# Co-Seismic Magnetic Field Perturbations Detected by Swarm Three-Satellite Constellation
## 3 Results of the Analysis  
From the launch of the mission on November 22\({}^{\mathrm{nd}}\), 2013 until April 2019, the Swarm satellite data have been analyzed to search when one or more satellites passed above the epicentral area during an earthquake occurrence. To be conservative, a simultaneous passage was defined by a circular area of 150 km around the epicentre and a time window that spanned 10 min before the event until 20 min after. With such criteria, 37 satellite passages (tracks) were selected. For each case, method 1 and 2 were applied, searching for possible \"interesting\" features in the residual of at least one method. We finally noticed that even if the algorithm selected the space-time tracks closer to the epicenter, in some cases, the satellites passed too early or too late above the epicentral area, losing the chance to detect a co-seismic signal. After visually inspecting the residual plots, 12 tracks were considered the most likely candidate containing a co-seismic effect. Figure 2 reports the distribution map of the earthquakes and the ionospheric disturbances possibly associated with the seismic events. Table 1 reports the list of the earthquakes that present a possible magnetic disturbance in the ionosphere with the Swarm satellite characteristics in the moment of disturbance detection. The measured time is that after the origin time of the earthquake in which the anomaly starts to be detected by the Swarm satellite. The table also reports the height in km of the ionospheric F2 electron density peak (hmF2) estimated by the IRI-2016 model [30] and the air temperature at 2 m retrieved from the European Centre for Medium-range Weather Forecast (ECMWF) ERA-Interim archive, which will be used for the discussion and interpretation of the results. Figure 2: Geographical distribution of the earthquakes (red star) and positions of the satellites when an ionospheric disturbance was detected (filled circleby a different colour for each satellite). The Swarm magnetic data during the earthquake occurrence are represented following the same order of Table 1, i.e., a chronological one, from Figures 3-13. Each figure is horizontally aligned with the origin time of the earthquake. In the upper part is shown in red the residual of the Y component of the magnetic field with respect to the IGRF-12 model. The Y component was selected as it is the most sensible for the internal perturbations ([22,31]). In the lower graph, a 0.25 Hz high-pass filter was applied, and the filtered data are shown in blue. Here, we did not subtract any polynomial, as the filter was sufficient to remove the residual trend. A black vertical line underlines the position where the satellite crosses the earthquake latitude if it is after the earthquake occurrence. An orange line represents the latitude of the satellite. Note that only the data inside -70\({}^{\circ}\) and +70\({}^{\circ}\) geographical latitude are shown. More details are provided in the Supplementary Materials, where Figures S1-S10 show the residuals of the other components as well as the total intensity of the magnetic field, as deduced using method 1 and method 2. Figure S11 also shows an example of a track without any evidence of possible co-seismic magnetic disturbance. The reasons for the lack of disturbance in the latter case could be several: the earthquake happened in the Atlantic ocean ridge and the ocean water depth at the epicentre is of several kilometres (about 3.4 km), thus possibly attenuating the mechanical effect (also considering the low magnitude of the event: Mw 5.5); another reason could be an intrinsic disturbance in the magnetic signal all over the track that could cover the detection of any small co-seismic effect.