## 3 Results
### Case 10 (8 September 2017)  
On 8 September 2017, the intense geomagnetic storm caused severe disturbances, with a minimum Dst index of \(-132\) nT (daily SSN:88). Numerous studies have investigated the ionospheric dynamics during this storm period (e.g., [25; 31]). In this case study, our focus is on exploring the association between SF occurrences in the bottomside ionosphere and the irregularities observed in the topside latitude Ne profile of the Swarm during the main phase of the storm.  
At 03:10 UT, intense SF conditions affected the ionosphere over Fairford, Dourbes, and Roquetes, as shown in Figure 11a. Over Fairford and Dourbes, MSF signatures were observed, with oblique traces propagating westward and northeastward over Fairford, while at Dourbes, the oblique traces were northeastward. Roquetes displayed an RSF signature without any oblique traces. During this period, the geomagnetic storm was highly active [31], and the GNSS d-TEC map (Figure 11b) revealed the existence of SF-associated MSTID signatures. To further investigate, we analyzed the ROTI map under SF activity, which showed a fluctuation of \(\sim\)0.2 TECU/min over Fairford with an hmF2 of 427 km, indicating significant ionospheric disturbance.  
Figure 10: On 19 May 2017: (**a**) SF event was observed over Juliusruh at 00:30 UT; (**b**) MSTID signatures in GPS d-TEC map (pink markers indicate locations of Juliusruh and Pruhonice); (**c**) ROTI fluctuations observed over Juliusruh and Pruhonice (pink markers); (**d**) Swarm B track over Juliusruh and Pruhonice; and (**e**) latitude Ne and (**f**) dNe profiles of Swarm B over Juliusruh and Pruhonice latitude.  
At 03:20 UT, Swarm B passed over Fairford, Dourbes, and Roquetes (Figure 11d-f). The latitude Ne profile from Swarm B (Figure 11e) showed intense fluctuations, reflecting the presence of irregularities in the topside ionosphere. A pronounced southward gradient was observed south of Roquetes, and deep Ne fluctuations south of Dourbes and Fairford indicated the irregularity structures. [PERSON] et al. 2022 [31] reported strong fluctuations in the Swarm B Ne profile, associating them with large-scale travelling ionospheric disturbances (LSTIDs) in the topside ionosphere. To validate the presence of these irregularities, we analyzed the dNe and found substantial fluctuations of \(\sim\)1.2 \(\times\) 10\({}^{4}\) elec/cm\({}^{3}\) around Dourbes and Fairford, confirming the association with SF activity. [PERSON] et al. 2019 [25] studied Swarm orbit degradation during storm main phases, noting precision errors exceeding 10 cm. Their findings suggested that increased thermospheric neutral density, more so than increased ionospheric plasma density, significantly affected orbit determination, leading to cascaded irregularity distributions in both the topside and bottomside ionosphere. [PERSON] et al. 2022 [31] further analyzed the Swarm latitude Ne profile during the storm period, identifying pronounced spatial and temporal variations in ionospheric characteristics over Europe. These variations were driven by MIT displacement and were often coupled with TIDs, resulting in complex irregularity patterns in the topside ionosphere. Their research highlighted the propagation of LSTID signatures from high to midlatitudes during all storm phases (initial, main, and recovery), with velocities between 300 m/s and 444 m/s and a duration of 3-5 h.  
Figure 11: On 8 September 2017, it was a geomagnetically active day (September Storm 2017): (**a**) observation of SF events over Fairford, Dourbes, and Roquetes at 03:10 UT; (**b**) MSTID in GPS d-TEC map (pink markers indicate locations of Fairford, Dourbes, and Roquetes); (**c**) ROTI fluctuations observed over Fairford, Dourbes, and Roquetes; (**d**) Swarm B track over Fairford, Dourbes, and Roquetes; (**e**) latitude Ne and (**f**) dNe profiles of Swarm B over Fairford, Dourbes, and Roquetes latitude.