# Co-Seismic Magnetic Field Perturbations Detected by Swarm Three-Satellite Constellation
## 5 Conclusions  
After a systematic analysis of the first 5.3 years of Swarm magnetic data around the Mw5.5+ earthquakes, we found that 12 Swarm satellite tracks recorded disturbances following earthquakes of magnitude Mw5.6 to 6.9. We underline that we limited our search for possible co-seismic effects to a subset of earthquakes selected in function of their depth and magnitude. Even if this subset could exclude some possibility to record some additional effect, we were driven by the necessity to exclude the deeper and lower magnitude events that are unfeasible to produce any seismic effect in the ionosphere. In addition, we do not expect particular difference with other selection criteria. However, more energetic seismic events were recorded during this period (included three Mw8+ earthquakes: two in Chile and one in Mexico), but we did not find possible co-seismic ionospheric disturbances produced by these large events. This is due to the fact that the Swarm satellites did not fly just over the epicentral area at the time of the earthquake occurrence. This explains why we found geomagnetic ionospheric disturbances only for almost medium magnitude earthquakes. The linear regression of the time versus the distance between the earthquake occurrence and the record of the disturbance by Swarm satellites seems to confirm that the seismic events possibly induce the disturbances. To explain these disturbances, a model that combines two different propagation ways has been proposed with promising result. The model considers an acoustic gravity wave as the way to propagate the earthquake disturbance up to the F2 ionospheric layer, i.e., the layer with the maximum ionisation. The impact of the acoustic gravity waves on this layer could induce some electromagnetic scattering that propagates in the upper ionospheric layer, reaching the Swarm satellites. The propagation velocities, according to our data, are (2.2 \(\pm\) 0.3) km/s with a 25 \({}^{\circ}\)C of surface air temperature for the acoustic gravity waves and (16 \(\pm\) 3) km/s for the electromagnetic scattering propagation. We note that the approximation to the same velocity for all the seismic events is a limitation and, in particular, it does not take into account, e.g., the different local time: future improvements could introduce the importance of the local time together with some other parameters that would take into account the different chemical and physical conditions of the atmosphere and ionosphere where the phenomena propagate. This double-velocity model has better statistical reliability, providing a lower AIC than the one with only one velocity. In order to validate the mostly subjective criteria chosen in this work to select the co-seismic anomalies, we developed an automatic algorithm that detects the co-seismic perturbations based on objective criteria (see Appendix A). The principal results of this paper were also confirmed by this automatic approach to detect the anomalies. Nevertheless, the automatic system was not yet able to achieve the quality with which a researcher can evaluate the signal of the satellite track to discriminate and choose the effects and causes of the detected disturbances. A future improvement of the automatic procedure can include the analysis of the signal frequency for not only searching a disturbance along the satellite track but also some particular frequencies without usually presenting in other parts of the track. \begin{table}
\begin{tabular}{c c c c c c} \hline  & **Number of** & **Number of** & **Residual Sum of** & & \\  & **Parameters K** & **Samples** & **Square RSS [s\({}^{2}\)]** & **AIC** & **AICc** \\ \hline Model 1 (one velocity) & 2 & 12 & \(1.09\times 10^{5}\) & 99.0 & 100.3 \\ Model 2 (two velocities) & 3 & 12 & \(6.07\times 10^{4}\) & 94.9 & 97.9 \\ \hline \end{tabular}
\end{table} 
Table 2: Results of the Akaike Information Criterion applied to the two used models.