Climatology of Spread Es over European Mid-Latitudes: Seasonal, Diurnal, and Spatial Variability During the Ascending Phase of Solar Cycle 25

Spread sporadic-E (Spread-Es) appears in ionograms as diffuse and range-spread Es echoes associated with spatial structuring of the reflecting layer. In this study, five years (2020–2024) of Digisonde ionograms from three European mid-latitude stations—Juliusruh (54.6° N, 13.4° E), Dourbes (50.1° N, 4.6° E), and Roquetes (40.8° N, 0.5° E)—were visually inspected to investigate the seasonal, diurnal, and interannual variability of Spread-Es during the ascending phase of Solar Cycle 25. The Daily sunspot number is included only to provide solar-activity context for the 2020–2024 observational interval; no statistical assessment of solar-activity dependence is attempted. Spread-Es occurrence exhibited a pronounced summer and nighttime enhancement at all three stations. The normalised summer occurrence was 16.9% at Juliusruh, 8.6% at Dourbes, and 12.5% at Roquetes, compared with winter values of 5.6%, 2.6%, and 4.8%, respectively. The maximum monthly–hourly occurrence rates were approximately 0.48, 0.35, and 0.40, respectively, while Roquetes showed a broader post-midnight distribution. A representative multi-station interval on 12–13 June 2022 was further examined using ionograms, Height–Time–Intensity (HTI) representations, and regional detrended total electron content (dTEC) observations. Spread-Es and spread-F signatures coexisted at all three stations, with Spread-Es simultaneously present from 21:25 to 22:13 UT. The corresponding dTEC maps showed predominantly southwestward-propagating regional perturbations with NW–SE-oriented phase fronts. The derived phase speeds were 180–210 m s−1, with periods of 48–64 min and horizontal wavelengths of approximately 518–806 km, characteristics consistent with nighttime MSTID activity over the European sector. Thus, the selected event shows the temporal coexistence of Spread-Es, spread-F, and regional MSTID-scale activity, while the available observations do not establish causal E–F electrodynamic coupling.