## 3 Automatic Detection Method
At the same time, variations in different plasma parameters like the electron density, electron temperature, as well as ion densities were also examined. Pictures of these parameters from the CSES satellite data processing system are displayed from top to bottom in Figure 5. In Figure 5, the detected electron perturbation listed in Table 3 as Pe1 is labelled with a black arrow in the top panel. From Figure 5, all parameters are totally characterized by a synchronous variation, first a decrease and then an increase, but not all perturbations were detected via the software with a specified space size of 2 min. The Kp index with eight three-hourly averaged values each day was examined from 20 January to 3 February 2024 (Figure 6) to avoid the ionospheric influence from solar activity. It is clear from Figure 6 that the Kp value on 23 and 24 January reached 3.7, indicating that ionospheric variations in the same period possibly originated from geomagnetic disturbances. For this reason, we also checked geomagnetic horizontal component H and geoelectric EW (east-west) component during 20-25 January 2024 at the ground-based Hongqian observing station (45.4\({}^{\circ}\)N, 84.8\({}^{\circ}\)E), only 80 km from the night orbit 33175 (See Figure 4a), and the fluctuations of these two parameters are shown in Figure 7. From Figure 7, obvious perturbed signals appeared for both the geomagnetic H component and geoelectric EW component in the same period when the electron density, as well as other ionospheric parameters, suffered an abrupt decease, which is called electron depletion or a plasma bubble [36]. This obvious depletion lasted only 1 min, less than 500 km in space. Orbits within 500 km from this plasma bubble orbit have also been examined during 21-25 January 2024 (a total revisiting period), but no similar phenomena happened. After that, three aftershocks with magnitudes of \(M_{\rm S}\) 5.2, \(M_{\rm S}\) 5.6, and \(M_{\rm S}\) 5.7 took place, respectively, on 25, 26, and 30 January (see Table 2 and Figure 3), approximately 700 km away from the location where the electron perturbation was searched on 24 January. \begin{table}
\begin{tabular}{l l l l l l l l l l l} \hline  & & **Pe1** & & & **Pe2** & & & **Pi2** & \\ \hline Date (y m d) & 2024 & 1 & 24 & 2024 & 2 & 3 & 2024 & 2 & 3 & \\ Time (h m s ms) & 20 & 49 & 48 & 975 & 20 & 50 & 28 & 465 & 20 & 50 & 26 & 464 \\ Orbit & 33175 & & & 33327 & & & & 33327 & & \\ Suborb & 1 & & & 1 & & & 1 & & \\ Latitude (\({}^{\circ}\)) & 47.7407 & & & 50.4960 & & & 50.3701 & & \\ Longitude (\({}^{\circ}\)) & 82.8527 & & & 82.0411 & & & 82.0913 & & \\ BkgdIon (\(\times 10^{6}\) cm\({}^{-3}\)) & 82,726.0 & & & 51,063.0 & & & 42,276.4 & & \\ Amplitude (\(\times 10^{6}\) cm\({}^{-3}\)) & 11,4377.0 & & & 63,603.4 & & & 53,827.6 & & \\ Trend & Increase & & & Increase & & Increase & & & \\ Percent (\%) & 38.3 & & & 24.6 & & & 27.3 & & \\ Time\_width (m s ms) & 1 & 30 & 0 & 1 & 9 & 0 & 1 & 12 & 7 \\ Extension (km) & 639 & & & 489 & & & 510 & & \\ \hline \end{tabular}
\end{table} 
Table 3: Information of night ionospheric perturbations detected via the software during 24 January–24 February 2024. [PERSON] et al. [24] have reported that obvious CSES ionospheric variations appeared 11 days and 6 days (considering a revisiting period of 5 days for the CSES satellite) prior to the 8 January 2022 Menyuan \(M_{8}\) 6.9 earthquake. At this time, neither electron perturbations nor ion perturbations were searched on the next revisiting night orbit 33251 on 29 January. Variations in several plasma parameters recorded in this orbit had also been checked and they are exhibited in Figure 8 with the same arrangement as in Figure 5. It is clear that no obvious irregularities had been found around the plasma bubble that appeared on the last revisiting orbit 33175. Figure 5: Variations in the ionospheric parameters along night orbit 33175 on 24 January 2024. (**a**) Electron density (top line) and electron temperature (bottom line). (**b**) He\({}^{+}\) density (top line) and O\({}^{+}\) density (bottom line).