In this study, probable signature of the mid-latitude electron density trough at the ionospheric critical frequencies (foF2) is investigated. For this purpose, hourly ionospheric critical frequencies data obtained from ionosonde stations for the year 1972 are used. These stations are situated between 45 degrees-65 degrees invariant magnetic latitudes (Lambda) of the Northern hemisphere. The foF2 data are examined depending on local time (LT), seasons,. and geomagnetic activity (K-p) variation. The results show that the foF2 troughs occur in the nighttime hours between dusk and dawn sector for all situations. The troughs under geomagnetic active (K-p > 2(+)) conditions are more prominent and apparent than that under geomagnetic quiet (K-p <= 2(+)) conditions. Under geomagnetic active conditions, the troughs and their minimum positions tend to appear at lower. for four seasons. Also, the average foF2 values of the trough minimum under geomagnetic active conditions have lower values than that under geomagnetic quiet conditions. It is observed that the seasonal variations have important effects on the structure of the foF2 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.