On the low correlation between long‐term averages of solar wind speed and geomagnetic activity after 1976

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Abstract
During solar cycle 20, the first full cycle with measurements of solar wind parameters, geomagnetic activity measured by Ap was found to correlate with the square of solar wind speed V, and activity measured by Dst was found to correlate with the product of V and the southward component of the interplanetary magnetic field, Bs. Both of these correlations break down during cycle 21. In the case of Ap, the much stronger variation of Bs in cycle 21 compared to cycle 20 makes clear that the Bs contribution to activity is important on yearly as well as shorter time scales. The product BsV² gives an excellent correlation with Ap over both cycles. In the case of Dst, the stronger variation of Bs in cycle 21 causes a stronger variation in BsV, which is not reflected in Dst, perhaps because Dst also depends upon solar wind dynamic pressure in a nonlinear way.