## 6 Discussion
### Assessment of the E-Field Causing Ionospheric Irregularities over the EQ Area  
On the Earth's night-side, the ionospheric plasma density is more sensitive to a possible signal of non-solar origin. A comparison of two time series of plasma densities measured by the LP instruments on board the Swarm satellites flying side by side at a distance of \(\sim\)150 km over the region of the strong EQ in Central Asia at 23 UT revealed a considerable difference in Ne. SwC, flying first, detected higher along-track _Ne_ values, while the second satellite (SwA) detected a drop of about 1.5 \(\times\) 10\({}^{4}\) el/cm\({}^{3}\). Comparison with other passages showed that the difference in plasma density was maximized on the day of the main shocks and aftershocks. This spatially localized ionospheric anomaly is attributed to the coseismic effect. Another notable feature is that this regional anomaly arose against the background of a global increase in Ne. A series of GIM-TEC maps suggests that this enhancement is caused by a plasma plume propagating from the dayside lower latitudes. TEC increased globally in response to the solar flares. There are two main possibilities for the formation of small or medium-scale ionospheric irregularities similar to those observed. The first possibility is an uplift or downlink of the F layer, while the second is a drift of the plasma irregularities in the horizontal plane. Figure 13: Time series of daily values of (**a**) latent heat flux and (**b**) AOD from 11 January to 1 February. Both parameters are averaged over 35\({}^{\circ}\)–45\({}^{\circ}\)N, 65\({}^{\circ}\)–95\({}^{\circ}\)E. The day of EQ occurrence is indicated by red line. Any transport of plasma implies the presence of an enhanced E-field in the ionosphere. In particular, eastward (westward) electric fields will convect the mid-latitude ionosphere upward (downward). Rapid changes in the ionosphere can also be associated with TIDs, which are manifestations of atmospheric gravity waves (AGWs) originated, among other sources, from earthquakes [7]. However, since the Swarm orbital height is above the peak of the F2 layer, the coseismic TID signatures may hardly be observed directly. In the frame of the LAIC concept [3; 65], drastic changes in Ne in the ionospheric heights can be produced by intense disturbance E-fields originating from the ground. For the plasma density change of 1.5 \(\times\) 10\({}^{4}\) el/cm\({}^{3}\) (the value observed by Swarm during the event under consideration), estimating the vertical displacement of the plasma using the International Reference Ionosphere (IRI) model [66] gives a height difference of \(\Delta\)h\(\sim\)30 km (online calculation [67]). The authors of [68] studied zonal and vertical disturbance dynamo drifts in low/middle latitudes associated with storm-time prompt penetration electric fields, which originate at high latitudes. Their observations have shown that temporal variations in the height of the nighttime F layer can reach tens of kilometers under the influence of E-fields of about 0.5-1 mV/m. These estimates can be compared with values obtained from Swarm observations on 22 January. The chain is as follows: \(\Delta\)Ne\(\sim\)1.5 \(\times\) 10\({}^{4}\) el/cm\({}^{3}\), \(\Delta\)h\(\sim\)30 km, E\(\sim\)0.5 mV/m. At 45\({}^{\circ}\) latitude, the vertical drift E\(\times\)B velocity is in the order of 10 m/s, so it takes about an hour for plasma to uplift 30 km. Modeling revealed that a 10 kV/m vertical E-field above ground would produce a few mV/m horizontal E-field in the ionosphere [69; 70; 71]. Since the actual observed near-ground atmospheric E-field is less than 100 V/m [72; 73], the E-field of lithospheric origin is too weak for measurable perturbations at ionospheric heights. An estimate based on approaches from other models [32; 74] allows an E-field of about 1 mV/m in the ionosphere if a 100 V/m vertical E-field appears near ground.