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Abstract:
Long-term changes are challenging and debatable research topics because of the estimation method and the social relevance in the climate change scenario. Studies on the long-term changes in the upper atmosphere yield more attention in the last three decades. The formation of the ionosphere and sporadic-E (Es) layer is caused by the solar and lower atmospheric forcing, respectively. Ionosphere and Es-layer can be used as a tracer to estimate the lower atmospheric impact and solar forcing on the upper mesosphere and thermosphere/ionosphere.
Using sixty-three and fifty-six years of continuous observations, we investigate the long-term oscillations and residual linear trends, respectively, in the E- and F-region ionosonde measured parameters (i.e. frequencies and height) over Juliusruh, Europe. The Lomb-Scargle periodogram (LSP) estimates long-term oscillations before the trend calculation. We found that the amplitude of the annual oscillation is higher than the 11-year solar cycle variation in the critical frequencies of the daytime E (foE) and Es (foEs) layers. The combination of the LSP estimated periods with their corresponding amplitudes and traditional regression analysis is 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, foF2, and hmF2 day (nighttime) show negative trends of -4.44±1.78 (-4.30±1.63) kHz/year and -413±47 (-574±75) m/year, respectively. In the E-region, foEs show a negative trend of -2.00±0.61 kHz/year.
