During periods of southward interplanetary magnetic field (IMF) we have examined the relationship between magnetic variations in the central polar cap and the IMF By and Bz components. The geomagnetic polar cap index PC that can be used as a measure of the flow across the polar cap has been derived using data from Thule in the IMS period. The results have been compared with IMP 8 measurements of the IMF and the solar wind velocity. The statistical analysis shows that the absolute value of the azimuthal component [By> contributes to the cross-polar cap flow in the same manner as the southward component Bs. The relative contributions of [By> and Bz have been examined and compared with the theoretical expression vBT sin2 &thgr;/2 for the merging electric fields. It is found that the contribution of [By> compared to Bz is only half as big in the observations as in the theoretical expression. The By effect on PC is compared to an earlier reported effect of By on the geomagnetic index AL (Murayama et al., 1980) and found to be quite different from this. This is discussed in relation to interpretations in terms of merging site asymmetry.