Potential effects of the IMF-orientation on the mid-latitude ionosphere are further investigated using critical frequencies foF2 from six ionosonde stations. For a period of 15 days around each inversion of B-Z, excluding all days with Ap >=6, a quiet standard diurnal variation was determined by day-by-day averaging for each hour UT. The regular diurnal, seasonal and solar cycle variations were then removed from the data by substracting from these the quiet standard value. The so obtained differences ofoF2 were sorted after the INF polarity. Distinct effects of northward and southward inversions were found so that a large part of the day-to day variability may be attributed to IMF B-Z polarity changes.