# Recent Advances and Challenges in the Seismo-Electromagnetic Study: A Brief Review
## 4 Seismo-Electromagnetic Phenomena in the Upper Atmosphere
The precursory worldwide signatures study found some clear concentrations of electron density and magnetic anomalies from more than two months to some days before the earthquake occurrences, and the epicenter location was related to the detected anomalies [112; 114]. There were also some reports about the detection of ionospheric signals in the region magnetically conjugate to the earthquake zone [115]. The China Seismo-Electromagnetic Satellite (CSES) mission, launched in 2018, is the first satellite in China to measure seismo-associated phenomena in the near-Earth electro magnetic environment, having a lot of application prospects in the study of seismology, geophysics, space sciences, and so on [91]. This was planned to operate for 5 years. During its five-year operation time, CSES records many kinds of data, such as multi-band waveform and the spectra of electric and magnetic fields; in situ plasma parameters (including the densities and temperatures of electrons and ions); electron density profiles and tomography; energetic particle flux and energy spectra. Many reports showed that the performance of the payload can satisfy the requirement of the scientific objectives of the mission [116; 117; 118; 119; 120; 121; 122]. Next, we discuss some of the last published results. [PERSON] et al. [123] reported the observations of unusual ionospheric irregularities by CSES before four earthquakes. [PERSON] et al. [65] investigated ionospheric perturbations in very low-frequency transmitters recorded by CSES, and they claimed that the perturbations to the satellite were more likely related to the overlapped electric field in the earthquake preparation area. An investigation into the characteristics of pre-earthquake ionospheric effects related to four shallow-focus earthquakes in Indonesia was conducted by [PERSON] et al. [124]. Significant positive perturbations 1-7 days before earthquakes were revealed. Recently, [PERSON] et al. [125] published the CSES data analysis of electron density (Ne) and temperature (Te), to support a temporal-spatial correlation between the earthquakes and the observation of positive Ne variations/negative Te variations before the seismic events. [PERSON] et al. [126; 127] and [PERSON] et al. [128] analyzed the pre-earthquake electromagnetic, ionospheric, or space electric field perturbations by machine learning. However, similarly to other reports on earthquake precursors, the anomalous variation of parameters has not been revealed in all studied earthquakes, and the perturbations seem to occur at various temporal and spatial distances to the earthquake. Other possible sources of the observed anomalies in the ionospheric and geomagnetic dynamics driven by solar activity may not be fully ruled out. On the other hand, the satellite orbits the earth at a high speed, and the observed anomalous variations cover a large zone even up to a 2000 km range. Overall, due to the relatively low resolutive power, the applications based on the satellite may face a challenge of the better constraint size and location of earthquake preparation areas. Even though the reported analyses from CSES are not conclusive now, the second mission of the CSES series, CSES-02, will be launched in 2023. It will have the same multi-instrumental payload and similar orbital parameters as the first satellite, but will allow for reducing the revisit time of the same geographic region. The large number of observations will help in studying the frequencies, amplitudes, and characteristics of ionospheric perturbations related to earthquakes and generation mechanisms. Among the seismo-electromagnetic phenomena in the ionosphere, anomalous changes in the ionospheric total electron content (TEC) are one of the most frequently reported phenomena preceding large earthquakes [129; 130; 131; 132; 133; 134; 135; 136; 137; 138; 139; 140]. TEC is a measure of integrated density along a straight line between the satellite and ground GPS stations. One TEC unit (TECU) is equal to \(10^{16}\) electrons/m\({}^{2}\).