## 3 Geomagnetic Activity and Ionospheric Weather
### Variation of TEC Measurements  
TEC (due to its definition) accounts for the entire ionospheric slab up to the plasma-sphere. TEC primarily depends on solar EUV radiation, which forms its regular diurnal cycle. As geomagnetic activity increases, the picture becomes more complex due to increased electric fields, plasma drift, outgoing ion flows, ionospheric currents, and auroral precipitation that vary over different spatial and temporal scales. To account for natural ionospheric variations, we used the Global Ionospheric Maps (GIM) of TEC. The GIM-TEC maps with a spatial resolution of 2.5\({}^{\circ}\times 5^{\circ}\) for each hour are continually produced by the Jet Propulsion Laboratory [50; 51]. In Figure 4, the Northern Hemisphere (0\({}^{\circ}\)-70\({}^{\circ}\)N) GIM-TEC maps for 18 and 23 UT on 21-23 January are presented. The epicenter of EQ is shown as a white dot in panels (b) and (e). The choice of these particular UTs is due to the fact that at 18 UT the main shock of EQ occurred, while at 23 UT the EQ area was crossed by the descending orbits of the Swarm satellites (as will be discussed below). The TEC distribution is dominated by diurnal variations, and they gradually decrease with increasing latitudes. At 18 UT, the EQ area is almost on the Earth's night side. Visually, the difference between the TEC distributions at 18 UT for the three consecutive days presented in Figure 4a-c is quite minor. At 23 UT (Figure 4d-f), the EQ meridian moved toward the sunrise terminator, resulting in a naturally larger TEC over the epicentre. The TEC map for 22 January, 23 UT (Figure 4e) differs significantly from the maps for adjacent days. Assuming the TEC distribution on 21 January, 23 UT, was undisturbed by a solar flare, on 22 January, 23 UT, a relative increase in TEC throughout the hemisphere was observed. The elevated ionospheric TEC plume tends to expand from the daytime to the nighttime and from the equator to higher latitudes. The global increase in plasma density can most likely be explained by a response of the ionosphere to the increased ionization rate associated with the solar flare effect. The arrival of the SW Pd front, although its magnitude is minor, may also somewhat contribute to TEC [52]. Any possible anomalous signatures in ionospheric Ne, as they may contain the effects of induced coseismic signals, should be considered against the background of natural intra-ionospheric disturbances caused by space weather. At 23 UT, 22 January, due to the solar flare, TEC increased globally, including the ionosphere over the EQ area. A rough reference point, in particular, for subsequent analysis of the Swarm observations, appeared to be much higher than could be expected based on geomagnetic indices alone. To quantify the evolution of TEC on a global scale and near the EQ epicentre, a 3-day time series of hourly values was constructed. Figure 5 shows TEC values averaged over the Northern Hemisphere (0\({}^{\circ}\)-70\({}^{\circ}\)N) and collected at a grid point of 40\({}^{\circ}\)N, 80\({}^{\circ}\)E for the period of 21-23 January. The times of the Swarm A/C satellite passages near the epicentre in ascending and descending orbits are indicated by vertical lines. Usually, the hemispheric TEC (Figure 5a) exhibits a regular diurnal variation with slightly greater values around 00 UT. At the end of 22 January, the hemispheric TEC began to increase rapidly, and at 23 UT, it reached a peak of \(\sim\)40 TECU, which is at least twice as high as on the previous or subsequent day. At the particular geographic location, the UT variation is quite large (\(\sim\)30 TECU). The nighttime TEC is about 10 TECU. At 23 UT, 22 January, a notable excursion to the higher values is seen (Figure 5b). This peak is small compared with the amplitude of daily UT variation; however, for the night-side ionosphere, it is more than 10% of TEC. It is also noteworthy that in Figure 5b, a decrease in TEC at \(\sim\)16 UT on 22 January can be identified.