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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. [ABSTRACT FROM AUTHOR]

Abstract: It is well known that ionospheric perturbations are characterised by strong horizontal gradients and rapid changes of the ionisation. Thus, space weather induced severe ionosphere perturbations can cause serious technological problems in Global Navigation Satellite Systems (GNSS) such as GPS. During the severe ionosphere storm period of 29–31 October 2003, reported were several significant malfunctions due to the adverse effects of the ionosphere perturbations such as interruption of the WAAS service and degradation of mid-latitudes GPS reference services. To properly warn service users of such effects, a quick evaluation of the current signal propagation conditions expressed in a suitable ionospheric perturbation index would be of great benefit. Preliminary results of a comparative study of ionospheric gradients including vertical sounding and Total Electron Content (TEC) data are presented. Strong enhancements of latitudinal gradients and temporal changes of the ionisation are observed over Europe during the 29–30 October storm period. The potential use of spatial gradients and rate of change of foF2 and TEC characterising the actual perturbation degree of the ionosphere is discussed. It has been found that perturbation induced spatial gradients of TEC and foF2 strongly enhance during the ionospheric storm on 29 October over the Central European region in particular in North–South direction exceeding the gradients in East–West direction by a factor of 2. [Copyright &y& Elsevier]

Abstract: New ionospheric activity indices are derived from automatically scaled online data from several European ionosonde stations. These indices are used to distinguish between normal ionospheric conditions expected from prevailing solar activity and ionospheric disturbances caused by specific solar and atmospheric events (flares, coronal mass ejections, atmospheric waves, etc.). The most reliable indices are derived from the maximum electron density of the ionospheric F 2-layer expressed by the maximum critical frequency foF 2. Similar indices derived from ionospheric M(3000)F 2 values show a markedly lower variability indicating that the changes of the altitude of the F 2-layer maximum are proportionally smaller than those estimated from the maximum electron density in the F 2-layer. By using the ionospheric activity indices for several stations the ionospheric disturbance level over a substantial part of Europe (34°N–60°N; 5°W–40°E) can now be displayed online. [Copyright &y& Elsevier]

A three-year study (2017, 2020, 2021) of diurnal and seasonal characteristics of nighttime spread F over eight Digisonde European stations covering an extended latitude and longitude range of the European mid-latitude ionosphere is presented. The findings of the study underline the significant seasonal differences between higher and lower latitude stations which is in line with the findings of previous studies. Moreover, we attempt to correlate spread F activity and Travelling Ionospheric Disturbances (TID) over Digisondes, identified on GPS detrended TEC maps from the DRAWING-TEC project at 10-min resolution for 2017. The similarity in spread F activity at nearby stations supports the hypothesis that spread F development over nearby stations can be attributed to common TID activity affecting both stations. • Mid-latitude spread F correlated with TIDs and F region uplifts, with Perkins instability as the primary seeding mechanism. • This paper focuses on the latitude effect on the diurnal and seasonal occurrence rate of mid-latitude spread F over Europe. • Differences in spread F leads to higher and lower mid-latitude stations are governed by different underlying mechanisms. [ABSTRACT FROM AUTHOR]