## 4 Spectral Patterns of Swarm Ne Disturbances during Nighttime of the 2016-1017 Winter
The time in Figures 4-6 is given in UTC along the horizontal axis, but easy recalculation can be performed to the local time (LOC) by adding +10 h. Figures 4-6 show the spectrograms of the selected, most significant passes over the region, in the sense that these Swarm tracks observed the most evident disturbances that were co-located with seismic activity in the region of Papua New Guinea between November 2016 and January 2017. The maxima of the PSDs of these disturbances detected by Swarm at a 300 km wavelength (Swarm Ne PSD) are summarized further in Figure 7 in Section 5. The days of 19 and 20 November 2016 are days of low ap and Dst indices and moderate seismic activity. A single week of relaxation from earthquakes equal to or stronger than M = 5.0 starts here, and the last earthquake with an M = 5.5 happened in the region on November 17, just before this quiet week. Swarm A finds a moderate disturbance on November 19 (Figure 4a) and a quite large, disturbed signal on November 20 (Figure 4b). Swarm C, flying close to Swarm A, also sees these signals (Figure 6a,b). In general, Swarm C signals are similar, but the one on November 20 is weaker, which may suggest a quite fast change in the signal in time (Figure 6b). Swarm B, which was visiting the region around four hours later on these days, observed a weaker disturbed signal on November 19 (Figure 5a), but one that was spectrally similar to that seen by Swarm A on November 20 in Figure 4b. These similarities can suggest to some degree that we see the same changing disturbance in Figures 4b, 5a and 6b. The relative longitudes of the Swarm A/C pair and Swarm B in the above case do not differ much (Figures 2a and 3a). The longitude of Swarm A on November 20 was around 150\({}^{\circ}\), whereas the longitude of Swarm B on 19 November was around 160\({}^{\circ}\). This is not significant, and it is theoretically possible that Figure 5a presents the spatiotemporal evolution of the signal shown in Figure 4b. The first earthquake with M = 5.0 occurred after a one-week period that was free from 5.0+ earthquakes and happened in the Papua New Guinea region on November 27, after a maximum ap increase to 5+ on November 25 and respective Dst variation. Figure 4c shows the quasi-trapezoidal shape of the PSD of a large Ne disturbance detected by Swarm A at an around 400 km wavelength, with a weaker disturbance at a wavelength that was shorter than 300 km. Figure 6c presents a weaker and moderately similar perturbation for Swarm C, which, however, occupies the same two wavelengths. Figure 5c shows an ionosphere disturbance sensed by Swarm B around 4 h later than Swarm A/C. This perturbation is latitude-shifted and weaker but also quite similar in its PSD shape and wavelengths to those in Figures 4c and 6c. One should take into account the different altitudes of the Swarm A/C pair and Swarm B, which can obviously contribute to differences in the disturbance shape between A/C and B. Although different altitudes of the satellites result in the observations of different parts of the Ne vertical profile, a probable consistency can be found between Figure 4c, Figure 5c, and Figure 6c, suggesting the same origin of the signals. Further large PSD values in Swarm A observations are found on 8 December 2016 (Figure 4d) when the seismic activity increased; this was five hours before one of the two largest earthquakes over the analyzed period. This earthquake induced a tsunami that was recorded by the National Oceanic and Atmospheric Administration (NOAA) tide gauges. The earthquake happened in Kira Kira, Solomon Islands, at 17:38 UTC (3:38 LOC) and had M = 7.8 and depth = 41 km. The passes of Swarm A and C were there around five hours before the mainshock (Figures 4d and 6d). The disturbance at that time was strong, very similar for A and C, and occupying longer wavelengths in the filtered frequency domain (300-400 km) for a significant time of the pass, but also some that were shorter than 300 km (Figures 4d and 6d).