# Recent Advances and Challenges in the Seismo-Electromagnetic Study: A Brief Review
## 4 Seismo-Electromagnetic Phenomena in the Upper Atmosphere
[158] statistically examined the ionospheric plasma frequency (or electron density) in ionograms recorded by a local ionosonde and found that the critical frequency of the F2-peak, foF2, significantly decreased 1-5 days prior to 13 M \(\geq\) 6.0 earthquakes in Taiwan during the period 1994-1999. [PERSON] et al. [159] analyzed the TEC anomalies before the earthquakes in Japan and found that the positive TEC anomalies were usually observed 1-5 days before Mw > 6.0 earthquakes with a focal depth of less than 40 km. [PERSON] et al. [135; 136] found that the electron density in the F-layer decreased by approximately 6 TECU for 3 days before the earthquakes in the low geomagnetic latitudinal regions. [PERSON] et al. [160] gave the statistical evidence that the electron density decreased 1-5 days before M \(\geq\) 6.0 earthquakes in mainland China. [PERSON] et al. [161] found that the occurrence rates of TEC disturbances within 5 days before the inland earthquakes were higher than those after earthquakes, and the biggest difference of spatial distribution lies in the epicenter area. He and [PERSON] [162] found possible ionospheric electron density enhancement detected before the large earthquakes by analyzing the vertical total electron content data observed by GNSS stations near epicenters, and these preseismic signals were mostly within 5 TECU. It is quite interesting to find that the TEC anomalies prior to large earthquakes can be either positive (Japan region) or negative (Taiwan region). In addition, TEC variability in seismically active regions is greater than in non-seismically active regions [142]. There are also some studies that focused on the changes associated with earthquakes in the magnetosphere, for example, a study of charged particle precipitation from van Allen radiation belts during a land earthquake and an ocean earthquake was reported [163]. Particle bursts (PBs) were found to be accumulated only around the earthquake day,reaching their maximum on the day of the earthquake and with a complete absence of such events on days away from the event day. Some statistical studies showed significant bursts of high-energy particles observed by SOHO satellite in the magnetosphere before earthquakes [164]. In general, since the magnetosphere is mostly affected by solar activity, it might be more complicated to study seismo-electromagnetic phenomena there. Regarding the preparation zone of the earthquake, the most widely used equation is \(R=10^{0.43M_{w}}\), where R is the radius (km) and \(M_{w}\) is the magnitude [165]. The equation gained only used 24 earthquake events and did not account for ionospheric anomalies, although the authors concluded that it could be used to estimate the effective precursor manifestation zone [165]. Based on data from Benioff (i.e., crustal) strains, [PERSON] et al. found that the exponent in the equation should be increased for \(M_{w}\geq 6.0\) earthquakes as they rupture the entire crust [166], so using the equation straightforwardly might result in errors. In addition, most scholars agreed that pre-earthquake TEC anomalies are most significant close to the epicenter [167], while other studies reported that they are greatest at the edge of the preparation zone [168]. [PERSON] and [PERSON] [169] revealed that the most perturbed ionospheric regions were located south of the epicenter and the zone was greater than 1000 km. Consequently, arguments have been put forward that earthquake preparation on the ionosphere/magnetosphere could have a larger radius [170]. The most complicated part of earthquake forewarning to use the ionosphere/magnetosphere perturbance is the large preparation zone based on the anomalies [142], so the surface observation should be combined to constrain potential earthquake locations.