## 3 Results and Discussion  
### Case-Study of Geomagnetic Storm on 7-8 September 2017  
In this section, we analyze the signatures of the storm-induced ionospheric irregularities at high and midlatitudes that developed during the geomagnetic storm on 7-8 September 2017, which were detected in the GOES GEO RO ionospheric soundings. This storm occurred during a low solar activity period of the 24 th solar cycle and can be classified as a moderate-to-severe level. Figure 2 demonstrates the geophysical conditions during this storm with variations in the interplanetary magnetic field (IMF) north-south component Bz, auroral electrojet (AE) index, and SYM-H index. These data were provided by the NASA OMNIWeb service ([[https://omniweb.gsfc.nasa.gov/ow_min.html](https://omniweb.gsfc.nasa.gov/ow_min.html)]([https://omniweb.gsfc.nasa.gov/ow_min.html](https://omniweb.gsfc.nasa.gov/ow_min.html)), accessed on 4 July 2024). The initial phase of this storm was characterized by the steady southward IMF Bz (Figure 2, top panel) and by a strong increase in the auroral electrojet (AE) index (Figure 2, middle panel) from \(\sim\)21 UT on 7 September to 04 UT on 8 September with a maximum value up to \(\sim\)1500 nT. The SYM-H index reached its minimum value of -145 nT around 1 UT on 8 September 2017 (Figure 2, bottom panel). In order to review the overall ionospheric response to this geomagnetic storm in terms of ionospheric irregularity development, we analyzed the IGS polar ROTI maps. Figure 3 shows a visualization of the IGS (International GNSS Service) polar ROTI map product for three days from 6 September to 8 September 2017. These daily ROTI maps depict an overall pattern of the spatial distribution of the ionospheric irregularities across the Northern Hemisphere's high and middle latitudes in the magnetic latitude-magnetic local time (MLAT-MLT) coordinate system. For the relatively quiet day of 6 September 2017 (Figure 3a), the ionospheric irregularities of low intensity were mainly detected near 75-80\({}^{\circ}\) MLAT on the dayside and \(\sim\)70\({}^{\circ}\) MLAT on the nightside. The next day, 7 September 2017, was characterized by an increased level of auroral activity, and the AE index had peaks above 500-1000 nT during 05-11 UT and after the storm onset at -21 UT. Figure 3b illustrates this intensification of auroral activity with a more distinct oval-like area of ionospheric irregularities detected over the polar region of the Northern Hemisphere between 65\({}^{\circ}\) and 75\({}^{\circ}\) MLAT. For the next day of geomagnetic storm development, 8 September 2017 (Figure 3c), the daily ROTI map shows a dramatic difference in comparison to the quiet-time day. In particular, the auroral ionospheric irregularities had high intensity (ROTI values equal or exceed 1.0 TECU/min, red color), and their spatial distribution formed a clear oval-like shape (similar to an auroral oval's form seen in the satellite-based UV  
Figure 2: Variations in IMF Bz and auroral electrojet (AE) and SYM-H indices during 6–8 September 2017.  
observations, e.g., (69; 70)) that largely expanded toward the middle latitudes. The auroral irregularities' oval expanded equatorward as far down as \(\sim\)57\({}^{\circ}\) MLAT for the nighttime sector and 60-65\({}^{\circ}\) MLAT for the dayside. Because the GPS receivers onboard GOES satellites were not configured optimally for RO, there is a small number of observations tracked below 1000 km tangent point altitude because these observations are considered \"useless\" for the GOES navigational solution due to the impact of the ionosphere. Thus, the final number of successful RO retrievals was quite limited for this reason. On average, there were only approximately five recoverable electron density profiles per day that met our minimum criteria for retrieval (continuous signal tracked from 2000 km to below 100 km tangent point).