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Abstract. In order to clarify the equatorial electrojet effects on groundmagnetic pulsations in central South America, we statistically analyzed theamplitude structure of Pc3 and Pc5 pulsations recorded during days considered quiet tomoderately disturbed at multiple equatorial stations nearly alignedalong the 10∘ magnetic meridian. It was observed that Pc3amplitudes are attenuated around noon at the dip equator for periods shorterthan ∼35 s. It is proposed that daytime Pc3s are related toMHD (magnetohydrodynamic) compressional wave vertically incident on the ionosphere, with thescreening effect induced by enhanced conductivity in the dip equator causingwave attenuation. Daytime Pc5s showed amplitude enhancement at allequatorial stations, which can be explained by the model of waves excited athigher latitudes and propagating equatorward in an Earth–ionospherewaveguide. However, a slight depression in Pc5 amplitude compared toneighboring equatorial stations and a phase lag in relation to anoff-equatorial station were detected at the dip equator. This wave amplitudedepression in the Pc5 frequency band cannot be explained by the ionosphericwaveguide model alone, and we propose that an alternative propagation modelthat allows ULF (ultra-low-frequency) waves to penetrate directly from the magnetosphere to lowlatitudes could be operating simultaneously to produce these features at thedip equator. Significant effects of the sunrise terminator on Pc3 pulsationswere also observed at the stations closest to the dip equator. Contrary towhat is reported at other longitudes, in central South America the sunriseeffect decreases the D∕H amplitude ratio. We suggest that thesedifferences may arise from the unique characteristics of this sector, with astrong longitudinal variation in the magnetic declination and precipitationof energetic particles due to the presence of the South Atlantic MagneticAnomaly (SAMA). The H-component amplification can be explained by enhancements ofthe zonal electric field near the magnetic equator driven by F-regionneutral winds and waves in the fast-mode of propagation during sunrise.

Latitudinal variation of Pc3–Pc5 geomagnetic pulsation amplitude across the dip equator in central South America