The 22-yr variation in geomagnetic activity has been reported by CHERNOSKY [(1966) J. geophys. Res. 71, 9651. It has been suggested that the observed 22-yr variation in the geomagnetic activity is related to the 22-yr cycle of the solar magnetic field. In order to provide a quantitative understanding, of this relationship, we first calculate [B(z)]GSEQ the annual average of the z-component of the interplanetary magnetic fields (IMF) in GSEQ (Solar Equatorial) coordinates, observed at 1 A.U. from 1967 to 1988. It is found that [B(z)]GSEQ does not correlate with the geomagnetic activity. We then calculate [B(y)]GSEQ, the y-component of the IMF in GSEQ coordinates and [B(z)]GSM, the z-component of IMF in GSM coordinates and found both of them to correlate with the 22-yr variation of the geomagnetic activity. We then suggest that the cause for the 22-yr variation of the geomagnetic variation is because [B(y)]GSEQ, the y-component of the IMF in the solar equatorial plane, has a 22-yr variation. In comparison with the 22-yr variation of geomagnetic activity with the effective component of IMF for geomagnetic activity, we calculated [B(z)]GSM and found that southward of [B(z)]GSM correlates with a higher geomagnetic activity. Then, adopting a simple model, we assume the IMF consists of a spiral magnetic field in the x-y plane of the GSEQ coordinates with no z-component (i.e. [B(z)]GSEQ = 0). However, the effective [B(z)]GSM which derives mainly from [B(y)]GSEQ does give a [B(z)GSM ranging from -0.08 to 0.15 nT. This simple model assumes that the effective B(z)(GSM) is related to geomagnetic activity in which there is no geomagnetic activity for B(z)(GSM) > 0 and the geomagnetic activity is proportional to \B(z)\ for B(z)(GSM), < 0. It is found from this simple model that the calculated geomagnetic activity due to the 22-yr variation of [B(z)]GSM can explain the phase and amplitude of the observed 22-yr variation of geomagnetic activity.
