referring to the seismo-ionospheric results obtained by previous work [6; 7; 8; 9; 11; 12; 13; 14; 15; 24]. The relative variations in ionospheric parameters are generally checked. It is usually considered to be an anomaly if the observed value is higher than 3.0 \(\times\) 10\({}^{10}\) m\({}^{-3}\) for ion density and 2.0 \(\times\) 10\({}^{10}\) m\({}^{-3}\) for electron density, with a little variation sometimes. At the same time, if the two parameters of ion density and electron density record abnormalities simultaneously, we consider these varieties are more available. However, variations in southern China usually gain less attention due to their locations near the equator, where ionospheric parameters vary strongly, and where there are magnetically conjugated areas of the Sumatra, where strong seismic activities occur frequently. At the same time, transformations in the magnetically conjugated areas of the Chinese mainland are usually also paid less attention due to the different seasonal ionospheric various backgrounds of the two opposite hemispheres. Figure 4: Variations in O\({}^{+}\) density during the periods of **(a)** April 28–May 4, **(b)** 5–11 May, **(c)** 12–18 May, and **(d)** 19–25 May 2021. The Maduo \(M_{\rm S}\) 7.4 EQ is labeled with a red dot. Figure 5: Variations in electron density during the periods of **(a)** April 28–May 4, **(b)** 5–11 May, **(c)** 12–18 May, and **(d)** 19–25 May 2021. The Maduo \(M_{\rm S}\) 7.4 EQ is labeled with a red dot. Figures 4a and 5a present the changes in these two parameters of all orbits covering the Chinese mainland from 28 April to 4 May 2021, three weeks before the Maduo EQ. It is clear that there was an enhancement either on the ion density or on the electron density near the epicenter of the impending event (see Figures 4a and 5a). However, we did not pay attention to these variations at that time, because similar variations also occurred in other broad areas with similar latitudes (see Figures 4a and 5a). We usually treat these kinds of changes as less important in our daily data processing work. The fluctuations we noted were the ones that appeared 5-11 May 2021, and they are presented in Figures 4b and 5b for ion density and electron density, respectively. These two obvious anomalies were of typical character. They were collective and obvious in our focus area. In addition, the ionic abnormity and the electronic abnormity occurred simultaneously in the same area in a solar quiet period, but there were none in other areas (see Figures 4b and 5b), which gained more probability as EQ precursors. So, we suggested that there would be strong seismic activities in the surrounding area of the anomaly within two weeks, but we could not specify a magnitude and a location for the impending event. However, we could not find continuous abnormalities appearing in this area over the next two weeks, although there were also simultaneous variations in these two parameters on the Chinese mainland due to the same reason shown in Figures 4a and 5a (see also Figures 4c,d and 5c,d). One difference is that the obvious wide anomalies in the north of China shown in Figures 4c,d and 5d were possibly associated with the high background ionospheric variations described in Figure 1.  
A large EQ with a magnitude \(M_{\rm S}\) 7.4 occurred on 22 May 2021 in the Maduo area, Qinghai province, with an epicenter located at 34.59\({}^{\circ}\) N and 98.34\({}^{\circ}\) E and a depth of 17 km, within two weeks after the obvious ionic and electronic variations recorded by the CSES satellite occurred during 5-11 May 2021. After the event, we retroactively investigated and revealed more details about this EQ and related ionospheric information to confirm their relationship.