The Am index is used to study the response of the magnetosphere to northward IMF. It is shown that most of the observed increase in activity can be explained by a correlation of the solar wind dynamic pressure with the IMF field strength such that the Am index varies only slightly with northward IMF strength when the solar wind dynamic pressure is held constant. There remains a smaller response of Am to solar wind velocity, which is usually attributed to a viscous interaction of the solar wind with the magnetosphere such as through the Kelvin-Helmholtz instability. The diurnal variation of Am during northward fields is also examined. A model invoking the Kelvin-Helmholtz instability on the flanks of the magnetopause have been used to predict the annual behavior of this diurnal variation. The predicted annual behavior is not found for northward IMF. The difference in diurnal variation from summer months to winter months, suggested to be caused solely by the Kelvin-Helmholtz instability, appears only for southward fields. Thus, while the Kelvin-Helmholtz instability may play some role in energy transfer into the magnetosphere and the resultant geomagnetic activity, this role is much less than that of dynamic pressure fluctuations and we find no evidence that it by itself plays any role in the annual variation of the diurnal variation of geomagnetic activity. ¿ American Geophysical Union 1990