## 5 Discussion  
In addition to the single case studies, some statistical studies were also recently carried out using CSES data. For example, [PERSON] et al. [16] used the O+ density and electron density data from CSES and the DEMETER satellite to search for seismo-ionospheric perturbations with different time resolutions. The results showed that the detection rate of the seismo-ionospheric disturbance increases as the data time resolution and the earthquake magnitude increase and as the focal depth decreases, proving that the CSES could effectively register seismo-ionospheric perturbations as the DEMETER satellite does [16]. [PERSON] et al. [36] statistically examined the -2.5 years of Ne data from CSES during the M 4.8+ EQs worldwide. They found that significant positive Ne variations related to earthquakes mainly occurred \(\sim\)1 to 7 days and \(\sim\)13 to 15 days before the earthquakes.  
Although numerous independent studies have suggested the seismo-ionospheric disturbances, the earthquake preparation process in the lithosphere and the seismic precursor transport process from the ground to space are still poorly understood due to various variables involved. The pioneering scientists established the Lithosphere-Atmosphere-Ionosphere Coupling (LAIC) models [4; 6; 10] mainly based on theoretical analyses and observations. [PERSON] et al. [37] reported a good correlation between CSES's observation and the full-wave-mode simulation on the VLF radio waves emitted by the ground-based transmitters, confirming the propagation capability of the electromagnetic waves from the lithosphere to the ionosphere. However, due to the complicated seismogenic process and the limited understanding of the seismo-ionospheric phenomena, there are many challenges in the physical process interpretation.  
[PERSON] et al. [38] put forward three critical problems in this field: (1) how to perform a reliable and appropriate observation with some precise physical quantities; (2) how to develop a robust methodology to reveal weak earthquake-related electromagnetic signals from the noisy background; and (3) how to develop plausible physical models based on theoretical analyses and/or laboratory experiments for the explanation of the earthquake-related electromagnetic signals observed under the field conditions.  
To obtain convincing seismo-ionospheric evidence, we pay considerable attention to data cleaning and data quality control before seismic information extraction. We have performed specific validations on CSES data, and some works about the data quality of the electromagnetic field [37; 39; 40] and the plasma data [31; 41] have been published.  
It is admitted that the seismic information analysis methods are still empirical. At present, we interpret data using four methods: the single-orbit analysis, the multi-orbit data analysis, the background map, and the comparative studies with multi-source data recorded from the ground to space. These methods are not sufficient to extract accurate seismic signals. Objectively and correctly extracting the feeble seismic information from the noisy  
Figure 9: The variation in VTEC over the 29 July 2021 M 8.1 and August 14 M 7.0 Alaska epicenters.  
electromagnetic background is still a challenging task, needing a complex multidisciplinary approach to clarify the short-term earthquake precursors by combining the observations at the different altitudes from the lithosphere to the ionosphere [6].