The horizontal spatial structure of Pc 3 pulsations observed at low geomagnetic latitude (22-46°) around dawn is studied statistically using data acquired by the Circum-pan Pacific Magnetometer Network (CPMN). It is found that while the phase of the H component of low-latitude Pc 3 pulsations remains largely unchanged with the passing of dawn, the D component undergoes a phase shift of ca. 180°. This phase variation across dawn is related to the abrupt change in the major axis orientation of polarization ellipses observed in previous studies. Both the H and D components have higher amplitude after dawn than before dawn. This horizontal amplitude and phase structure is well explained by the response of a nonuniform ionosphere around dawn to incident Alfvén waves, where the secondary electric field caused by charge accumulation at the dawn terminator plays an important role in deformation of the current system. Enhancement of the DIH ratio is also observed just after dawn at very low latitudes (22°). As Alfvén waves are not excited efficiently at very low geomagnetic latitudes, including the magnetic equator, the observed horizontal structure in such regions may be due to a large-scale current system originating at higher latitudes.
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