The possible effects of the orientation of the IMF on the ionosphere has been studied by Tulunay (1995) using foF2 data from 15 ionospheric stations in Europe over the COST 238 area. The results showed that a good amount of the day to day variability of the mid-latitude ionospheric F region could be related to changes in orientation of the southward IMF Bz. This variability is quantified as the maximum change of delta foF2. This paper investigates the effects of By distribution on the ionospheric critical frequencies. (C) 1997 COSPAR. Published by Elsevier Science Ltd.