## 3 Results
### Case 6 (20 January 2016)  
On 20 January 2016, it was another geomagnetically quiet day (minimum Dst \(\sim\) 4 nT, daily SSN:62). Around 03:30 UT, both Fairford and Dourbes Digisondes detected MSF ionogram signatures, as depicted in Figure 7a. The stations predominantly experienced oblique traces, indicating the presence of MSTIDs. At Fairford, irregularities propagated northeastward and eastward, whereas Dourbes recorded southwestward propagation. These observations are supported by MSTIDs associated with the SF activity evident in the GNSS d-TEC map shown in Figure 7b.  
In this case study, Swarm A-C satellites passed over Fairford and Dourbes, as illustrated in Figure 7d. The latitude Ne and dNe profiles for this pass are presented in Figure 7e,f. In Figure 7e, a pronounced southward gradient in Ne is evident near the Dourbes latitude, while a strong northward gradient is observed near Fairford. Notably, between latitudes of 40-48\({}^{\circ}\) N, significant fluctuations in the latitude Ne profile indicate the manifestation of MSTIDs on the topside ionosphere. Figure 7f shows substantial dNe fluctuations (\(\sim\)1 \(\times\) 10\({}^{4}\) elec/cm\({}^{3}\)) over Fairford and Dourbes.  
A noteworthy observation is the phase shifts recorded between the latitude Ne profiles of Swarm A and C. These shifts occur when the Swarm A-C satellites encounter MSTID wavefronts. Even though the satellites are closely spaced, the latitude Ne profiles of Swarm A and C exhibit distinct phase differences when moving through MSTID signatures detected in GPS d-TEC maps. This scenario provides insight into MSTID propagation azimuths, as  
Figure 6: (**a**) observation of SF events over Dourbes and Roquetes at 23:20 UT; (**b**) MSTID signatures in GPS d-TEC map (pink markers indicate locations of Dourbes and Roquetes); (**c**) ROTI fluctuations observed over Dourbes and Roquetes (pink markers); (**d**) Swarm B track over Dourbes and Roquetes; and (**e**) latitude Ne and (**f**) dNe profiles of Swarm B over Dourbes and Roquetes latitude.  
measured by both satellites. [PERSON] and [PERSON] [37] introduced a straightforward method to infer TID propagation directions using these in situ Ne measurements from the Swarm A-C pair. Their approach was validated by comparing TID cases observed in detrended GNSS d-TEC maps across multiple continents. In this study, we demonstrate the propagation direction of MSTID (indicated by brown dotted lines in Figure 7f) over Fairford and Dourbes. As shown in Figure 7a,b, analyzing the phase difference between Swarm A-C Ne profiles in Figure 7f reveals a northeastward MSTID propagation over Fairford and a southward direction over Dourbes.  
To further investigate the association between bottomside and topside ionospheric irregularities, we analyzed ROTI fluctuations under SF activity at 03:30 UT using the GNSS ROTI map presented in Figure 7c. Over Fairford, a ROTI value of \(\sim\)0.15 TECU/min was recorded with an hmF2 of 352 km, while Dourbes showed \(\sim\)0.1 TECU/min (hmF2 \(\sim\) 329 km), both coinciding with MSF and dNe fluctuations of \(\sim\)1 \(\times\) 10\({}^{4}\) elec/cm\({}^{3}\). This observation implies the potential colocation of irregularities in both the bottomside and topside ionosphere. Consistent fluctuations suggest a shared source affecting both layers simultaneously, as proposed by [PERSON] et al. 2022 [30].  
Figure 7: (a) observation of SF events over Fairford and Dourbes at 03:30 UT; (**b**) MSTID signatures in GPS d-TEC map (pink markers indicate locations of Fairford and Dourbes); (**c**) ROTI fluctuations observed over Fairford and Dourbes (pink markers); (**d**) Swarm A–C track over Fairford and Dourbes; (**e**) latitude Ne and (**f**) dNe profiles of Swarm A–C over Fairford and Dourbes latitude with the brown dotted lines represent the propagation directions of MSTIDs.