## 2 The CSES Satellite and Its Primary Application in EQ Prediction Practice
### Application of CSES Data  
The CSES ion (O\({}^{+}\)) density and electron density data were put into service as candidates for EQ precursory searching work in China at the end of April 2020, after a series of running tests and data calibrations. Data utilized here are available online: [[https://www.leos.ac.cn/accessed](https://www.leos.ac.cn/accessed)]([https://www.leos.ac.cn/accessed](https://www.leos.ac.cn/accessed)) on 1 March 2022 with the time resolution of 1 s for O\({}^{+}\) density and 3 s for electron density. We know there are some disadvantageous factors during seismo-ionospheric response discrimination, such as the ionospheric anomaly surrounding the equator and the South Atlantic Magnetic Anomaly (SAMA). Fortunately, the Chinese mainland is located within the low-mid latitude in the northern hemisphere, an advantageous area to detect EQs, and the appearance probability of ionospheric perturbations within one week could up to be 80%, although with a lower detection rate [14; 15]. We also examined the seasonal background variations in ionospheric parameters, such as O\({}^{+}\) density. Figure 1 shows the averaged seasonal ionic changes at night on the Chinese mainland with a 2\({}^{\circ}\times 4^{\circ}\) grid in latitude and longitude, and the daily Kp index available online: [[http://sigi.unistra.fr/data_download.php](http://sigi.unistra.fr/data_download.php)]([http://sigi.unistra.fr/data_download.php](http://sigi.unistra.fr/data_download.php)) accessed on 1 March 2022 remained below three during 2020. It is well known that China is located at the middle latitudes of the northern hemisphere, where with a low ionospheric background, the total fluctuation of the ionosphere is relatively stable. The strengthening of the sun in the summer can induce corresponding ionospheric perturbations compared with the spring, which leads to a higher background value in the summer as shown in Figure 1a,b. In addition, there is a relatively high ionospheric area in northern China corresponding to a geomagnetic anomaly [34]. However, the Kp index is an important parameter we have to consider, aiming to avoid the global effect on the ionosphere from solar activities. A strong magnetic storm occurred on 26 August 2018, an example that illustrates this issue. The CSES recorded this event comprehensively. Figure 2 exhibits the fluctuations in the Kp index from 20 to 31 August 2018. One can see clearly from Figure 2 that a strong magnetic storm occurred on 26 August 2018, and its Kp index was up to seven. This enhanced event had a strong influence on the ionospheric densities. Figure 3 comparatively displays the variations in the ion density recorded on 26 and on 24 August 2018. Figure 3 shows that the ionosphere can register more effect from solar activities during the daytime than the nighttime (see Figure 3a,b). Compared with the magnetic quiet period, ionospheric irregularities can gain strength when a strong magnetic storm occurs (See Figure 3). Thus, only nighttime data were utilized during our daily use in order to avoid the influence from solar activities. If the daily averaged Kp index was equal to or more than 3 and the 3-hourly averaged Kp index was more than 4, the corresponding data were usually eliminated. However, we usually accepted all data if the number of days with Kp \(\geq\) 3 was more than 2 in order to keep the completeness of the data (considering the 5-day revisiting period of the CSES satellite). Of course, an ionospheric anomaly appearing during this time is less reliable. Figure 1: Averaged seasonal variations of O\({}^{+}\) density for the periods of (**a**) January–March, (**b**) April–June, (**c**) July–September, and (**d**) October–December 2020. Figure 2: Variations in the Kp index from the middle to the end of August 2018. We process the data week by week in order to keep the rhythm of a weekly consultation of the EQ prediction work.