Observations using Arecibo Observatory’s highly sensitive Incoherent Scattering Radar (AO-ISR) show ionospheric descending layers from as high as $\sim$400 km, much higher than earlier studies, with continuity down to 90 km. The AO-ISR was operated to observe the ion-line and plasma-line with coded-long-pulse for high temporal and spatial resolution of 35/10 seconds and 300 m, respectively, during 01-06 February 2019. We found multiple layering structures descending from 400 km to 90 km in all these six days. These layers are traditionally called intermediate descending layers (IDLs) ($>$130 km and below F-peak), upper semi-diurnal daytime $\&$ nighttime layers (110 km-130 km), and lower diurnal layers($<$110 km). We have denoted the new daytime descending layers above the hmF2 as top-side descending layers (TDLs). All these layers are collectively named ion descending layers (IonDLs) since all of them are connected with some discontinuity at the F1-peak (i.e., 170 km), except for the daytime lower-diurnal layer. The most pronounced IonDLs occur in the twilight times. IonDLs mainly occur in shear zones of the vertical ion drifts and are favored by downward ion drifts, and their descent speeds increase with increasing altitude. The estimated phase velocities of the waves in the F-region are comparable with the descending speed of the IonDLs. Furthermore, IonDLs/IDLs occur with and without spread-F events but intensified spread-F events raise their beginning altitude. The TDLs and IDLs are driven by gravity waves with time periods of 1.5-4 hours.

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.