Investigation of characteristics of ionospheric vertical plasma drift during sunset over the mid-latitude station Nicosia, Cyprus

This study investigates the characteristics of the vertical component of the ionospheric F2 layer plasma drift over the mid-latitude station of Nicosia, Cyprus ( \(\hbox {35}^{\circ }\hbox {N}\) ${\text{35}}^{\circ }\text{N}$ , \(\hbox {33}^{\circ }\hbox {E}\) ${\text{33}}^{\circ }\text{E}$ , at magnetic dip angle, \(\hbox {I}=\hbox {51.7}^{\circ }\) $\text{I}={\text{51.7}}^{\circ }$ ), during pre- and post-sunset hours using digisonde data from November 2013 to October 2014. The observations demonstrate significant seasonal dependence in the vertical plasma drift during post-sunset hours. Vertical velocity shows a regular upward enhancement and notable oscillations during the evening in all seasons, with time variation dependent on the seasonal variability of sunset. The upward enhancement during sunset is typically attributed to chemical loss. However, the present study shows that the critical plasma frequency does not decrease significantly during sunset. Therefore, the observed velocity enhancement may not be solely attributed to recombination effects after sunset. It might be influenced by wind-driven electric fields. Wavelet analysis reveals wave activity around sunset, suggesting that the oscillations may be excited by the Solar Terminator.