## 4 Spectral Patterns of Swarm Ne Disturbances during Nighttime of the 2016-1017 Winter
Additionally, the PSD of the large Ne peak in Figure 4d appears like a mirror reflection of the PSD in Figure 4c, which suggests the observation of a very similar perturbation of the ionosphere crossed by Swarm A from two opposite directions. The signal observed by Swarm C (Figure 6d) is very similar to that observed by Swarm A (Figure 4d), and we can be sure that it refers to the same disturbance. The Ne signals were different 4.5 h later, which was half an hour before the earthquake and before Swarm B visited the region (Figure 5d). Figure 5d shows the more disturbed Ne signal of Swarm B, which has clearly separated short wavelengths around 150 and 180 km. This fact strongly suggests new perturbations aside from those found 4.5 h before by Swarm A/C. These perturbations can originate from different physical phenomena, and their presence can be confirmed with other observations of physical and chemical parameters, including the local ones. At least, the possibility of a higher emission of radon can be taken into account, as it is generated continuously in any geological structure, and its highest emission can also be found just before the strong earthquakes [2]. Figures 4e and 6e still show quite large disturbances on the next day (December 9) for Swarm A and C, which occupy the wavelengths around 300-400 km. Figure 5e presents a much more dispersed and weaker residual signal of Swarm B on December 9. We should, however, remember the longitudes, which differ more in the case of Swarm A/C than in the case of Swarm B (Figures 2a and 3a). The footprint of the Swarm A/C pair on December 9 is quite far from the corresponding one on the previous day. Figures 4f and 6f present an increased PSD at various frequencies of residual Ne on 17 December 2016, which was 40 min. after the major earthquake occurred at 10:51 UTC (20:51 LOC) close to the island New Britain in Papua New Guinea, having M = 7.9 and a depth of 103 km. The PSD pattern in Figure 4f has a similar dispersion in frequency to that in Figure 5d for Swarm B, because it occupies at least three separated spectral bands. The common point of Figure 4f, Figure 5d, and Figure 6f is that all of these figures describe Swarm passes in the temporal proximity of major earthquakes. A characteristic feature of these disturbances is a high PSD magnitude at various wavelengths, including higher wavelengths somewhere between 100 and 300 km, which are probably wavelengths for gravity waves propagating to the ionosphere [20; 63]. It should be pointed out that due to the complicated geometric relations and high Swarm speed, with respect to most seismic  
waves [13], we can interpret that Swarm measures a snapshot of the existing waves at each given moment. The selection of the wavelengths that are sensitive and corresponding to ionospheric disturbances in the Swarm signal has a preliminary character here. Thus, consideration of relative speeds of Swarm and gravity waves would be time-consuming and not necessarily effective, with no detailed knowledge of the expected disturbing wave width at Swarm altitudes. Summarizing the day of December 17, we can assess that the PSD of the disturbed signal from Swarm C in Figure 6f follows the same rule as Swarm A in Figure 4f and the one from Swarm B in Figure 5d, i.e., the signal is composed of many wavelengths. A very similar complex spectrum can also be observed for Swarm B in Figure 5f, but the PSD is much weaker there. However, a suspicion of a similar origin of the disturbances in Figure 5f must be verified through additional data investigation. On 18 December 2016, the signals seemed to occupy many frequency bands, but at the same time, they are dispersed over a larger area, which is noticeable in Figure 4g (Swarm A) and Figure 5g (Swarm B) but is less pronounced in Figure 6g for Swarm C. The spectra of the most noticeably dispersed disturbances in Figures 4g and 5g are very similar, but different from that in Figure 6g.