## 4 Spectral Patterns of Swarm Ne Disturbances during Nighttime of the 2016-1017 Winter  
From the winter set of the 91 trajectories within the three-month period, eight are selected for each Swarm spacecraft to illustrate the spectral characteristics of the most important moments in the spectrograms (see labels on Swarm passes in Figures 2 and 3). Besides that, the measures of the disturbance scale for the entire three-month comparisons with seismicity are also determined from the spectrograms as the maximum PSD of Ne disturbances at the selected wavelength (from here, simply called Swarm Ne PSD). The  
Figure 3: (**a**) Selection of descending (nighttime) Swarm B tracks over Papua New Guinea between 1 November 2016 and 30 January 2017. The tracks are selected to have the closest possible time of the day and longitude to the first selected track just before M = 7.8 mainshock. The red triangles denote earthquakes above magnitude 6.5. (**b**) Selection of Swarm B tracks at the same latitudes and local time in a more seismically quiet region. The red triangles denote earthquakes above magnitude 4.0. (**c**) Selection of Swarm B tracks at the high and middle latitudes. The red triangles denote earthquakes above M = 4.5.  
orbital tracks from the three Swarm satellites are selected for consecutive days at a similar time of the day (\(\pm\)4.4 h), e.g., one for each of the 91 days from 1 November 2016 to 30 January 2017. The Ne data collected along these orbital arcs are high-pass filtered and then spectrally analyzed via STFT. All spectrograms describing a varying PSD of ionospheric Ne are sampled for further joint processing with seismicity, solar conditions, and the geomagnetic index. However, as three months produce 273 trajectories, this section shows the spectrograms from three Swarm satellites only for the selected eight most important days from the winter solstice in the region of Papua New Guinea (Figures 2a, 3a, 4, 5 and 6). In order to avoid a dominating PSD at long wavelengths, which often disturb viewing shorter wavelengths in spectrograms or scalograms, the data are high-pass filtered, which removes the signals with wavelengths that are longer than 500 km. The approximate recalculation from the period of 65 s. to wavelength is based on the speed of the Swarm at its flight altitude, which is around 7.6 km/s. This 500 km wavelength threshold is fixed approximately, but its choice is based on the most frequent width of seismic gravity waves located in the ionosphere close over their lithospheric source. This width usually does not exceed 500 km [9,20,63]. On the other hand, the wavelengths of similar widths are detectable in TEC series before the earthquakes as well [64,65]. The latter fact provides information about the usefulness of such selected thresholds in the detection of preseismic, co-seismic, and post-seismic irregularities. Moreover, the use of a reasonably short lower bound of the filter must be selected to exclude natural low-frequency Ne signals, which are dependent on the Sun and geomagnetic activity. This is important, especially at equatorial latitudes in the region of equatorial ionization anomaly (EIA). The high-pass filter is based on the decomposition of the signals via DFT. Figures 4-6 present the spectrograms calculated with the use of STFT and the Tukey window with its main parameter r = 0.4 (Equation (3)). The width of the Tukey window is set to 180 s (\(\sim\)1400 km), and the period/wavelength range is set from 10 s (\(\sim\)80 km) to 120 s (\(\sim\)920 km). This relatively long window size slightly blurs the moment of the occurred disturbance but enables a significantly better spectral resolution of the residual Ne spectrogram at longer wavelengths. It should be pointed out here that scales of the spectrograms, as relative quantities, are omitted to avoid redundant information, but are exactly the same.