# Co-Seismic Magnetic Field Perturbations Detected by Swarm Three-Satellite Constellation
## 1 Introduction
### Swarm Satellite Magnetic Data  
In this paper, the data from the ESA Swarm mission have been investigated. The main goal of the mission was to measure and monitor the Earth's magnetic field with global coverage and better understand the electromagnetic phenomena on our planet [17]. Swarm is a three-identical satellite constellation successfully launched on 22 November 2013. The mission is still in orbit, and the present objective is to extend the mission for an entire solar cycle of 11 years. Most of the instruments continue to work in nominal conditions. Currently, more than five years of data are available. The three satellites follow quasi-polar orbits: two of them (called Alpha and Charlie) fly close to each other at a lower orbit (around 450 km) and the other one, called Bravo, currently has a separation of about 90\({}^{\circ}\) longitude, and it is placed at a higher orbit, around 510 km. In the first months of the mission, all the satellites orbited close to each other. Each satellite is equipped with the following payloads: Vector Field Magnetometer (VFM), Absolute Scalar Magnetometer (ASM), electric field instrument (EFI), accelerometers, GNSS receivers and laser retroreflectors. The VFM and ASM measure the orientation and intensity of the magnetic field. These sensors are placed on a 4-m length boom to avoid possible interferences with the satellite platform. The VFM sensor, placed half-way along the boom, is a fluxgate magnetometer with three-axis compact spherical coil with a three-axis compact detector coil inside. The magnetic field intensity calibration of the VFM is based on the data acquired by the ASM instrument placed at the end of the boom. For the Charlie satellite, the calibration is set on the data of Alpha ASM, as its ASM has not functioned since November 2013. The orientation of the VFM was precisely monitored with three star-tracker cameras placed on the same optical bank of the magnetometer. The Swarm mission has already provided vast results; among them, a map of the lithospheric magnetic field with a resolution up to 250 km [18] and the oceanic tidal effect on geomagnetic field and electromagnetic induction generated by oceanic circulation (e.g., [19; 20; 21]). Furthermore, some of the authors of this paper found a series of anomalies in Swarm magnetic field and electron density data that could be related to the preparation phases of some earthquakes around the world (e.g., Mw7.8 Nepal 2015 [22], Mw8.2 Mexico 2017 [23], 2018, Mw6.5 Italy 2016 [24], Mw7.3 Iran 2017 [25], Mw7.5 Indonesia 2018 [26]). A recent paper by [PERSON] et al., 2019 [27] provided for the first time a worldwide statistical correlation of Swarm electron density and magnetic anomalies with earthquake occurrences, finding  
Figure 1: Earthquakes extracted from USGS catalogue Mw5.5+ from 2013-11-25 to 2019-04-12 (black) and those selected for the analysis (red). (**a**) Geographical distribution, (**b**) earthquakes magnitude/depth plot. The blue line shows the limit criterion used to select the earthquakes: for instance, Mw5.5 earthquakes have a depth threshold of 20 km as well as Mw7.0 of 150 km and Mw = 8.0 around 240 km. some distinct concentrations of anomalies that preceded the earthquakes from some days to several months; additionally, a linear dependency was established between the logarithm of the anticipation time of the anomaly and the earthquake magnitude, confirming for satellite data the empirical Kikitake law [28], which was already introduced for ground precursors. The Swarm magnetic data have been downloaded from the free accessible FTP repository (swarm-diss.e.esa.int) in Level 1B as CDF files with the filename SW_OPER_MAGx_LR_1B_[...], where x stands for A, B or C satellite and the filename is completed with the starting and ending timestamp of the file and the data version (408 for most of this work and 505 for the most recent data).