On the influence of solar activity on the mid-latitude sporadic E layer

To investigate the influence of solar cycle variability on the sporadic E layer (Es), hourly measurements of the critical frequency of the Es ordinary mode of propagation, fo Es, and of the blanketing frequency of the Es layer, fb Es, recorded from January 1976 to December 2009 at the Rome (Italy) ionospheric station (41.8° N, 12.5° E), were examined. The results are: (1) a high positive correlation between the F10.7 solar index and fo Es as well as between F10.7 and fb Es, both for the whole data set and for each solar cycle separately, the correlation between F10.7 and fb Es being much higher than the one between F10.7 and fo Es; (2) a decreasing long-term trend of the F10.7, fo Es and fb Es time series, with fo Es decreasing more rapidly than F10.7 and fb Es; (3) clear and statistically significant peaks at 11 years in the fo Es and fb Es time series, inferred from Lomb-Scargle periodograms.

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