Jianping Huang, Junjie Song, Yantao Zhang, Yuan Yao, Zhong Li, Wenjing Li, Hengxin Lu, Xingsu Li, Yumeng Huo, Ruiqi Yang
Abstract. Anomalous electromagnetic phenomena in the ionosphere before seismic activity have been identified as potential indicators for earthquake early warning. Data from the CSES-01 satellite were analyzed using the STL decomposition method to break down electric field time series into longitudinal, latitudinal, and residual components. To quantify disturbance intensity, we use the C-value, a spectrum-fitting index derived from the power-law relationship between electric-field power spectral density and frequency. The longitudinal and latitudinal components reveal the electric field's double periodicity, characterized by a V-shape from south to north and a bimodal shape from east to west. After isolating conventional periodic disturbances with a strength of 0.87, unconventional disturbances in the residual component were examined to identify seismic precursor anomalies. Electric field power density disturbances associated with the 11 May 2023, Tonga Islands magnitude 7.6 earthquake were extracted. Multiple significant anomalies in the ionospheric electric field were detected within 20 d prior to the earthquake: an initial anomaly with a C-value exceeding 3.5 appeared 20 d before; a persistent anomaly with a peak C-value of 3.9 occurred 13 to 11 d prior; a sharp increase to a peak C-value of 4.4, three times the standard deviation, was observed 7 d prior; disturbances decreased until a resurgence 4 d prior, with a peak C-value of 3.5 lasting 2 d; the C-value returned to baseline 1 d before the earthquake. The STL-C method effectively differentiates various causal disturbances in the ionospheric electric field, offering novel approaches and insights for studying seismic precursors.