It has been generally accepted that there are two models for the propagation mechanism of Pc3 pulsations observed on the ground near the magnetic equator, as follows: (1) Compressional waves, propagating along the equatorial plane of the magnetosphere across the ambient magnetic field lines, arrive at the equatorial ionosphere and couple with the magnetic perturbations on the ground through the ionosphere (compressional wave model); (2) Alfvén waves or compressional waves, propagating into the high-latitude ionosphere, generate large-scale ionospheric current oscillations there, and then they leak to the low latitude and cause Pc3 pulsations near the magnetic equator (ionospheric current model). In order to clarify which of these models is more suitable for Pc3 pulsations on the ground near the magnetic equator, the longitudinal structures of their coherence, amplitude, and phase were statistically studied by using three longitudinally separated subequatorial stations, CRI (Φ = 3.01°, A = 273.44°), GUA (Φ = 5.60°, A = 215.50°), and MUT (Φ = 6.24°, A = 192.17°). We found a nearly inphase structure in the 0730-1700 LT sector and a nearly 180° phase shift across 0730 LT for high-coherence Pc3 events in the H component. The longitudinal phase structure can be explained by the ionospheric current model; in this case, the 180° phase shift across 0730 LT means a meridional current along the dawn terminator supplied from the source at higher latitudes. The compressional wave model cannot explain the 180° phase shift across 0730 LT if the ionosphere is assumed as an infinitely thin sheet; however, the compressional wave model still remains as a candidate if we treat the ionosphere as that having a finite thickness. The data at hand do not let us conclude which of the two models is more appropriate; still, the longitudinal phase structure, shown in this paper, is a new finding that has never been reported and is important for understanding how Pc3 pulsations propagate from the magnetosphere to the equatorial ionosphere.
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