Climatology of Satellite Traces over European Mid-Latitudes and Their Association with Travelling Ionospheric Disturbances

Satellite traces (STs) are additional ionogram echoes associated with off-vertical reflections from spatially structured F-region plasma. This study investigates the climatology of STs over the European mid-latitude ionosphere using Digisonde ionograms from Roquetes, Dourbes, and Juliusruh during 2020–2024. STs were manually identified from ionogram sequences and their annual, seasonal, and monthly–hourly occurrence characteristics were examined. At all three stations, STs occurred predominantly at night, with a clear enhancement during late spring and summer. The highest monthly–hourly occurrence rates were observed during the evening sector, reaching approximately 0.32 at Roquetes, 0.29 at Dourbes, and 0.21 at Juliusruh, with lower occurrence extending into the post-midnight hours. A representative multi-station event on 6–7 August 2024 was analysed using Digisonde-derived F-region tilt and regional detrended total electron content (dTEC) observations. During this interval, STs were observed while the local F-region tilt varied and a nighttime medium-scale travelling ionospheric disturbance (MSTID), characterised by predominantly southwestward propagation and NW–SE-oriented phase fronts, traversed the European sector. For the 6–7 August 2024 event, the combined observations are consistent with STs arising from off-vertical reflections from temporally varying and tilted F-region isoionic surfaces during MSTID passage, while not implying that all ST events observed during 2020–2024 were associated with MSTIDs.