## 3 Results
### Case 2 (17 February 2015)  
On 17 February 2015, geomagnetic activity was moderately disturbed, with a minimum Dst index of \(-\)44 nT (daily SSN: 39). Figure 3a-f provides a comprehensive illustration of this case study. At 22:45 UT, a mixed spread F (MSF) ionogram signature--characterized by spreading in both altitude and frequency [45]--was observed over Juliusruh, while an FSF signature was detected over Pruhonice. The third Digisonde station, San Vito, recorded multiple reflected echoes (MREs), a known TID signature identified in ionograms [45]. In Figure 3a, over Juliusruh, oblique traces propagated from northeast to south. At Pruhonice,  
Figure 2: **(a)** observation of SF events over Fairford, Dourbes, and Roquetes at 01:00 UT; **(b)** MSTID signatures in GPS d-TEC map (pink markers indicate locations of Fairford, Dourbes, and Roquetes); **(c)** ROTI fluctuations observed over Fairford, Dourbes, and Roquetes (pink markers); **(d)** Swarm B track over Fairford, Dourbes, and Roquetes; and **(e)** latitude Ne and **(f)** dNe profiles of Swarm B over Fairford, Dourbes, and Roquetes latitude.  
similar oblique traces were detected, though propagating southeast. GNSS d-TEC maps (Figure 3b) confirm MSTID signatures coinciding with SF in these directions, indicating consistent propagation patterns across the region.  
Figure 3e,f displays the corresponding topside irregularity signatures recorded by Swarm B at 22:46 UT. A pronounced fluctuation in the latitude Ne profile was observed, stretching from south to north over Juliusruh, with a dNe amplitude of approximately 2 \(\times\) 10\({}^{4}\) elec/cm\({}^{3}\). In contrast to Juliusruh, no significant fluctuations were detected over Pruhonice or San Vito. The ROTI map (Figure 3c) also shows, under the SF activity, elevated ROTI values exceeding 0.1 TECU/min exclusively over Juliusruh.  
This observation suggests that ionospheric irregularities over Juliusruh were localized. According to [PERSON] et al. 2021 [46], various instabilities can develop along the equatorward or poleward walls of the Midlatitude Ionospheric Trough (MIT), leading to small-scale plasma density irregularities within the MIT region. Notably, temperature gradient drift instabilities often generate irregularities along the equatorward wall of the MIT, which can then be advected equatorward under the influence of MSTIDs. Possibly, these small-scale irregularities, generated within the MIT and carried equatorward by MSTID, contributed to the MSF formation over Juliusruh, where hmF2 reached 400 km. [PERSON] et al. 2022 [30] showed that the gradient in the topside effective scale height during satellite overpasses can serve as an indicator of small-scale irregularities. During this MSF event, these irregularities  
Figure 3: (**a**) observation of SF events over Juliusruh and Pruhonice whereas MREs registered over San Vito at 22:45 UT; (**b**) MSTID in GPS d-TEC map (pink markers indicate locations of Juliusruh, Pruhonice, and San Vito); (**c**) ROTI fluctuations observed over Juliusruh, Pruhonice, and San Vito; (**d**) Swarm B track over Juliusruh, Pruhonice, and San Vito; and (**e**) latitude Ne and (**f**) dNe profiles of Swarm B over Juliusruh, Pruhonice, and San Vito latitude.  
were likely uplifted and fragmented into smaller-scale structures, as evidenced by the steep fluctuations observed in the latitude Ne profile of Swarm B.