Long‐Term Variations and Residual Trends in the E, F, and Sporadic E (Es) Layer Over Juliusruh, Europe

Using 63 and 56 yr of continuous observations, we investigate the long‐term oscillations and residual linear trends, respectively, in the E‐ and F‐region ionosonde measured parameters of frequencies and height over Juliusruh, Europe. Using the Lomb‐Scargle periodogram (LSP) long‐term variations are estimated before the trend calculation. We found that the amplitude of the annual oscillation is higher than the 11‐yr solar cycle variation in the critical frequencies of the daytime E (foE) and Es (foEs) layers. In the F‐region, except for daytime hmF2, and nighttime foF2, the amplitude of the 11‐yr solar cycle variation is higher than the annual oscillation. The combination of the LSP estimated periods and their corresponding amplitudes and traditional regression analysis are used to construct a model for E‐ and F‐region ionospheric parameters. The modeled estimates are in good agreement with the observations. The trend calculation is derived by applying a least‐squares fit analysis to the residuals, subtracting the model from the observation. In the F‐region, both day (nighttime) foF2 and hmF2 show negative trends of −4.44 ± 1.78 (−4.30 ± 1.63) kHz/yr and −413 ± 47 (−574 ± 75) m/yr, respectively. The Piecewise linear trend of foF2 provides negative and positive trends in 1964–1996 and 1997–2019, respectively. In the E‐region, foEs show a negative trend of −2.00 ± 0.61 kHz/yr. The present investigation suggests that the greenhouse cooling effect and negative trend in the atomic oxygen (O) as well as wind shear variability could be the main drivers for the observed negative trends in the hmF2, foF2, and foEs, respectively.