The investigation of the time dependence on the ionospheric mid-latitude foF2 variability and comparison with the relevant results of the Ariel 4 satellite ambient electron density during the declining phases of the solar cycle 20 and solar cycle 21

In this study, hourly critical frequency (foF2) data that are taken from seven ionosondes stations are situated at between 30-70 invariant magnetic latitude of the north hemisphere are investigated changes depending on the seasons and latitudes for different geomagnetic activity conditions during years which contain 1972-1976 and 1982-1986 periods that decreased the sunspot number at 20th and 21th solar cycle. The obtained results are compared with electron density data measured by Ariel 4 satellite at about 550 km altitude and determined the effect of the structure that is called as mid-latitude electron density trough on foF2. According to the results from the analysis, a decrease occur at the lower and upper decile values of foF2 and δfoF2 with increasing invariant magnetic latitude for all seasons of 20th and 21th solar cycle. In addition, the mode values of δfoF2 are same for all stations and δfoF2 values exhibit a distribution that skewed to the left because of the lower decile values of δfoF2 diverge from the mode values. The hourly changes of δfoF2 exhibit positive deviations during daytime and negative deviations during night hours. The distribution of δfoF2 values and the results that are obtained from superposed epoch analysis show that changes in the level of magnetic activity have important effect on the daily changes of foF2 values. Variations according to invariant magnetic latitudes of the average foF2 values show that structures similar to trough occur only during night local time groups for both geomagnetic activity conditions and all seasons. However, the troughs that are determined under geomagnetic active condition (Kp>2+) are more prominent and apparent than that under geomagnetic quiet condition (Kp<=2+). In addition, the troughs and their minimum positions tend to occur at lower invariant magnetic latitudes in all four seasons under geomagnetic active condition. The seasonal changes have important effects on the structure of trough and its formation. It is determined that June period is the season that is least observed of the middle latitude electron density trough, whereas December period is the season that is the most observed of the middle latitude electron density trough for both geomagnetic activity conditions of 20th and 21th solar cycle. The statistical analyses that are performed between averaged foF2 data and averaged electron density data for night hours that are observed troughs during December period of 20th solar cycle show that there is an important correlation between two condition that are measured at different altitudes. The results of this study show that changes in the magnetic activity level and structure that is called magnetospheric plasma boundary at about 550 km altitude of mid-latitude have important effect on electron density changes in lower altitudes of the ionosphere. Also, it is determined that variations according to invariant magnetic latitude for different local times, different seasons and different geomagnetic activity conditions of the electron density data measured at about 550 km and foF2 data at lower latitudes have quite similar structure to each other. KEYWORDS: Ionosphere, magnetosphere, geomagnetic activity, mid-latitude electron density trough, critical frequency.