The enhancement of Pc1 wave activity after magnetic sudden impulses (SIs) is well established by ground-based and satellite measurements. This phenomenon is explained by compressions of the magnetosphere under the action of interplanetary shocks. In this paper we use long-term ground-based observations of Pc1 to study the variation of the Pc1 wave activity before SIs. We conclude that there is a tendency for an increase of the rate of Pc1 occurrence as the time gets closer to the SI. We interpret this effect as an impact of the interplanetary foreshocks on the Earth's magnetosphere. ¿ 2000 American Geophysical Union