# Recent Advances and Challenges in the Seismo-Electromagnetic Study: A Brief Review
## 4 Seismo-Electromagnetic Phenomena in the Upper Atmosphere  
The seismo-electromagnetic phenomena in the upper atmosphere are usually detected by using satellite observations and/or ground-based transmitter signals [17; 87; 88; 89; 90; 91]. Since the 1990s, [PERSON] et al. continuously published some research papers that confirm the correlation between very low frequency (VLF) (3-30 kHz)/low frequency (LF) (30-300 kHz) radio signals and earthquakes [92; 93; 42; 94]. They found significant shifts in the terminator times and phase before the main shock for the signal's propagation characteristics. Their statistical study suggested that the VLF/LF ionospheric perturbation in terms of amplitude (trend) show a statistically significant precursory behavior (3-5 days before the earthquake), and the enhancement of dispersion (fluctuation) is clearly visible for M \(\geq\) 6.0 earthquakes. The statistical results also confirm and support previous results by [PERSON] et al. [95]. The VLF from the direction of the source of earthquakes was reported by [PERSON] et al. [96], though without establishing statistical significance as 'earthquake-preceding phenomena'. Five megahertz-sampling rates of VLF recorders were developed in Japan, and [PERSON] et al. determined the source location using the arrival time difference [97]. They examined the data for 5 days before and 1 day after an earthquake on 25 June 2017, and found seven similar pulses located within 20 km of the hypocenter. There was no light in that time window. There are also two sites observing VLF pulses, ultra-low frequency, and atmospheric electric fields at Kuroshio-town in Kochi, Japan [98]. In the past ten years, the application of ground-based and satellite signals in seismic monitoring was developed in China, and VLF/LF artificial source signal monitoring has become one of the main technical means of seismic disturbance analysis [99; 100]. The French satellite DEMETER was the first seismo-electromagnetic satellite devoted to investigating seismo-electromagnetic phenomena. The pre-earthquake disturbances in the ionosphere were observed by the DEMETER satellite. [PERSON] et al. [101] and [PERSON] et al. [17] demonstrated several examples of variations in the plasma parameters recorded by DEMETER satellites over the epicenters of earthquakes before their occurrence. [PERSON] et al. [102] analyzed the DEMETER data and their results indicated that the electron concentration, temperature, and oxygen ion changed rapidly 5 days before the Wenchuan earthquake. [PERSON] et al. [103] investigated the electron and ion density variations before strong earthquakes (M \(>\) 6.0) using DEMETER and GPS data. They found that the anomalous deviations prior to earthquakes had different signs from case to case, and their amplitude depended on the magnitude of the earthquake. There were positive and negative anomalies in both DEMETER and GPS-TEC data for 1-5 days before all the studied earthquakes during quiet geomagnetic conditions. These results were verified by subsequent studies [104; 105; 106; 107; 108; 109], suggesting that the ionosphere might have some response prior to large earthquakes. Additionally, the VLF field intensity observed by the DEMETER satellite was found to be statistically significant [110]. The field intensity was found to take a minimum at the time when the satellite was closest to the epicenter of the upcoming earthquake. The pre-seismic changes were observed by satellite only after stacking many cases due to the noises. After the DEMETER satellite ended in 2010, the European Space Agency launched the Swarm satellite in 2013. [PERSON] et al. [111; 112; 113] reported the seismo-ionospheric anomalies from Swarm satellite data around the time of the strong earthquake and the worldwide statistical correlation of eight years of satellite data with M5.5+ earthquakes.