## 2 Data and Methods  
We analysed the magnetic field data measured by the three identical satellites belonging to the _Swarm_ constellation, called Alpha, Bravo and Charlie, respectively. They were launched by a single rocket on 22 November 2013 and are still in a quasi-polar orbit. After a few months of test, in-orbit calibration and commissioning, the satellites were put in the final orbital configuration: Alpha and Charlie fly almost in parallel at a lower orbit (in 2019 about 440 km above Earth surface) with a small separation of about 1.4 degrees, while the third satellite Bravo flies at a higher orbit (around 510 km in 2019) with a longitudinal shift that precedes along the mission time, and in 2019, it was about 90 degrees with respect to the orbit of the other two satellites. The orbital configuration was selected to take into account the different goals of the mission, mainly to measure the Earth's magnetic field and its variations, and in particular, to measure the Field Aligned Currents (FAC) and discriminate them from the lithospheric field. The satellites are equipped by several instruments to measure the Earth's magnetic field, to monitor the ionospheric plasma environment and to determine the orbit and orientation of the satellites as best as possible (e.g., by Global Navigation Satellite Systems-GNSS, laser retroreflector, accelerometers). In this work, we analysed the data of the Vector Field Magnetometer (VFM) and the Absolute Scalar Magnetometer (ASM) placed at the middle and at the end of a four-meter boom, respectively, both located at the back of each satellite. ESA downloads the raw data from _Swarm_ satellites to the Kiruna and Svalbard stations and processes them in almost real-time (with a delay of 3-4 days only). The Agency provides calibrated magnetic open access data at Level 1b, where the measurements are provided not only in the instrumental frame but are also oriented in the Earth frame system NEC (North, East, Centre) at the original sampling frequency of 50 Hz (HR = High Resolution) and resampled at 1 Hz at the GPS o'clock seconds (LR = Low Resolution). In this work, we analysed the LR Magnetic _Swarm_ product of all the satellites from 1000 days before the Ridgecrest mainshock. The data are provided with a quality check by means of 4 Flags: Flag_B and Flag_F are related to the quality of the measurement of each VFM magnetic field component and to ASM scalar intensity, respectively. Flag_attitude indicates if the pointing and attitude systems of satellites are working properly, and Flag_platform provides some information about the general status of the satellite platform, including, for example, the indication of the activation of the thrusters. In order to extract magnetic anomalies possibly related to the major seismic events, we need to remove the main magnetic field. We then apply an approach successfully used in previous works and well described in the Methods section of [15] under the name of the MASS (MAGnetic _Swarm_ anomaly detection by Spline analysis) algorithm. In particular, the magnetic field data are analysed by a numerical approximation of the temporal derivative, and then a cubic-spline is fitted and subtracted to remove the long trend. Finally, a moving window (generally of 7 degrees in latitude) investigates the obtained residuals. The anomalies are defined by a threshold (named k\({}_{t}\)) on the root mean square (rms) of the moving window compared with the Root Mean Square (RMS) of the whole track between \(-\)50\({}^{\circ}\) and \(+\)50\({}^{\circ}\) magnetic latitude. Only the tracks acquired in quiet geomagnetic field conditions (\(|\)Dst\(|\)\(\leq\) 20 nT and a\({}_{p}\)\(\leq\) 10 nT) and with instruments and satellites in nominal conditions (checked by Flags) are taken into account to search for anomalies. Finally, the algorithm can also produce a figure of the residual of the magnetic measurements (X, Y, Z and F) of the track, together with some orbital information and geomagnetic indices Dst and a\({}_{p}\) during the satellite passage.