## 3 Results and Discussion
### Case-Study of Geomagnetic Storm on 7-8 September 2017
For the period of 7-8 September 2017, we have several fortunate occultation events from the GOES-16 geostationary GPS receiver with the tangent points located over the high and midlatitudes. To retrieve the signatures of the ionospheric irregularities, we analyzed the rate of change of the total electron content on the GOES GPS receiver-GPS transmitter links for successful radio occultation events when the electron density profiles were retrieved according to the methodology from (62). Prior to the main phase of this geomagnetic storm, four successful GOES RO events took place. Two events occurred at high latitudes of the Northern Hemisphere, near Greenland at 00:45 UT and 09:20 UT, and two in the Southern Hemisphere with tangent points (magenta dots) at midlatitudes over the Atlantic Ocean at around 01:42 UT and at high latitudes over Antarctica around 15:51 UT on 7 September 2017 (Figure 4). Figure 4: _Cont._  
Figure 3: The daily MLT-MLAT ROTI maps for the Northern Hemisphere for **(a–c)** 6–8 September 2017. 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. The blue color indicates no or very weak ionospheric irregularities, whereas the red color shows severe ionospheric irregularity occurrence. The left panels on Figure 4 show a global distribution of the ionospheric irregularities' occurrence as detected by ground-based GNSS ROTI observations before the storm's main phase and for the temporal periods close to the GOES RO events. In addition, each geographical map shows projections of the GOES-16 position (black dot near the equator at 75.2\({}^{\circ}\) W longitude) and central line-of-sight link in the direction between the GOES GPS receiver and GPS transmitter (black line), which extends beyond the map due to the GEO-GPS geometry and location of the GPS satellite on the opposite side of the Earth (Figure 1). The right panels on Figure 4 show variations in the rate of the TEC (ROT) as derived from the GOES GPS measurement for several successful occultation events spanning an altitudinal range of 100-1000 km. The first set of ROTI maps for 7 September 2017, corresponding to the pre-storm conditions, shows quite typical results with a very low intensity of ionospheric irregularities  
Figure 4: The rate of the TEC from the GOES GPS RO observations as a function of altitude (right) and ground-based ROTI maps corresponding to these occultation events (left) for **(a–d)** several successful occultation events for pre-storm conditions on 7 September 2017. The gray shading on the maps shows the nighttime, and the thick black line shows the magnetic equator. The maps also show the position of GOES-16 at 72.5\({}^{\circ}\) W (large black dot), GOES RO tangent point projections (magenta dots), and line-of-sight link in the direction between the GEOS GPS receiver and GPS transmitter (black line). (low ROTI values correspond to the scale blue marks) practically everywhere globally, with recognized intensification in ionospheric irregularities' occurrence at high latitudes near the polar cap. The altitudinal variation in the ROT shows some variations in the TEC on the links between the GPS receiver onboard the geostationary GOES-16 and GPS transmitter with the first peak around 100-150 km, which correspond to the ionospheric E layer, and the second one close to the F2 layer ionization maximum, around 400 km. But the magnitude of these variations is quite small and does not exceed 0.2 TECU/min. These elevated ROT values can be explained by the normal quiet (or slightly disturbed) conditions when particle precipitations can occur at high latitudes and in the polar cusp region, which was confirmed by the corresponding ground-based ROTI maps (Figure 4a,c). Figure 5, organized in the same way as Figure 4, presents the results corresponding to the main phase of the geomagnetic storm.