## 4 Local Variations of Ne in the Topside Ionosphere during Seismic Activity
### Ionospheric Disturbances over the EQ Area  
Along-track satellite observations provide a brief overview of the surrounding EQ regions. To detect local Ne anomalies over the area surrounding the EQ epicenter, the Swarm passages in the region 30\({}^{\circ}\)-45\({}^{\circ}\)N, 64\({}^{\circ}\)-94\({}^{\circ}\)E were selected. The spatial scale of this area roughly corresponds to the Dobrovolsky radius of influence, R (km) = 10\({}^{0.430}\)[54]. For the event under study, R \(\approx\) 1000 km. A rectangle is centered on the meridian of the EQ epicenter with a side of 15\({}^{\circ}\). The latitudinal extent was chosen to be half as long as the longitudinal one. The geometry and size of the rectangle were chosen for two reasons. Firstly, active faults are elongated in latitude. Secondly, it is desirable to catch more satellite passages over the selected area (the rotation of the Earth during the time between consecutive passages is about 25\({}^{\circ}\)). At the orbital altitude, a spatial window is taken larger than that controlled by the ground-based parameter R. Within this window, the along-track measurements show the exact coordinates of the Ne anomaly. Relative to the epicentre, the latitudinal boundaries are slightly shifted to the south. This is because the seismo-electromagnetic signal in the ionosphere is supposed to be propagating along geomagnetic field lines [55], which in the northern hemisphere tilt south. According to the IGRF model [56], the ground-level latitude of 40\({}^{\circ}\)N is mapped to about 35\({}^{\circ}\)N at the orbital height. It was previously noted that the affected area extends approximately 600 km in the equatorial direction from the epicenter located at 40\({}^{\circ}\)N [1]. Figure 7 illustrates the Swarm satellites passing in the proximity of the epicenter of the 22 January EQ (thick lines) along with the other nearby tracks on this day (thin lines). Passages of the tandem of SwA/SwC satellites were at ~12 and ~23 UT; the Swarm_B passages occurred at ~08 and 14 UT. The time characteristics of satellite passages through the EQ area are presented in Table 1. Typically, the SwA/SwC trajectories cross this area twice a day: once along the ascending branch and once along the descending one. Since the orbit drifts in longitude, the distance between successive passages is about 25\({}^{\circ}\), giving rise to gaps in the satellite coverage of the seismically investigated region. Figure 7: Swarm orbit projections for 22 January. The trajectory of SwA, SwB, and SwC is shown as a thin line in blue, red, and green, respectively. Trajectory segments that fall into a rectangular area around the epicenter (white star) are highlighted with thick lines: orange (purple) for SwA/SwC (SwB). To investigate the temporal evolution of Ne before, during, and after the earthquake, for the 15-day period centered on the day of the main shock, the data from SwA and SwC flying in pairs were selected. For each day from 14 January to 28 January, the Ne data were extracted, respectively, between 30\({}^{\circ}\)-45\({}^{\circ}\)N and 64\({}^{\circ}\)-94\({}^{\circ}\)E, for the descending (night-side half-orbits) and ascending (day-side half-orbits) nodes. Then the time series of the successive passages was formed. Figure 8 presents a stacked panel of the Ne variations from SwA and SwC vs. latitude in the range 30\({}^{\circ}\)-45\({}^{\circ}\)N as actually observed along successive descending (Figure 8a) and ascending (Figure 8b) orbits on 14-28 January. The Ne for the EQ date, 22 January, is shown on a larger scale in a separate panel (c). As Figure 8a shows, in the early morning, the Ne ionospheric values are close to 0.7 \(\times\) 10\({}^{5}\) el/cm\({}^{3}\). On 18 and 19 January, Ne dropped below 0.5 \(\times\) 10\({}^{5}\) el/cm\({}^{3}\) (on January 20, there were no appropriated passages over the selected area). On 22 January, the day of the main shock occurrence at 18 UT, Ne jumped up to 1.5 \(\times\) 10\({}^{5}\) el/cm\({}^{3}\).