Long-term variations and residual trends in the E, F and sporadic E (Es)
layer over Juliusruh, Europe
Abstract
In the present study, using sixty-three and fifty-six years of
continuous observations, we investigate the long-term oscillations and
residual trends, respectively, in the E- and F-region ionosonde measured
parameters over Juliusruh, Europe. Using the Lomb-Scargle periodogram
(LSP) long-term variations are estimated before the trend estimation. 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. A weak semi-annual oscillation is also
identified in the foE. In the F-region, except for daytime hmF2, and
nighttime foF2, the amplitude of the 11-year solar cycle variation is
higher than the annual oscillation. The LSP estimated periods and their
corresponding amplitudes are used to construct a model E- and F-region
ionospheric parameters that are in good agreement with the observation.
The linear trend estimation is derived by applying a least-squares fit
analysis to the residuals, subtracting the model from the observation.
Except for the daytime foF2, all the other parameters like nighttime
foF2, day and nighttime h’F, and hmF2 show a negative trend. Present
results suggest that the greenhouse effect is a prime driver for the
observed long-term trend in the F-region. Interestingly, weak negative
trends in the foE and foEs are found which contradicts an earlier
investigation. The present study suggests that the changes in the upper
stratospheric ozone and mesosphere wind shear variability could be the
main driver for the observed weak negative trends in the foE, and foEs,
respectively.