A simple analytic model for the directly driven, adiabatic solar wind--magnetosphere--ionosphere coupling is derived on the basis of frontside reconnection and force balance in the tail. Magnetosphere--ionosphere coupling occurs through Alfv¿n waves which are driven by changes of electric fields in the magnetosphere. The model provides an algorithm which calculates the directly driven part of the auroral electrojets from an observed solar wind variation. For two days in May 1979 the cross correlation between the prediction and observation of AE is greater than 0.9. This suggests that most of the geomagnetic activity during these two days was directly driven by the solar wind. ¿ American Geophysical Union 1993