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In this study, probable signature of mid-latitude electron density trough at the ionospheric critical frequencies is investigated. For this purpose, hourly ionospheric critical frequency (foF2) data obtained from ionosonde stations for the year 1972 is used. These stations are situated at between 45o - 66oinvariant magnetic latitudes (Λ) of the Northern hemisphere. foF2 data is investigated in terms of local time (LT), seasons, Λ and geomagnetic activity (Kp) variations. The results show that the troughs occur at nighttime hours between dusk and dawn sector for all situations. The troughs at geomagnetic active (Kp>2+) conditions are more prominent and apparent than that at geomagnetic quiet (Kp≤2+) conditions. The troughs and their minimum positions tend to occur at lower Λ for all four seasons at geomagnetic active conditions. Also, the average of foF2 values in the minimum points of the troughs has lower values at geomagnetic active conditions. It is observed that the seasonal variations have important effects on the structure of trough, especially for December Solstice. The results of this study will contribute a better description and prediction of the mid-latitude electron density trough features for the ionospheric F2 peak heights and better understanding the role of the mid-latitude electron density trough on the ionospheric variability.
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