## 4 Discussion and Conclusions  
By an automatic analysis of the anomalous _Swarm_ three-satellite tracks, it has been possible to detect an increase of anomalies around 200 days before the 6 July 2019 California mainshock. Such an increase of anomalies is considered as possibly related to the preparatory phase of the California Ridgecrest earthquake. The result has been validated after comparison with two equivalent areas centred at the same geomagnetic latitude and with a longitude that corresponds to US East Coast (82.5\({}^{\circ}\) W) and to Europe-Spain (3.3\({}^{\circ}\) W). The comparison is essential to exclude possible global perturbations of the geomagnetic field. The detected anticipation times are well compatible with those expected by the Rikitake empirical law, recently confirmed for satellite data by the statistical studies conducted by [PERSON] et al. (2015). Other increments of anomalies have been detected at about 500 and 365 days before the earthquake, but increases of the cumulates have also been detected in the comparison regions at similar times. We noted that such increases at about 500 and 365 days in the US East Coast comparison area are  
Figure 4: Local time distribution of the analysed windows. **(a)** Passages of the three satellites above the area in geomagnetic field quiet time; **(b)** local time distribution of the anomalies in the earthquake area; **(c,d)** local time distribution of the anomalies in the US East Coast and Europe comparison areas, respectively. The histograms in panels **(b–d)** highlight the anomalies around the jumps indicated by the coloured arrows in Figures 2a and 3a. even higher than above epicentral area. This made us conclude that these anomalies are a regional phenomenon in US, but it is not likely due to the preparation phase of the Ridgecrest earthquake. An open question is whether only one type of pre-earthquake Lithosphere-Atmosphere-Ionosphere Coupling (LAIC) process exists or whether more phenomena could be involved during the earthquake preparation phase. The latter hypothesis seems to be more reliable, possibly explaining why some anomalies are closer in time to the event. In this paper, a magnetic anomaly in the Y component appears 15 min before the mainshock, while similar anomalies appear 9 days before the 2016 M7.8 Ecuador earthquake and 3 days before the 2016 M6.0 Italy earthquake, as found by [PERSON] et al. [10] and [PERSON] et al. [12], respectively. For the same earthquake, [PERSON] et al. [19] found a chain of Lithosphere-Atmosphere-Ionosphere anomalies even increasing in number toward the event. This work is complementary in the sense that the anomalies depicted in this paper covered the whole preparation phase investigated in [PERSON] et al. [19], with some anomalies with long anticipation time (Figures 2 and 3) until an early anomaly before the earthquake (shown in Figure 1). It is worth noting that the coverage of the three _Swarm_ satellites is not uniform in time, so the analysed region is revisited from each satellite about twice per day (one during night-time and the other one in the daytime). Therefore, a broader satellite constellation could hopefully permit us to have more chances to detect such phenomena, and to have better time coverage worldwide, i.e., all the active seismic zones. The higher number of anomalies in the epicentral area with respect to the comparison ones suggests that at least some of the anomalies in the epicentral area could be due to the earthquake preparation phase. This result is similar to that obtained in the previous investigation conducted by [PERSON] et al. [14; 15] in the frame of the ESA funded project SAFE (SwArm For Earthquake study). Moreover, the concentration of anomalies in a different local time with respect to those detected in the comparison areas can be considered a further hint in supporting this hypothesis.