## 3 Results
### Case 1 (4 February 2015)  
On 4 February 2015, geomagnetic activity remained quiet (minimum Dst \(\sim-11\) nT; daily SSN: 96). Around 01:00 UT, SF was observed over Fairford, Dourbes, and Roquetes Digisondes. Figure 2a-f illustrates the key features of this event (Case 1). In Figure 2a, we see that all three Digisondes exhibit FSF ionogram signatures. At midlatitudes, plasma instabilities are often triggered either by the formation of unstable sporadic E layers (spread Es) or by MSTIDs driven by GWs. Since no evidence of sporadic E layers was observed in the ionograms, we analyzed GNSS-derived d-TEC maps for MSTID signatures, as shown in Figure 2b. MSTIDs were indeed detected over Fairford, Dourbes, and Roquetes around 01:00 UT, preceding or coinciding with SF.  
As MSTIDs propagate, they can induce perturbations that fragment into smaller-scale structures, potentially affecting the topside ionosphere [30]. To explore this, we examined the latitude Ne profile recorded by Swarm B as it passed over these Digisonde sites around 01:05 UT (Figure 2d). Figure 2e shows the Swarm B Ne latitude profile along its track, while Figure 2f highlights fluctuations (dNe), revealing significant topside irregularities. Specifically, the Swarm B Ne exhibited high fluctuations over Fairford and Dourbes, with lower fluctuations observed over Roquetes. A fluctuation amplitude of dNe \(\sim\) 1.5 \(\times\) 10\({}^{4}\) elec/cm\({}^{3}\) was noted in the topside over Fairford and Dourbes (delineated by a dotted box) in connection with the FSF signatures, where hnr2 reached \(\sim\)425 km. Over Roquetes, a lower fluctuation of dNe \(\sim\) 0.5 \(\times\) 10\({}^{4}\) elec/cm\({}^{3}\) was observed, corresponding to RSF at hnr2 \(\sim\)370 km.  
[PERSON] et al. 2019 [44] investigated the RODI from Swarm Ne measurements and compared it with the ROTI derived from GNSS L-band data, demonstrating that RODI can effectively identify irregularities linked to scintillation events observed from GNSS receivers on the ground. They found a notable similarity between RODI fluctuations and ground-based ROTI values. Following this, we analyzed ROTI fluctuations near Digisonde stations under SF activity, as shown in Figure 2c, around 00:50 UT. Over Dourbes, ROTI values of \(\sim\)0.15 TECU/min [23] were registered, consistent with Swarm B Ne fluctuations of \(\sim\)1.5 \(\times\) 10\({}^{4}\) elec/cm\({}^{3}\), supporting findings of a link between ground-based and topside ionospheric irregularities [44].  
Additionally, Figure 2a reveals oblique traces representing a southwest-to-northeast flow of plasma instabilities, visible as a transition from yellow to blue traces (red arrows in the ionograms of Figure 2a), over Fairford and Dourbes. This pattern is indicative of MSTIDs, as corroborated by Figure 2b. MSTIDs, which are often triggered by GWs, can cause undulations that contribute to the formation of SF and can extend to influence the topside ionosphere as small-scale irregularities [30]. In this case, we observed pronounced plasma fluctuations in the Swarm latitude Ne profile (Figure 2e) south of Fairford and Dourbes.  
In summary, these observations suggest a localized link between bottomside irregularities and topside fluctuations, as evidenced by simultaneous Ne fluctuations detected in Swarm profiles, providing further insight into the dynamics linking the bottomside and topside ionosphere.