## 4 Discussion  
While previous studies [3; 4; 8; 9; 18] have examined various mechanisms, origins, distributions, and latitude dependencies [18; 45] of midlatitude irregularities, significant gaps remain in understanding the generation of topside irregularities [32; 33; 34; 35; 36] and their connection to bottomside disturbances. Section 1 discusses mechanisms such as Perkins instabilities [8; 9; 10], AGW-induced MSTIDs [11; 15; 16; 17; 18], and UA-NDP [13; 14] that contribute to the formation of irregularities in the bottomside ionosphere. Using data from six Digisonde stations across the European midlatitude region (2015-2017) and coinciding Swarm satellite passes, we analyzed latitude-based Ne profiles to detect topside irregularities. Additionally, GNSS d-TEC and ROTI maps were used to study MSTIDs and ROTI associated with SF activity. Ten specific SF cases and associated topside irregularities were identified and studied in detail. In this analysis, nine out of ten cases (except Case 8) showed that the upper midlatitude regions (latitude \(>\) 45\({}^{\circ}\) N) were more affected by SF than the lower midlatitude regions (latitude \(<\) 45\({}^{\circ}\) N) [18]. Most of these cases involved FSF or MSF, predominantly associated with topside irregularities observed as fluctuations in Swarm Ne data. This suggests that the dynamics of the midlatitude ionosphere are heavily influenced by high-latitude dynamics, particularly the MIT. The widely accepted mechanism for MIT formation involves plasma \"stagnation\" due to interactions between high-latitude plasma convection and midlatitude corotation flow, with additional contributions from sub-auroral ion drift (SAID), sub-auroral electron temperature enhancements, and Joule heating [46] (pp. 1-2). Theoretical studies have suggested that small-scale instabilities may arise along the equatorward and poleward walls of the MIT, such as temperature gradient drift instabilities on the equatorward wall, which can generate plasma irregularities. These irregularities may then be transported equatorward, contributing to FSF/MSF formation in the upper midlatitude region. This process is accompanied by F-layer uplift, as observed in Cases 1, 2, 4, and 10 (though Case 10 occurred during a magnetically disturbed period). The uplift supports bottomside irregularities, which, under the influence of asymmetric geomagnetic field lines and gravity waves (GWs), drift to higher altitudes and impact the topside Ne, triggering topside ionospheric irregularities. [PERSON] et al. 2023 [18] concluded that higher midlatitude regions are primarily affected by FSF, which represents elevated bottomside irregularities. MSF, in contrast, is a transitional state where RSF evolves into FSF. The frequent association of FSF/MSF with topside irregularities in this study suggests a strong likelihood that these irregularities originate from the equatorward wall of the MIT Case 8, a unique exception, involving irregularities at lower midlatitudes with no corresponding irregularities in the upper midlatitude ionosphere, as detailed in Section 3.8. The present study demonstrates the consistent manifestation of MSTIDs with SF-associated topside irregularities across all ten cases. MSTIDs, often initiated by GWs, contribute to undulations that foster the formation of SF and extend their influence into the topside ionosphere as small-scale irregularities. [PERSON] et al. 2022 [30] highlighted that gradients in topside effective scale height during satellite passes are indicative of such small-scale irregularities. They attributed these irregularities to asymmetric geomagnetic and electric field lines, which facilitate upward plasma motion, cascading medium- and large-scale irregularities into smaller-scale disturbances observable in Swarm latitude Ne profiles. During SF events, MSTIDs likely uplift and fragment these irregularities into smaller structures, as evidenced by strong fluctuations in Swarm data.