## 5 Conclusions  
In this work, we analyzed the characteristics of ionospheric plasma perturbations before four strong EQs with magnitudes ranging from Ms 6.9 to Ms 7.4 occurred in southeast Asia and northern Oceania (in the south of the circum-Pacific seismic belt) from August to October 2018, which were observed by PAP and LAP onboard CSES-01 and were supported by the ULF waveform results of EFD. The different variations of \(No^{+}\) and \(V_{z}\) under intense geomagnetic storms and strong EQ were compared. Our findings support the possibility of applying in-situ plasma observation of LEO satellites to examine ionospheric pre-EQ signatures, which could be useful to understand seismo-ionospheric precursors better. The main results of this study are summarized as follows:  
1. The \(No^{+}\), \(Ne\), \(Te\), and \(V_{z}\) values show significant synchronous disturbances at the same position near the epicenters from 1 to 15 days before the series of strong EQs occurred in southeast Asia and northern Oceania within 50 days. Regarding time and space characteristics, these plasma perturbations are possibly explained as seismo-ionospheric precursors.
2. The \(No^{+}\) and \(V_{z}\) seemed like promising parameters for analyzing ionospheric disturbances excited by strong seismic activity. Local ionospheric perturbations caused by strong EQs are often accompanied by a change in \(V_{z}\), which is not easily influenced by the geomagnetic storm. Meanwhile, the variation of \(No^{+}\) could reflect the combined effects induced by strong EQ and intense geomagnetic storms on the ionospheric plasma environment.
3. Based on the long-term data analysis results of CSES-01, the absolute value differences of plasma in different locations are large. It may be more effective to extract precursory seismic anomalies by long-term monitoring of the critical areas and checking the relative abnormal changes of the revisiting orbits.  
**Author Contributions:** [PERSON], [PERSON] and [PERSON] contributed to the conception and methodology. [PERSON] analyzed the data and wrote the original draft. [PERSON], [PERSON] and [PERSON] wrote some programs. [PERSON], [PERSON] and [PERSON] checked and organized the database. [PERSON] reviewed the expression. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research is supported by the National Natural Science Foundation of China (grant number 41874174), the Natural Science Foundation of Beijing (grant number 8184091), and the ISSI-BJ project.  
**Data Availability Statement:** The CSES-01's plasma observation data used in this paper can be downloaded from the CSES data service website: [[https://www.leos.ac.cn](https://www.leos.ac.cn)]([https://www.leos.ac.cn](https://www.leos.ac.cn)) (accessed on 1 September 2022). The earthquake data were downloaded at: [[http://news.ceic.ac.cn](http://news.ceic.ac.cn)]([http://news.ceic.ac.cn](http://news.ceic.ac.cn)) (accessed on 10 February 2023) and [[https://earthquake.usgs.gov](https://earthquake.usgs.gov)]([https://earthquake.usgs.gov](https://earthquake.usgs.gov)) (accessed on 8 May 2023), and the geomagnetic index data were downloaded at: [[http://wdc.kugi.kyoto-u.ac.jp](http://wdc.kugi.kyoto-u.ac.jp)]([http://wdc.kugi.kyoto-u.ac.jp](http://wdc.kugi.kyoto-u.ac.jp)) (accessed on 5 March 2023).  
**Acknowledgments:** This work utilizes the data from the CSES mission. We acknowledge the experts who helped improve the quality of this paper.  
**Conflicts of Interest:** The authors declare no conflict of interest.