The annual variation of magnetic activity and its variation with universal time are analyzed separately for days sorted according to the polarity of the interplanetary magnetic field (IMF). A coherent interpretation of the observed effects is obtained (1) by taking into account the variations of the southward component of IMF in GSM (geocentric solar magnetospheric) coordinates, as was suggested by Russell and McPherron (1973) (the effect expected from this mechanism is calculated and compared to the observed ones) and (2) in the case of Am, by taking into account the annual and diurnal variations of the angle between the dipole axis and the earth-sun line.