A theory is put forward to account for the horizontal structure and polarization of the magnetic field of low-frequency ULF waves on the ground at low latitudes. The theory is based on the condition that the component of the rotation of magnetic field normal to the Earth's surface vanishes in the neutral atmosphere and also on an assumption that the phase in the H component of the ground magnetic field is constant latitudinally at low latitudes. When the H component phase is independent of the latitude, the D component amplitude increases monotonically from the magnetic equator to higher latitudes. The D component is 180¿ out of phase in the northern and southern hemispheres as well as east and west of longitude of the wave source. The sense of rotation of the magnetic perturbation vector in the H-D plane is clockwise and counterclockwise for the eastward and westward propagations, respectively, in the northern hemisphere and vice versa in the southern hemisphere. The major axis of the H-D plane ellipse orients in the northeast and northwest directions west and east of the source longitude, respectively, in the northern hemisphere and vice versa in the southern hemisphere. For compressional MHD disturbances emitted from a distant source region, their wave fronts surround the Earth in the inner plasmasphere owing to the slowdown of the Alfv¿n speed just inside the plasmapause and wind around it near the Earth. This suggests that the latitudinally constant phase of the H component is a common feature in low-latitude ULF waves. ¿ 1998 American Geophysical Union