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Possible ELF/VLF Electric Field Disturbances Detected by Satellite CSES before Major Earthquakes

The electric field data of ELF/VLF frequency bands recorded by space Electric Field Detector (EFD) on satellite CSES were utilized to analyze the abnormal electromagnetic (EM) emission associated with seismic activities. Two adjacent earthquakes (EQ), which are the Mw6.9 EQ on 7 July and the Mw7.2 EQ on 14 July 2019 in Indonesia, were selected as examples. The disturbance of the electric field in the ULF/VLF band was extracted by using observational and comparative analysis methods. The results of this study indicate the following. (1) The significant electric field anomalies in the ELF/VLF band (mainly from about 49 to 366 Hz) were detected near the epicenter, exactly in the northeast, of two strong low-latitude earthquakes by the electric field detector of CSES. (2) The electric field disturbances were mainly detected by satellite CSES over the epicenters at night, i.e., along the scending orbits. (3) These abnormal enhancements will gradually diminish as the frequency increases. (4) The electric field anomalies started to appear in the northeast of the epicenters before the mainshocks and gradually moved closer to the sources after them. At the same time, a clear magnetically conjugated feature also gradually appeared before the first earthquake, but then faded away when approaching the next one.