## 3 Swarm and Ancillary Data  
The region of Papua New Guinea is located at the junction of four main tectonic plates: the Australian, Pacific, Eurasian, and Philippine Sea Plate. There are also several smaller plates, which makes this region especially seismically active. The main time period selected for the analysis due to the strong seismic activity is the winter period covering November and December of 2016 and January of 2017. The two additional three-month periods for the validation are the spring period from February to April of 2016 and the summer period extending from May to July of 2016. The general M = 5.0 threshold for the selection of earthquakes in this work is assumed approximately on the basis of existing works on the sensitivity of ground GNSS data to seismic activity (see: [14, 62]), as well as from the recent works on the sensitivity of Swarm magnetic data to earthquakes [37, 42]. November of 2016 is relatively seismologically quiet and includes a week during which no earthquake of M \(\geq\) 5.0 was recorded in the investigated region. December 2016 in Papua is very active seismologically, with two earthquake events having magnitudes of 7.8 and 7.9, both triggering tsunamis. January is not as quiet as November, including more earthquakes with M \(\geq\) 5.0. It includes also one having a M = 7.9; however, this was deep, with no following aftershocks. It should be pointed out that based on the earthquake number, the tectonic plate junction seems to have been more relaxed in January 2017 than before December 2016. February-April of 2016 and May-July of 2016 are seismically lower active periods compared with to winter. However, in this especially active region, these periods also include a significant number of earthquakes exceeding 5.0 and 6.0 magnitudes. The earthquake records for all three seasons in the Papua New Guinea region are selected using the geographical window of 10\({}^{\circ}\)N-30\({}^{\circ}\)S and 130\({}^{\circ}\)E-170\({}^{\circ}\)E. Additionally, two other regions of Swarm data analysis are selected for validation and confutation purposes. These regions use geographical windows selecting Swarm Ne data at the equatorial latitudes first, but at a significantly different longitude, and second, at an absolutely different latitude, far from the equator. For the African region, we applied the window of 10\({}^{\circ}\)N-30\({}^{\circ}\)S and 0\({}^{\circ}\)E-40\({}^{\circ}\)E. The region of Aleutian Islands was selected using the window of 30\({}^{\circ}\)N-70\({}^{\circ}\)N and 160\({}^{\circ}\)E-130\({}^{\circ}\)W. We used geodetic latitudes, as the difference with respect to geomagnetic ones is less significant here than the selection of the place that is free from a latitudinally aligned tectonic plate boundary, which is Africa. The data in both ancillary regions cover the primary period of Papua New Guinea data from November 2016 to January 2017. The list of earthquakes is extracted from the Search Earthquake Catalog, which is available from the USGS website. There were almost 200 earthquakes in the Papua New Guinea region with a magnitude equal to or above M = 5.0 from the beginning of November 2016 to the end of January 2017 (Figure 1). A significant part of these earthquakes included aftershocks and occurred just after 8 and 17 December 2016. The orbits of Swarm satellites, which have an inclination between 87\({}^{\circ}\) and 88\({}^{\circ}\), pass the boundary of two major subducting tectonic plates (Australian and Pacific) at an angle that is close to 80\({}^{\circ}\). The analysis of the Swarm data in the Papua New Guinea region is based on the LP data from Swarm Level 1b Electric Field Instrument (EFlxLP). The longitude (\(\lambda\)) of the Swarm satellite pass does not repeat the next day at the same time, and moreover, it is not exactly the same. Therefore, we selected Swarm passes from consecutive days of the three-month periods, having close longitudes and being recorded at a similar time of the day.