During solar cycle 20, the first full cycle with measurements of solar wind parameters, geomagnetic activity measured byApwas found to correlate with the square of solar wind speedV, and activity measured byDstwas found to correlate with the product ofVand the southward component of the interplanetary magnetic field,Bs. Both of these correlations break down during cycle 21. In the case ofAp, the much stronger variation ofBsin cycle 21 compared to cycle 20 makes clear that theBscontribution to activity is important on yearly as well as shorter time scales. The productBsV² gives an excellent correlation withApover both cycles. In the case ofDst, the stronger variation ofBsin cycle 21 causes a stronger variation inBsV, which is not reflected inDst, perhaps becauseDstalso depends upon solar wind dynamic pressure in a nonlinear way