## 5 Swarm Ne Disturbances during Nighttime of the 2016-2017 Winter and M5+ Earthquakes
The comparative analysis of Swarm Ne PSD with respect to the series of earthquakes in the Papua New Guinea region confirms the high sensitivity of the Swarm LP instrument and even the dynamic character of the high-frequency ionospheric Ne changes at around a 500 km altitude. In order to slightly average rapid the variations in residual high-frequency Swarm signals and look at their longer-term behavior, the moving average from 10 days is presented together in Figures 7-15. It is easy to guess from this averaged Swarm Ne PSD (bold blue lines in Figure 7) that a more long-term Swarm high-frequency signal increase starts several days before the time of the two major earthquakes in the case of Swarm A/C (Figure 7a,c). In the case of Swarm B, this long-term signal increase is not so rapid at the beginning, but it also starts before December 2016 (Figure 7b). These findings correspond to the findings that are related to a precursory character of Swarm signals in [43; 48]. In order to validate Swarm LPs sensitivity over a seismically active region, an alternative region that is free from tectonic activity is also selected. It is intentionally selected to be far from the Pacific Plate and located over the African Plate, west of the Papua New Guinea region. This region does not include latitudinally arranged seismically active tectonic plate boundaries. Additionally, there were almost no earthquakes in the winter of 2016/2017  
Figure 7: Maxima of PSDs of residual Ne sampled at 300 km wavelength from individual tracks on different days (light-blue area plot) for (**a**) Swarm A, (**b**) Swarm B, (**c**) Swarm C in the nighttime in the winter of 2016–2017. The figures also present 10-day moving averages of Swarm Ne PSD (bold blue lines), earthquakes with M \(\geq\) 5.0 (magnitude multiplied by 10—black stems with dots, depth—black stems with circles), cumulative sum of seismic energy within 10-day period described in Section 5 (bold black), 10-day average of ap index multiplied by 2 (bold green), 10-day average of Dst index multiplied by 2 (orange line), F10.7 (red line), confirmed EPB number divided by 10 (red bars), and unconfirmed EPB number divided by 10 (blue bars). Swarm passes denoted with blue dots have spectrograms in Figures 4–6. The label in the right-bottom corner describes the range of times of the day for the selected tracks. at the tectonic junction between the African, Somalia, and Lwandle plates. The tracks are selected there at a possibly similar period of the same days, i.e., passing the region at evening and night hours. The T selected tracks have a very similar range of geodetic latitudes to the Papua New Guinea region, which also leads to similar geomagnetic latitudes in the case of these two regions. We have selected three months of tracks using the same rules of Swarm pass selection defined in Section 3, and these new tracks are shown in Figures 2b and 3b. The seismic relaxation, which was not checked before the track selection, turned out to be more perfect than expected. The scale of Swarm Ne PSD and its 10-day average applied here (Figure 8) is the same as in Figure 7. The main finding in Figure 8 is a very small level of disturbances at a 300 km wavelength in Ne signals from the Swarm A/C pair (Figure 8a,c). Figure 8b, aside from some surrounding data gaps in Swarm B observations, shows a potentially precursory disturbance to a very shallow earthquake on January 27. Summarizing Figure 8, the level of Swarm Ne disturbances resulting from almost no stronger seismic activity is, as expected, very low in the case of the African region that is located at exactly the same latitudes as Papua New Guinea. Moreover, the highest increase in Ne PSD in Figure 8b even unexpectedly correlates with the only shallow M \(\geq\) 5.0 earthquake. The low Swarm Ne PSD at a 300 km wavelength in Figure 8 confirms that the Sun being at low latitudes and an EIA related to the equatorial latitudes cannot trigger large disturbances alone.