## 3 Results and Discussion
### Case-Study of Geomagnetic Storm on 25-26 August 2018
As the geomagnetic storm started after \(\sim\)17:30 UT on 25 August followed by a rapid increase in auroral activity with the AE index rising from 500 nT to 1500 nT during the main phase of the storm, the global ROTI map depicts a significant intensification of the ionospheric irregularities' occurrence at high latitudes of both hemispheres (Figure 9). Later, as the storm developed, the belt-like area with the strong ionospheric irregularities of auroral origin largely expanded in size and moved equatorward toward the midlatitudes as far as \(\sim\)60\({}^{\circ}\) MLAT in the American and European sectors (Figure 9c, left). Here, we have three successful GOES occultations corresponding to the storm's main phase development during 25 August (Figure 9). The first one occurred at \(\sim\)19:31 UT with the tangent points located at \(\sim\)25\({}^{\circ}\) W longitude near the Antarctica region. The second RO event occurred at \(\sim\)21:22 UT with tangent points near 60-70\({}^{\circ}\) W close to Antarctic Peninsula. The third one took place at 23:39 UT near 73\({}^{\circ}\) N, 20\({}^{\circ}\) W in the Northern Hemisphere. During the first occultation (Figure 9a), we have rapid TEC variations described by the ROT with a magnitude up to 0.5 TECU/min. These intense ROT variations were primarily observed above the F2 layer ionization peak within the altitude range of 450-600 km. For the second  
Figure 7: The daily MLT-MLAT ROTI maps for the Northern Hemisphere for **(a–c)** 25–27 August 2018. The maps cover 50–90\({}^{\circ}\) N MLAT with 10\({}^{\circ}\) latitude circles; the magnetic local noon/midnight is at the top/bottom and dusk/dawn. occultation at ~21:22 UT (Figure 9b), the intense fluctuations in the altitudinal ROT were registered from ~450 up to 1000 km of the tangent point altitude. For the third occultation at ~23:39 UT near Greenland (Figure 9c), the intense fluctuations in the altitudinal ROT were mainly concentrated within a 400-450 km altitudinal range. The ROTI maps corresponding to these cases confirmed the presence of ionospheric irregularities near the area of the tangent point location along the line-of-sight links directed from the GOES location to the GPS transmitter on the opposite side of the Earth. During the next day, 26 August 2018, the storm main phase lasted until ~07 UT and then the recovery phase started. For this day, we have six successful GOES RO events (Figures 10 and 11). The first event occurred at ~00:50 UT with tangent points over Western Europe (Figure 10a). The altitudinal ROT from the GOES data indicated the presence of TEC gradients with a peak intensity near 0.5 TECU/min in the topside ionosphere within an altitudinal range of 650-900 km. The corresponding GNSS ROTI map (Figure 10a, left) shows the presence of intense auroral irregularities in the eastern European sector, located farther than the tangent points. Two other events happened relatively close to each other during the storm's recovery phase: one at 13:57 UT near 145\({}^{\circ}\) W longitude in the Alaska region and the second one at 14:44 UT near 120\({}^{\circ}\) W longitude in Canada (Figure 10b,c). For the case of ~13:57 UT, the intense ROT fluctuations were predominant within the altitudinal range of 600-900 km, whereas for the second case at ~14:44 UT, the fluctuations appeared mainly in the bottomside ionosphere at tangent point altitudes between 100 and 350 km. The ROTI maps (Figure 10b,c, left) provided confirmation of the ionospheric irregularities'  
Figure 8: The rate of the TEC from the GOES GPS RO observations as a function of altitude (right) and the ground-based ROTI maps corresponding to these occultation events (left) for (**a**,**b**) several successful occultation events for the pre-storm conditions on 25 August 2018. The gray shading on the maps shows the nighttime, and the thick black line shows the magnetic equator.