# Recent Advances and Challenges in the Seismo-Electromagnetic Study: A Brief Review
## 4 Seismo-Electromagnetic Phenomena in the Upper Atmosphere
Various GPS stations can be utilized for ground-based TEC observations, including the International GNSS Station (IGS), the Crustal Movement Observation Network of China (CMONC), the GPS Earth Observation Network (GEONET), and so on. Two-dimensional TEC maps derived from ground-based observations are widely used, such as the data from the Center for Obit Determination of Europe (CODE), and they are regarded as one of the most precise available Global Ionosphere Maps (GIM) and treated as a high-accuracy reference [141]. In contrast, space-borne TEC data are rare due to the lack of processing TEC data in the satellite platform. However, satellite systems can overcome the problem that ground stations are not available in the ocean [142], and act as a supplement to GIM-TEC. The method to detect anomalies in the ionosphere includes the mean/median statistical envelope method [143; 144], cubic fit analysis [140], pattern recognition analyses [145; 139], etc. The mean/median statistical envelope methods use the N-day mean or median techniques to detect anomalies, becoming a common and popular method. However, there is no clear justification for the selection of N-day used in studies, and the method faces the challenge of accounting for TEC changes owing to different sources such as space weather and seismic effects. The confidence level, which defines the normal ionospheric variations, also needs to be ascertained in the mean statistical envelope methods. The cubic fit analysis was adopted by [PERSON] to obtain the anomalies about five TECU several hours/minutes before the 2011 Tohoku earthquake. The study indicated that similar preseismic TEC anomalies were seen in the 2010 Chile earthquake (Mw8.8), the 2004 Sumatra (Mw9.2), and the 1994 Hokkaido (Mw8.3) earthquake, but not in smaller earthquakes [140]. The anomalies even increased to 10 TECU before the Sumatra earthquake (Mw9.2). Some scholars point out that it might yield erroneous interpretations about anomalies [147; 148]. [PERSON] and [PERSON] introduced the correlation analysis method and detected the preseismic TEC anomalies before the 2011 Tohoku earthquake and the 2016 Kumamoto earthquake, whose characteristic patterns do not depend on the choice of extrapolation functions for nonlinear regression [149; 150]. In the research on seismo-electromagnetic phenomena in the ionosphere, the effects of other sources should be considered because the TEC perturbations are the integrated product of space weather and other kinds of sources such as earthquakes, tsunamis, volcanic eruptions [151], etc. The choice of space weather indices could be justified based on the location, depending on the data availability and station coverage [152]. When defining the space effect levels, some consensus values should be used based on the authoritative parameters from space physics, and all relevant space weather indices should be verified for quiet or low activity in order to ensure the space-related influences on ionospheric perturbations [142]. Some technological methods were developed to eliminate the effect of space weather activity without the use of arbitrary space indices to detect pre-earthquake TEC anomalies, including wavelet analyses [153], singular spectrum analysis [151; 154], etc. In addition, demonstrating the three-dimensional structure of ionospheric disturbances can better reshape the morphological characteristics [155; 156]. In general, it is very important to establish a reliable normal background field, exclude the interference effects, and identify reliable anomalies related to seismic activity [157]. In the review report of [PERSON] et al. [16], the total electron content (TEC) over the epicenter usually increased or decreased for approximately 4-6 h on 1-5 days before earthquakes, and the TEC anomalous amplitude was related to the earthquake magnitude and local time, ranging from \(\pm\)30% to \(\pm\)100%. Meanwhile, some statistical investigations into the relationship between earthquakes and TEC anomalies were performed. [PERSON] et al.