###### Abstract  
This work presents an analysis of the ESA _Swarm_ satellite magnetic data preceding the Mw = 7.1 California Ridgecrest earthquake that occurred on 6 July 2019. In detail, we show the main results of a procedure that investigates the track-by-track residual of the magnetic field data acquired by the _Swarm_ constellation from 1000 days before the event and inside the [PERSON]'s area. To exclude global geomagnetic perturbations, we select the data considering only quiet geomagnetic field time, defined by thresholds on Dst and \(\mathrm{a_{p}}\) geomagnetic indices, and we repeat the same analysis in two comparison areas at the same geomagnetic latitude of the Ridgecrest earthquake epicentre not affected by significant seismicity and in the same period here investigated. As the main result, we find some increases of the anomalies in the Y (East) component of the magnetic field starting from about 500 days before the earthquake. Comparing such anomalies with those in the validation areas, it seems that the geomagnetic activity over California from 222 to 168 days before the mainshock could be produced by the preparation phase of the seismic event. This anticipation time is compatible with the [PERSON] empirical law, recently confirmed from _Swarm_ satellite data. Furthermore, the _Swarm_ Bravo satellite, i.e., that one at highest orbit, passed above the epicentral area 15 min before the earthquake and detected an anomaly mainly in the Y component. These analyses applied to the Ridgecrest earthquake not only intend to better understand the physical processes behind the preparation phase of the medium-large earthquakes in the world, but also demonstrate the usefulness of a satellite constellation to monitor the ionospheric activity and, in the future, to possibly make reliable earthquake forecasting. Swarm; earthquakes; Ridgecrest; ionospheric seismo-induced disturbances; earth magnetic field +
Footnote †: journal: _Swarm_:along the same direction and connected fault plane [1]. This region is seismically very active (e.g., [2]); we note that about 310 km northward with respect to the Ridgecrest epicentre, on 28 December 2016, three M5.5+ earthquakes happened in the same day (M5.6 at 16:18 UTC, M5.6 at 16:22 UTC and M5.5 at 17:14 UTC, respectively). In this paper, we search for possible electromagnetic satellite signals before the earthquake occurrence. [PERSON] et al. [3] found a clear magnetic disturbance at the ground in the ULF band of 0.05 Hz\(-\)0.20 Hz prior to the M7.1 Loma-Prieta earthquake that occurred in California on 17 October 1989. The data were taken from a ground magnetic observatory, very close (7 km away) to the impending earthquake epicentre. Despite the promising observation that came from a ground observatory, it is still possible to search for this type of anomalies in magnetic satellite data. Some of the first works that provided pieces of evidence in satellite data for electromagnetic disturbances that preceded the occurrence of earthquakes in the world came from the DEMETER satellite (e.g., [4; 5; 6; 7]). In the last years, our research group has proposed some electromagnetic satellite anomalies from the European Space Agency (ESA) _Swarm_ constellation, which is composed of three identical satellites in orbit from 22 November 2013 [8], and from the CSES-01 (China Seismo-Electromagnetic Satellite) dataset, prior to medium (M6.0-M7.4)-large (M7.5+) earthquakes in the world [9; 10; 11; 12; 13; 14]. More recently, a Worldwide Statistical Correlation (WSC) analysis was applied on 4.7 years of _Swarm_ magnetic field and electron density data, finding a significant correlation of concentrations of ionospheric anomalies with the worldwide shallow M5.5+ earthquakes in the same period [15]. Besides, they found that the largest concentrations of anomalies precede large earthquakes, with each anticipation time increasing with the magnitude of the seismic event, also confirming the Rikitake law [16] for electromagnetic pre-earthquake anomalies from satellite data.