A flux gate magnetometer with an amplitude resolution of 6 pT/LSB was installed at Huancayo, Peru (12¿S, 750¿W in geographic coordinates, 0.8¿N, 335.6¿ in geomagnetic coordinates) in December 1985 and recorded field variations for 1 year with 3-s sampling intervals. Horizontal field variations of Pc 3--4 pulsations (periods from 150 to 30 s) were studied and it was found that the oscillations were observed predominantly in the H component (magnetic north-south), though oscillations in the D component (magnetic east-west) are superimposed temporarily at local sunrise. The D component signals, which last for ≈2 hours after local sunrise, bring about a large tilt of the polarization azimuth angles in the H-D plane without a significant change in the ellipticity. The azimuth at sunrise lies in the NW-SE quadrant during local summer (November to January), while it lies in the NE-SW quadrant for local winter (March to September). On the basis of the observed seasonal change of the azimuth angles, we present a model of the ionospheric current system setup by the incident Pc 3-4 pulsations in the dayside region of the dawn terminator at the dip-equator. ¿ American Geophysical Union 1993