Seismic-ionospheric effects prior to four earthquakes in Indonesia detected by the China seismo-electromagnetic satellite

In this study, we investigated the characteristics of pre-earthquake ionospheric effects related to four successive shallow-focus earthquakes (M6.4 on July 28, 2018, M6.8, 5.9, 6.9 on August 05, 09 and 19, 2018) in Indonesia by the China Seismo-Electromagnetic Satellite (CSES). By adopting the moving median method (MMM), the ionospheric electron density (Ne) detected by the Langmuir probe (LAP) onboard CSES presented remarkable enhancements along the orbits #2660-0 on July 27, #2721-0 and #2728-1 on July 31, #2827-0 and #2735-1 on August 7, #2934-0 on August 14, respectively, which were just 1, 5, 2 and 5 days before the four earthquakes. Moreover, the disturbances mainly appeared in the north and north-east within the distance of 2000 km from each epicenter. By comparing the disturbed orbits with their corresponding four nearest and four revisiting orbits as well as the predicted values from the International Reference Ionosphere 2016 model (IRI-2016), the results powerfully illustrated that CSES observed the ionospheric precursors preceding the four Indonesian earthquakes. To make the space-borne results more convincing, we further studied the grid total electron content (TEC) data from the Center for Orbit Determination in Europe (CODE), which exhibited similar anomalies in both the temporal and spatial distributions. In the end, the superposed epoch analyses (SEA) method was conducted on 35 M >= 5.8, depth <= 50 km earthquakes in Indonesia during 2007-2017 by using CODE TEC. The statistical results demonstrated that there were significant positive anomalies 1-7 days before earthquakes within 2000 km from the epicenters in this region. The positive plasma perturbations detected by CSES were highly suggested as the seismic-ionospheric effects before earthquakes.