## 4 Local Variations of Ne in the Topside Ionosphere during Seismic Activity
### Ionospheric Disturbances over the EQ Area
Such an abrupt Ne three-fold enhancement may be expected because this feature is likely a part of a large-scale expansion of TEC on the day-side (cf. Figure 4b). In the context of a possible coseismic effect, we pay attention to a smaller-scale anomaly in Ne, which appears when comparing data from two satellites flying nearby. During all days except 22 January, the Ne variations from SwA and SwC look quite similar. SwC observes a localized drop in Ne at \(\sim\)35\({}^{\circ}\)N, while Ne from SwA peaks. Below, this feature will be discussed in more detail. Passages through the more sunlit (17 LT) ionosphere revealed much larger _Ne_ values (Figure 8b). The satellites observed a gradual decrease in Ne with increasing latitude. Intense ionization by solar UV radiation effectively smoothed out any small-scale irregularities in plasma density. It is only noteworthy that, among the days under consideration, 22 January is characterized by the lowest Ne values. The reason for this feature is not clear but may be related to the local redistribution of ionospheric plasma. From Figure 8, it is clear that the Ne variations measured along the descending orbits of SwA/SwC, when the winter ionosphere is in darkness, are more informative in the sense of a possible coseismic signal than the sunlit ionosphere. They deserve further analysis. To detect the anomalies possibly associated with a strong EQ, we calculated the difference (\(\Delta\)Ne) between the SwA and SwC Ne values. With this approach, the background field is automatically eliminated. SwC flew 7 s ahead of SwA, and the distance between satellites was 1.4\({}^{\circ}\) in longitude, giving a total distance of about 150 km. We subtract the samples at the same acquisition time (thus at different geographical latitudes). To reveal the specific changes in the plasma density on 22 January compared with other days, Figure 9 shows a stacked plot of \(\Delta\)Ne for 14-28 January. From Figure 9, it can be seen that \(\Delta\)Ne is usually slightly positive (i.e., the SwA Ne exceeds the SwC Ne, which is likely due to the calibration of LPs), with irregular fluctuations in the range of \(<\)10\({}^{4}\) el/cm\({}^{3}\). On 22 January, the shape of the \(\Delta\)Ne variation was quite different. It looks like a peak that is almost twice as high and exceeds 0.15 \(\times\) 10\({}^{5}\) el/cm\({}^{3}\), with a maximum at \(\sim\)35\({}^{\circ}\)N. At the edges of the latitudinal segment, \(\Delta\)Ne is negative. The amplitude of the \(\Delta\)Ne peak is about 10% of the background level compared with the enhanced Ne on 22 January and \(\sim\)20% compared with other days. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sat** & **Orbit** & **Appr. UT** & **Appr. LT** \\ \hline \multirow{2}{*}{Swarm\_A, Swarm\_C} & ascending & 12 & 17 \\ \cline{2-4}  & descending & 23 & 05 \\ \hline \multirow{2}{*}{Swarm\_B} & ascending & 02 & 08 \\ \cline{2-4}  & descending & 14 & 20 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of satellite passages through the selected area 30\({}^{\circ}\)–45\({}^{\circ}\)N, 65\({}^{\circ}\)–95\({}^{\circ}\)E on 22 January. This signature implies the presence of horizontal ionospheric irregularities and steep gradients in Ne. Another possibility is a lifting or lowering of the ionospheric layer with respect to the satellite altitude. Rapid changes in the plasma density via the production/recombination processes are unlikely at midlatitudes [57]. The change in Ne may only be due to transportation processes. Although there is no possibility to distinguish whether the magnetic field is perpendicular to the magnetic field, the magnetic field is perpendicular to the magnetic field. Figure 8: Stacked plot of the Ne variations (in \(10^{5}\) cm\({}^{-3}\)) along the SwA (blue) and SwC (red) trajectory between 30\({}^{\circ}\)N and 45\({}^{\circ}\)N on 14–28 January: (**a**) descending orbits, 05 LT; (**b**) ascending orbits, 17 LT; (**c**) descending orbit on 22 January.