## 3 Research Methodology  
The data recorded by the CSES can be divided into up-orbit data (night side) and down-orbit data (day side), corresponding to approximately 02:00 and 14:00 local time, respectively. To minimize unnecessary perturbations, we selected nighttime data (up-orbit data) for processing and analysis due to the notable influence of solar radiation on the ionosphere during the daytime, which affects the extraction of seismic information [28]. We selected orbital data under the condition of space magnetic disturbance (Dst \(\geq-30\) nT and Kp \(<3\)). More than 4000 orbits were selected from over 160,000 orbits in this study. The seismogenic area of an Ms \(>5.0\) earthquake is \(-142\) km, according to Equation (1). The speed of the CSES is 7.8 km/s, which suggests that it traverses the earthquake area within 18 s. The orbit time of the CSES ranges from 2028 to 2078 s. The perturbation anomaly of the ionosphere generates a certain offset effect, which can produce a shift up to 10\({}^{\circ}\). Therefore, in this paper, the minimum study period was divided into 10 s intervals. That is, every 10 geomagnetic three-component vector data, i.e., the horizontal north-south component, horizontal east-west component, and vertical component, were performed with the Fourier transform to obtain spectral data, respectively, as the FGM sampling rate is 1 Hz. Ground-based data were selected from the second-sampled instrument of GM4 with a period of 5-100 s, i.e., data with a frequency ranging from 0.01 to 0.2 Hz, to extract seismic anomalies [8]. Therefore, in this study, filtering analysis was performed after the above Fourier transform to obtain the dynamic spectrum of the geomagnetic three-component vector data (frequency: 0.01-0.2 Hz) recorded by the HPM probe. The filtering analysis meant that the amplitudes with frequency \(>0.2\) Hz or \(<\)0.01 Hz were all 0. We selected the 0.01-0.2 Hz geomagnetic three-component dynamic spectra recorded by the CSES from 90 days before to 30 days after a given earthquake.  
Polarization values N of the observations from 90 days before to 30 days after an earthquake were obtained, along with average polarization values M (with M as the background value) and the mean square S of the polarization values over the 4-year period, according to Equations (2) and (3), respectively. The time series of the perturbation magnitude F of the observations relative to the background values was obtained for analysis according to Equation (4).  
\[\mathrm{H}=\sqrt{\mathrm{H_{x}^{2}+H_{y}^{2}}} \tag{2}\]  
\[\mathrm{Y_{zh}}=\left|\frac{\mathrm{Z}}{\mathrm{H}}\right| \tag{3}\]  
where H\({}_{x}\) is the north-south spectrum of the geomagnetic horizontal component, H\({}_{y}\) is the east-west spectrum of the geomagnetic horizontal component, H is the spectrum of the geomagnetic horizontal component, Z is the spectrum of the geomagnetic vertical component, and Y\({}_{zh}\) is the polarization value (the ratio between vertical component Z and horizontal component H) [7; 8; 9].  
\[\mathrm{F}=(\mathrm{N}-\mathrm{M})/\mathrm{S} \tag{4}\]  
where N is the polarization value from 90 days to 30 days after a given earthquake, M is the background value, i.e., the average of the polarization values over the 4-year period, and S is the mean square of the polarization values over the 4-year period.