The diurnal (universal time) modulation of the geomagnetic activity is investigated, using the am index along with interplanetary plasma and magnetic field data collected by a variety of spacecraft from 1965 to 1987. A direct comparison with satellite measurements of the Bs component of the interplanetary magnetic field (IMF) is made. Our results confirm the Russell-McPherron (R-M) model, which attributes the above phenomenon to the varying southward component (Bs) of the IMF in the geocentric solar magnetospheric system. Moreover, we compared the contributions of the equinoctial and R-M mechanisms to the observed am modulation, showing that the latter effect is dominant. ¿ American Geophysical Union 1995.