We now examine Pc 3-4 waves observed by Polar Magnetic Field Experiment [Russell et al., 1995] on event day May 11 as well as the control day May 14 using dynamic power spectra of the Polar magnetic field while in the dayside magnetosphere. Figure 3 shows the dynamic power spectra of dBy GSM component (mainly the transverse component) from 07 to 10 UT on May 11 and 14. The Polar orbit can be found in Figure 2 of our earlier paper [Le et al., 1999]. During this time period on both days, Polar was in the dayside magnetosphere at ~ 1100 local time, and its ionospheric footprint moved from 64o N to 77o N in magnetic latitude. This is the high-latitude region where strong Pc 3-4 pulsations are seen under favorable IMF orientations. As we have stated the IMF cone angles were very similar for these two intervals, 40o for May 11 and 38o for May 14, both favorable for occurrence of Pc 3-4 waves in the dayside magnetosphere. However, Polar only observed strong Pc 3-4 pulsations in the dayside magnetosphere under the nominal solar wind conditions on May 14. In the bottom panel of Figure 3, enhanced wave power in Pc 3-4 band is evident. The wave frequency decreased gradually due to the decreasing of resonant frequency of the magnetic field lines as the spacecraft traveled away from the Earth. In contrast, the dayside magnetosphere was extremely quiet and Pc 3-4 waves were nearly absent on May 11.