In order to clarify characteristics of low-altitude Pc 3 magnetic pulsations with 10--45 s periods, we carried out a simultaneous observation with standardized rulfmeters (ring-core-type ULF flux gate magnetometer) at conjugate stations, Moshiri in Japan, and Birdsville in Australia (L~1.5). Amplitude of low-latitude Pc 3's with 20 min to 1 hour duration changed concurrently at the conjugate stations. Diurnal amplitude variations of the Pc 3's which appear primarily in the morning hours are found to be related with the shaded and sunlit ionospheres. There are three distinct intervals in the diurnal variation of the Pc 3 polarizations at the low-latitude stations. (1) Before sunrise, the major axes of the Pc 3 polarization ellipses associated with coherent H- and D-wave packets are almost always in the NW-SE quadrant at both the northern and southern stations. (2) In the sunlit ionosphere during 0630--1100 local time (LT), the Pc 3 pulsations show predominantly ''mirror'' polarizations of standing field-line oscillations, i.e., predominantly left-handed (right-handed) with the major axes in the NW-SE (NE-SW) quadrant in the northern (southern) hemisphere. (3) In the afternoon, the D component of Pc 3 is often smaller than and not coincident with the H component. Therefore mixed polarizations are observed, but the major axes are almost oppositely directed at the conjugate northern and southern stations. Sometimes the polarization reversal across local noon appears simultaneously at the low-latitude conjugate stations. We suggest that Pc 3 source waves, originating outside the magnetosphere, are propagating westward in the morning and eastward in the evening and can couple with various HM oscillations in the plasmasphere.