By considering only those periods when the delay time from the interplanetary observing position to the magnetosphere is less than about 5 min, it is found that irrespective of substorm activity: (a) the 45-min average value of interplanetary Bz predicts the latitudes of the poleward and equatorward boundaries of polar cusp electron precipitation with rms errors of 1.34° and 1.16° respectively; (b) both boundaries move equatorward by about 5° as Bz varies from 0 to −6γ, the cusp remaining about 4° wide; (c) the amount of flux added to the polar cap is about 9.2% of the total southward flux impingent on the magnetosphere in the previous 45 min; and (d) as Bz becomes more positive, the equatorward boundary moves only slightly more poleward (½° between Bz = 2γ and Bz = 6γ), while the poleward boundary moves significantly toward higher latitudes, resulting in a cusp approximately 7° wide for Bz = 6γ. This dependence of the width of the cusp on positive Bz suggests that reconnection between the IMF (interplanetary magnetic field) and open geomagnetic field lines (which are southward near the dayside magnetopause) is facilitated when the IMF is northward, allowing deeper penetration of magnetosheath plasma into the high latitude magnetosphere.