Evaluation of Oblique and Fork Traces in Ionograms as Indicators of MSTIDs and Their Relationship to the Development of Spread-F at European Mid-Latitudes

Oblique and Fork-shaped traces in ionosonde’s ionograms have been identified as manifestations of nighttime and daytime Medium-Scale Traveling Ionospheric Disturbances (MSTIDs), respectively. The MSTIDs associated with these singular patterns have been verified by analyzing the evolution of detrended total electron content (dTEC) perturbation maps derived from the Global Navigation Satellite System (GNSS) signals. This has made it possible to create a database of MSTIDs characterized by their dominant period, horizontal propagation speed, wavelength, and propagation direction. This also allowed comprehensive case studies and statistical analyses of their temporal occurrence, direction of propagation, seasonal dependence, and its association with Spread-F (SF) phenomena. The results reveal that approximately 92% of the “Oblique” traces and 86% of the “Fork” traces in ionograms correspond to nighttime and daytime MSTID activity, respectively, as observed in the dTEC maps. Oblique traces are predominantly observed during southwestward propagating MSTIDs and exhibit a marked seasonal occurrence during summer and winter. Fork traces occurred predominantly during equatorward propagating MSTIDs and are most frequently observed in winter. A statistically significant association has been found between daytime MSTID activity (Fork traces) and the subsequent appearance of SF, with conditional probabilities ranging from ~87% to ~78% at mid-to-high latitudes and from ~76% to ~61% at mid-to-low latitudes. However, when overall MSTID activity was considered—taking into account both the disturbances associated with the daytime Forks and those associated with nighttime Oblique traces—the statistical association with subsequent occurrence of SF increased to approximately 91%. These results show a strong statistical relationship but do not establish a direct causal link between an individual daytime MSTID activity and nighttime SF. Instead, the occurrence of MSTID is seen as an indicator of ionospheric conditions that are favorable for the development of SF at mid-latitudes. These findings offer new insights into the ionogram-based characterization of MSTID signatures and their association with nighttime SF over Europe. The results highlight that MSTID occurrence serves as a strong indicator of favorable conditions for the development of instabilities in the mid-latitude ionosphere, offering valuable implications for the possible modeling of the ionosphere and the prediction of ionospheric weather.