# Retrospective Study on Seismic Ionospheric Anomalies Based on Five-Year Observations from CSES
## 1 Introduction  
Strong earthquakes (EQs) are one of the greatest destructive natural disasters faced by human beings, posing significant threats to human life and property safety. Every year, more than 5 million EQs occur worldwide, including about 100 EQs with a magnitude over 6 (M \(\geq\) 6), around 18 EQs with M \(\geq\) 7, and one or two strong EQs with M \(\geq\) 8 [1]. Therefore,it is important to perform sufficient research on EQs and to attempt early warning and forecasting for strong EQs. With the development of space technology, anomalies associated with EQs are increasingly observed in the atmosphere and ionosphere, in addition to the lithosphere [2], especially when more short-impending anomalies are observed in the ionosphere. More and more seismologists have explored the seismic abnormality of ionospheric parameters through case studies [3; 4] and statistical research [5; 6; 7]. A large number of studies have confirmed the relationship between EQs and ionospheric anomalies [8], and reported that seismic anomalies can manifest anywhere from a few minutes to 30 days prior to a significant earthquake. For instance, [PERSON] [9] observed anomalies in ionospheric TEC approximately 40 min before the earthquake in NE Japan. [PERSON] et al. [10] documented anomalies two days prior to the Cucapah earthquake. [PERSON] et al. [11] identified anomalies six to seven days prior to the Morocco earthquake in September 2023. Additionally, [PERSON] et al. [12] reported anomalies occurring over a period of 1-30 days. [PERSON] et al. [13] statistically determined that about 94% of earthquake-related TEC anomalies occur approximately seven days prior to the event. Furthermore, ionospheric disturbances within a 1000 km radius from the epicenter have been identified as significant [3; 5], and in most cases, [PERSON]'s radius remains the critical distance at which anomalies are observed [14; 15]. Currently, the ionospheric anomalies related to EQs have been widely studied and discussed by the international scientific community, and electromagnetic satellites are considered one of the most effective methods for studying EQ precursors [16], bringing new opportunities to promote the capacity of EQ prevention and disaster reduction. The Lithosphere-Atmosphere-Ionosphere Coupling (LAIC) mechanism aims to research the processes of seismic signal coupling, providing theoretical support for ionospheric anomaly observation research [17; 18]. The launches of several electromagnetic satellites have provided abundant data for researching seismic ionospheric anomalies. Specifically, the DEMETER satellite, which was successfully launched in June 2004 in France [19], opened up a new era of seismic ionospheric observation and has greatly boosted the development of this research field. Following the steps of DEMETER, China successfully launched its first seismic electromagnetic satellite in February 2018, which is called the China Seismo-Electromagnetic Satellite (CSES-01), providing a new platform for near-earth space observation and seismic ionospheric research [20]. Currently, the second satellite (CSES-02) is being prepared and will be launched around the end of 2024 or early 2025 [21]. With two satellites (CSES-01 and CSES-02) in orbit simultaneously, more abundant observational data are expected in future years. Despite decades of research, including space technology, anomaly extraction, and LAIC mechanism, a series of remarkable results have been achieved, but several problems still require further resolution. For example, the anomalous characteristics of the same parameter from different EQ cases may vary depending on the different processing methods and criteria of recognition; the relationships of anomalies among different parameters are unclear [22]. The LAIC cannot fully explain the abnormal phenomena because of the differences between the observation and the LAIC simulation [23].