## 3 Results and Discussion
### Case-Study of Geomagnetic Storm on 25-26 August 2018  
Following a similar approach, we examined the GOES GEO RO data to identify the signatures of the storm-induced ionospheric irregularities for the case of the August 2018 geomagnetic storm. This event occurred on 25-26 August 2018, and it was one of the biggest geostorms in the 24 th solar cycle (SYM-H min -206 nT). Figure 6 shows the geophysical conditions with variations in the IMF Bz and AE and SYM-H indexes during 25-27 August 2018. The IMF Bz southward turning occurred after 14 UT on 25 August 2018. The main phase of the geomagnetic storm started to develop after \(\sim\)17:30 UT on 25 August. The SYM-H index reached its minimum of \(-\)206 nT at \(\sim\)07 UT on 26 August. The auroral electrojet AE index exceeded 500 nT after 18 UT on 25 August, and the AE peaks with values up to 1500-2000 nT were registered during 02-09 UT on 26 August 2018. Figure 7 presents the IGS polar ROTI maps illustrating the overall pattern of the storm-induced ionospheric irregularity development during the August 2018 geomagnetic storm. For the rather quiet day of 25 August 2018 (Figure 7a), the area affected by the ionospheric irregularities was within 80\({}^{\circ}\) MLAT on the dayside and 70-72\({}^{\circ}\) MLAT on the nightside, with the highest intensity of the ROTI occurring between 19 and 22 MLT. For the day of the main phase of the geomagnetic storm, 26 August 2018 (Figure 7b), the daily ROTI map shows a significant contrast in comparison to the previous day. In particula  
Figure 6: Variations in IMF Bz and auroral electrojet (AE) and SYM-H indices during 25–27 August 2018.  
irregularities had high intensity (ROTI values equal to or exceeding 1.0 TECU/min, red color on the map), and their spatial distribution formed a clearer oval-like shape largely expanded in size. The auroral irregularities' oval expanded equatorward as far down as \(\sim\)55\({}^{\circ}\) MLAT for the nighttime sector and 60-65\({}^{\circ}\) MLAT for the dayside. For the day with the storm's recovery phase, the daily ROTI maps depicted that the auroral irregularities' zone was greatly diminished in size and intensity (Figure 7c). For the period of 25-26 August 2018, we also obtained several successful occultations from observations provided by the geostationary GPS receiver onboard GOES-16. Figure 8 presents the details of the ionospheric irregularities' signatures in the GOES GPS observations specified by the ROT and the ionospheric irregularities' occurrence on a global scale as specified by ground-based ROTI measurements before the geomagnetic storm onset at \(\sim\)17:30 UT on 25 August 2018. The corresponding ROTI maps illustrate the lack or extremely weak presence of ionospheric irregularities (blue-colored ROTI values) across the globe, with some increased activity in the high latitudes in the dayside region. There were two GOES RO events that occurred at 02:55 UT and 16:48 UT on 25 August 2018 in the opposite hemispheres near 60\({}^{\circ}\) N and 60\({}^{\circ}\) S, respectively. The altitudinal-temporal variations in the ROT along the GOES RO links were also quite small. During both occultations, the ROT values indicate some fluctuations in the TEC on the links connecting the geostationary GOES to the GPS transmitters on the opposite side of the Earth. The largest ROT values with a magnitude of 0.1-0.15 TECU/min were concentrated at \(\sim\)100-150 km altitude near the E layer and at \(\sim\)300-400 km altitudes corresponding to the typical location of the F2 layer ionization peak. Similar to those observed in the September 2017 pre-storm cases, these ROT variations can be explained by the normal quiet-time plasma density gradients on the GOES-GPS links passing through the polar region.