## 4 Discussion
The data are available at [[http://vt.superdarn.org/](http://vt.superdarn.org/)]([http://vt.superdarn.org/](http://vt.superdarn.org/)) [62]. Figure 6 shows the maps for the electric potential from SuperDARN for 18:30 UT (a), 18:40 UT (b), 19:00 UT (c), 19:30 UT (d), 20:00 UT (e), 20:30 UT (f), 21:00 UT (g), 21:30 UT (h). Green dashed curve shows the Heppner-Maynard boundary. From Figure 6 one can see that, after 18:30 UT, the expansion of auroral oval started. In the Irkutsk sector (look for Lake Baikal at the map bottom), the Heppner-Maynard boundary reached 50\({}^{\circ}\) magnetic latitude at \(\sim\)19:00 UT and persisted at this position until \(\sim\)21:30 UT. Over that time, the boundary approached and then moved away from the facilities without reaching them. However, the boundary was very close to the radiation patterns of the instruments that we used (Figure 3). The ionosonde has wider radiation patterns. So, it was able to record more reflections from small-scale structures due to the expanded oval than the Irkutsk incoherent scatter radar was. The difference in the observation time seems to be related to the radiation pattern difference. This point proves the hypothesis that the oval expansion is the reason for generating small-scale irregularities. Another hypothesis is that small-scale irregularities are caused by the passage of a large-scale disturbance. We assume that a small-scale irregularity source is originated in the auroral area and then travels down to mid-latitudes. To prove this, we used TEC variation maps (see Experimental Facilities). On these maps, the TEC variation amplitude is plotted. The point positions correspond to the ionospheric pierce points (300 km height). Figure 7 shows the 10-20 min TEC variations maps for 18:30 UT (a), 19:00 UT (b), 19:30 UT (c), 20:00 UT (d), 20:30 UT (e), 21:00 UT (f). Figure 8 provides similar maps, but for 20-60 min TEC variations. For reference, we provide the movies that illustrate the global spatial-temporal distribution of the 10-20 min and 20-60 min TEC variations for 17:00-24:00 UT at 1-min time resolution (see Supplementary Materials, Videos S2 and S3). There are always sporadic disturbances in the auroral oval that do not form any pronounced structures. We can clearly see this before the storm from Videos S2 and S3. After the storm onset at 18:30 UT, we see a large-scale well-defined negative disturbance that covers all longitudes. Later, we see a positive disturbance. We can see a long negative front at 19:30 UT reaching the northern part of the Irkutsk region. Maps of 20-60 min TEC variations show an extended sharp \"edge\" in the European-Asian region. The \"edge\" dynamics corresponds to that for the Heppner-Maynard boundary from the SuperDARN data. The 10-20 min TEC variation dynamics (Video S2) show that, in the European-Asian region, the irregularities of the auroral origin (\"irregularity cloud\") approached 50\({}^{\circ}\)N. The same spatial limit is seen from maps of 20-60 min TEC variations. This agrees with the results in Figure 2 (left panel), where TEC variations exceeding 0.1 TECU were recorded only for the IPPs located northward 52\({}^{\circ}\)N. These auroral-origin irregularities, presumably, are manifestation of delayed instability development in the auroral oval. The recorded TEC variations (Figure 2b, Figure 7, Figure 8) can be related to the movement of the boundary with an increased ionization level. We should also note that the polar cap features smaller TEC variations. Along with the propagation of the auroral-origin boundary, we can see the polar cap expansion. After \(\sim\)19:20 UT, we can observe an appearance of large-scale TID, close to the position of the \"edge\" related to the oval expansion. Thus, there were observed both auroral oval expansion and TID generation on the auroral oval boundary. We should note that it is difficult to definitely separate these two phenomena due to the complexity of the considered event.