## 4 Discussion  
### General Comparison for Magnetic Field and Ne Results  
Comparing the results of the electron density and magnetic field measured aboard the satellite, it is possible to note that the _Swarm Ne_ results are not statistically significant. One reason for this could be the large corresponding number of detected anomalies, which is about 80,000. It will be interesting in future works to apply the same technique to CSES data, as, due to the satellite's Sun-synchronous orbit, it would permit to have _Ne_ deep local nighttime (02:00 LT) data, always avoiding the effects of the solar irradiation (for example after sunset, when the ionosphere can show perturbation even until midnight or little later [38]). Therefore, the fixed local time reduces the signal variation in _Swarm_ due to its gradual orbital precession. In addition, the crossing of critical local times close to  
Figure 4: 8-year _Swarm_ Ne data WSC analyses separated for frequency content in the anomalies correlated with M5.5+ earthquakes for 120 days (left) and 1000 days (right) around the EQ occurrence, in particular: (**A**) signal period from 2 s to 10 s and analysis from \(-90\) days to +30 days with respect to the earthquake, (**B**) signal period from 2 s to 10 s and analysis from \(-500\) days to +500 days with respect to the earthquake, (**C**) signal period from 10 s to 25 s and analysis from \(-90\) days to +30 days with respect to the earthquake, (**D**) signal period from 10 s to 25 s and analysis from \(-500\) days to +500 days with respect to the earthquake, (**E**) signal period from 25 s to 50 s and analysis from \(-90\) days to +30 days with respect to the earthquake, and (**F**) signal period from 25 s to 50 s and analysis from \(-500\) days to +500 days with respect to the earthquake. The color bar represents the density of the anomalies in terms of the number of anomalies per square degree.  
the terminator can create perturbations or even false anomalies due to the shadow/light passage of the _Swarm_ satellite along the same orbit.  
_Swarm_ satellites show better results for the magnetic field. This could be due to the intrinsically higher quality of magnetic instrumentation in comparison with that of Electric Field Instruments and to the satellite design (in particular, the star cameras on the same optical bench as the fluxgate magnetometers). Physically, this could also be explained as the magnetic field is usually less affected by the local time than _Ne_. Hence, its precession in _Swarm_ satellites (see the discussion above) seems not to affect the usability of its measurements to search for possible pre-earthquake ionospheric disturbances.  
In addition, a specific comparison of the coefficients found by the empirical Kikitake law in this work for the _Swarm_ magnetic field and electron density data and the ones found by [PERSON] et al. [29] is provided in Figure S9 of the Supplementary Materials. We confirm the previous results and find that almost all the results are within the trend proposed in 1987 by [PERSON] [37] for earthquake precursors of the quasi first kind (i.e., earth-currents, resistivity, radon, underground water, and other geochemical elements) or only the geomagnetic precursor (from the ground). This result is promising because it means that, with the availability of new satellite data, we are confirming an empirical trend obtained decades ago for ground precursors and extending its validity to ionospheric precursors, as recently found in [PERSON] et al. [29].