Abstract. Physical meaning of the equinoctial effect for semi-annual variation in geomagnetic activity is investigated based on the three-hourly am index and solar wind parameters. When the z component of the interplanetary magnetic field (IMF) in geocentric solar magnetospheric (GSM) coordinates is southward, am indices are well correlated with B_sV_x², where B_s is the southward component of the IMF and V_x is the solar wind velocity in the sun-earth direction. The am-B_sV_x² relationship, however, depends on the range of V_x²: the am in higher ranges of V_x² tends to be larger than am in lower ranges of V_x² for the same value of B_sV_x² for both equinoctial and solstitial epochs. Using the data sets of the same V_x² range, it is shown that distribution of points in the am-B_sV_x² diagram at the solstitial epochs overlaps with that at the equinoctial epochs and the average am values in each B_sV_x² bin in solstitial epochs are closely consistent with those in equinoctial epochs, if V_x² for each point at solstices are reduced to V_x²sin² (Ψ) where Ψ is the geomagnetic colatitude of the sub-solar point. Further, it is shown that monthly averages of the am index in the long period is well correlated with the values of sin²(ψ) for the middle day of each month. These findings indicate that the factor that contributes to the generation of geomagnetic disturbance is not the velocity of the solar wind, but the component of the solar wind velocity perpendicular to the dipole axis of the geomagnetic field. The magnitude of the perpendicular velocity component varies semi-annually even if the solar wind velocity remains constant, which is considered to be the long-missed key factor causing the equinoctial effect.