## 6 Discussion
### Assessment of the E-Field Causing Ionospheric Irregularities over the EQ Area
The discrepancies between the modeling results are due to different approaches to boundary conditions in the upper atmosphere and different approximations of the vertical profile of air conductivity. Both parameters are poorly defined. Note that on 22 January, the air conductivity was likely increased due to the flux of solar protons. More case studies are critical to revealing typical and specific characteristics of the ionospheric response to large earthquakes and quantitatively testing the models. The peak-like pattern of the _Ne_ disturbance observed by the Swarm tandem on 22 January is in qualitative agreement with the model predictions. According to the model proposed in [1], the horizontal distribution of the perturbed plasma density in the F-region is divided into regions of increased and reduced concentration. Both regions move towards the equator with increasing altitude. The ionosphere simulation code SAMI3 [75] used to study TEC variations caused by the E-field from the surface charges of the earthquake fault zone also demonstrated splitting [17]. These authors found that plasma density enhancement (reduction) was on the west (east) side of the epicenter. The models and observations suggest that the E-field penetration efficiency at night is higher than during the day, which strongly depends on the size of the ground-level vertical E-field localization. The _Ne_ anomalies observed by the Swarm satellite occurred exactly on the day of the main shock and a series of aftershocks. The statistical and case studies convincingly demonstrated that the pre-seismic signals may be detected in the ionosphere above the earthquake preparation zone for several days or even months (e.g., [25; 76; 77; 78]). This approach was considered promising for searching for earthquake precursors. However, in our particular event, no relevant anomalies during the preceding period of reasonable duration were found. On a relatively long-term time scale, it seems quite difficult to distinguish cooseismic ionospheric anomalies from ionospheric variability due to variable space weather conditions. Also, the question remains open as to why the pre-seismic signal disappears for some time and does not continuously evolve into a coseismic signal. We believe that the large global increase in Ne, from \(\sim\)0.5 \(\times\) 10\({}^{5}\) (the typical level for nearby days) up to about 1.5 \(\times\) 10\({}^{5}\) el/cm\({}^{3}\) observed by both satellites on 22 January (Figure 8), cannot be considered a seismo-induced signature. By coincidence, a series of moderate solar flares accompanied by X-ray and solar proton fluxes occurred immediately before and after the main seismic shock. The solar irradiance increased the ionization rates and the amount of plasma in the ionosphere. At the same time, the geomagnetic indices (the parameters that are commonly used to identify periods of geomagnetic quietness suitable for searching for pre- or coseismic anomalies in the ionosphere) remained almost undisturbed. It is because the SW disturbance (a magnetic cloud but not the irradiance or solar energetic particles) was weak, and thus the ionospheric auroral electrojets and ring current (the electric currents that are responsible for ground geomagnetic disturbances) were not affected. In this regard, we note that the idea exists that space weather disturbances may play a role in the occurrence of earthquakes. Some evidence of such a link, albeit with a prolonged time delay between a geomagnetic storm and a seismic event, is presented in [79]. Another idea, supported by numerical simulations, is that solar flares can cause changes in the density of telluric currents in faults in the Earth's crust, capable of triggering earthquakes [80].