## 4 Discussion  
Using the data from Figure 4, we can conclude that, during the geomagnetic storm, 500-5500 m small-scale irregularities were present in the ionosphere. The absence of the GPS/GLONASS amplitude signal scintillations means that there were no intensive irregularities with corresponding scales. The ionospheric irregularity amplitude exponentially decreased as the irregularity size decreased [1; 53]. That is why there were no significant 100-300 m irregularities. We also examined mechanisms for the existence of such intensive small-scale irregularities. We had two hypotheses. The first possible mechanism is an expansion of the auroral oval toward mid-latitudes, so we would observe auroral phenomena at mid-latitude stations. Particle precipitation results in local increases and decreases in the plasma density more than 10 km wide. Such structures can be cascaded into smaller scales through instability processes and turbulent diffusion [54]. The 0.1-30 km irregularities are generated at high latitudes mainly due to ExB instability [55]. Also, the thermomagnetic instability can produce F-region 1 km-scale irregularities at high-latitudes [56]. The second mechanism suggests generation of small-scale irregularities due to passage of a medium-scale (MS) TIDs [57]. For example, generating medium-scale irregularities due to the large-scale disturbance passage was mentioned by [PERSON] et al. [16]. [PERSON] et al. [16] found medium-scale 2-10 min irregularities, when a well-distinguished strong disturbance of the 40-min time scale passed by. There are a number of papers by [PERSON] focusing on the spread-F caused by TIDs. [PERSON] and [PERSON] [58] showed that, during the passage of a large scale TID, the average ionospheric F2 layer height rises. [PERSON] [17] showed that the spread-F observed in equatorial regions several hours after increased geomagnetic activity is directly related to ionosphere height rise. Studies by [PERSON] et al. [59] indicated that a favorable condition for the instability growth is induced rise of the ionosphere height. The \"rise-in-the-ionospheric-layer-height\" mechanism, which makes the spread-F observable at equatorial stations, may also produce the spread-F at mid-latitudes [58]. In fact, gravity waves are the prevailing source for both MS TIDs and small-scale irregularities [60]. Gravity waves induce currents that drive plasma instability. To find a proper mechanism, we used, first, the OVATION model data and the SupernDARN data to estimate the oval boundary dynamics and its correspondence with the observed small-scale irregularity appearance. Second, we studied the global distribution of TEC variations. On the one hand, the global TEC variations maps can reveal TID origin and propagation. On the other hand, the maps provide some evidence of small-scale structure development. Third, we analyzed the changes in PPP error over the globe. An increase in the PPP error and its localization can provide additional information about the source of the recorded small-scale irregularities. To study oval expansion we used the OVATION prime energy flux data (not shown here) ([[http://iswa.gsfc.nasa.gov](http://iswa.gsfc.nasa.gov)]([http://iswa.gsfc.nasa.gov](http://iswa.gsfc.nasa.gov)); [[https://sourceforge.net/projects/ovation-prime/](https://sourceforge.net/projects/ovation-prime/)]([https://sourceforge.net/projects/ovation-prime/](https://sourceforge.net/projects/ovation-prime/))) [61]. For the time interval of interest, the oval boundary did not reach 52\({}^{\circ}\)N for the Asian sector. However, the OVATION is a model and cannot represent the actual precipitation (especially during such intensive storms). Based on SuperDARN data, [PERSON] et al. [27] studied the polar convection for this very storm and concluded that the auroral oval expanded to \(\sim\)50\({}^{\circ}\)N (we should note that it is magnetic latitude). We also used the SuperDARN data to study this expansion in detail.