This study uses the global ionospheric total electron content （TEC） grid data provided by the Jet Propulsion Laboratory （JPL） of the International GNSS Service （IGS） Center to conduct analysis of electromagnetic anomalies. Based on geographic coordinate information, the nearest TEC grid data to the M_S5.7 earthquake in Songyuan, Jilin Province （45.27°N, 124.71°E） in 2018 was selected for calculation and analysis. Since pre-seismic ionospheric anomaly regions are not all concentrated above the epicenter, they also occur in regions at a certain distance from the epicenter. Therefore, the research grid points were selected from the nine closest grid points around the epicenter.

Through comparative calculation of the traditional quartile method and the sliding quartile method, we find that the sliding quartile method is less susceptible to special phenomena than the traditional quartile method in the calculation results. It can more objectively reflect the actual background of the observation points, yield more distinct results, and is consistent with previous studies. By applying the sliding quartile method, we conduct time series analysis on the four study grid points closest to the Songyuan earthquake epicenter and find that the impact of polar substorms on ionospheric observations （on days −11 and −5 relative to the earthquake） can be eliminated. The ionospheric observations first increased from day −14 to 8 and then decreased from Day −8 to 1 in the week before the earthquake, which is consistent with the study by Wang et al （2014）. Meanwhile, the amplitude of ionospheric anomalies increases as the epicentral distance decreases. As Songyuan is located further north, it is more susceptible to polar magnetic substorms. After excluding the effects of space weather, there is a high probability that positive ionospheric anomalies persist for five days and negative ones for two days, which are associated with the earthquake.

The analysis of seismic ionospheric spatial distribution anomalies shows that: ① The closer the northern ionospheric disturbances are to the intense phase of polar substorms, the stronger they become; ② Pre-seismic TEC observations first increased, then decreased, and rebounded in the post-seismic period, with the anomaly amplitude increasing as the epicentral distance decreases; ③ In terms of ionospheric spatial distribution, positive and negative anomalies are temporally clustered and exhibit regular characteristics, with an overall "N"-shaped pattern. Anomalies over and in the vicinity of the epicenter persisted for more than two hours, characterized by a small spatial coverage, low intensity and distinct localization.

The lithosphere-atmosphere-ionosphere coupling （LAIC） mechanism governs energy propagation during seismic gestation and occurrence. By combining analyses of deep lithospheric resistivity, atmospheric and ionospheric datasets for the Songyuan earthquake, we identify a close correlation among the three spheres during the seismogenic and coseismic periods: fracturing of the lithospheric medium, energy release and electrical variations exert a coupling effect on the atmosphere and ionosphere. Negative ionospheric TEC anomalies are likely directly related to changes in the atmospheric electric field caused by negative charges released during rock rupture and upward electron transport via the vertical atmospheric electric field. Positive TEC anomalies mainly result from the release of positive charges in rock masses, thereby accelerating collisional ionization of atmospheric ions and altering the ion density distribution in the ionosphere. The atmospheric thermosphere and ionosphere also exhibit mutual influence and synchronous variations—particularly, high-amplitude positive ionospheric TEC anomalies show good synchronization with atmospheric thermal infrared anomalies in terms of intensity enhancement and spatial distribution patterns.

Currently, the application of pre-seismic ionospheric detection in China is mainly concentrated in the western regions for earthquakes with M_S≥6.0, with few studies conducted in eastern China, where earthquakes are fewer and weaker. In recent years, Songyuan has become one of the most seismically active areas in Northeast China. This study analyzes the largest earthquake in Songyuan over the past five years: the M_S5.7 Songyuan earthquake in Jilin Province on 28 May 2018 （with an epicentral depth of 10 km）. It presents the application of a short-term ionospheric TEC prediction method in Songyuan, supplementing the seismic data of eastern China for ionospheric anomaly-based seismic prediction and accumulating experience for the application of pre-seismic ionospheric detection.