On the seasonal and solar cycle variation of the ULF fluctuations at low latitudes: A comparison with the ionospheric parameters

A long term analysis (1996–2015) of the occurrence and characteristics of the wave events (Pc3; f = 20–100 mHz;H and D component), detected at a low latitude ground-based station (L’Aquila, Italy), and the comparison with the ionospheric parameters reveal some interesting aspects of their seasonal and solar cycle variation. In general, the daytime wave activity, which basically consists of penetrating upstream waves and, at higher frequencies, of resonances of local field lines, appears more intense (with a more relevant percentage of D events) in winter than in summer, suggesting a seasonal dependence of the attenuation and rotation of the downgoing signal through the ionosphere and, during winter, less efficient ionospheric conditions for the onset of resonance processes (typically occurring along the H component). This situation persists during solar minima, while, during solar maxima, the summer occurrence rate of events exceeds the winter one: this aspect might be related to the more frequent impact on the magnetosphere of energetic solar wind structures during solar maxima; in summer, given the favourable ionospheric conditions, they might determine a much more frequent manifestation of relevant resonance events. Rapidly increasing after midnight, the wave activity reveals a pre-sunrise peak followed by a minimum, on average coincident with the foF2 minimum, and the winter pattern appears to be delayed ~ 1 h compared to the summer one.