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

Abstract

In order to clarify the equatorial electrojet effects on ground magnetic pulsations in central South America, we statistically analyzed the amplitude structure of Pc3 and Pc5 pulsations recorded during days considered quiet to moderately disturbed at multiple equatorial stations nearly aligned along the 10 • magnetic meridian. It was observed that Pc3 amplitudes are attenuated around noon at the dip equator for periods shorter than ∼ 35 s. It is proposed that daytime Pc3s are related to MHD (magnetohydrodynamic) compressional wave vertically incident on the ionosphere, with the screening effect induced by enhanced conductivity in the dip equator causing wave attenuation. Daytime Pc5s showed amplitude enhancement at all equatorial stations, which can be explained by the model of waves excited at higher latitudes and propagating equatorward in an Earth-ionosphere waveguide. However, a slight depression in Pc5 amplitude compared to neighboring equatorial stations and a phase lag in relation to an off-equatorial station were detected at the dip equator. This wave amplitude depression in the Pc5 frequency band cannot be explained by the ionospheric waveguide model alone, and we propose that an alternative propagation model that allows ULF (ultra-low-frequency) waves to penetrate directly from the magnetosphere to low latitudes could be operating simultaneously to produce these features at the dip equator. Significant effects of the sunrise terminator on Pc3 pulsations were also observed at the stations closest to the dip equator. Contrary to what is reported at other longitudes, in central South America the sunrise effect decreases the D/H amplitude ratio. We suggest that these differences may arise from the unique characteristics of this sector, with a strong longitudinal variation in the magnetic declination and precipitation of energetic particles due to the presence of the South Atlantic Magnetic Anomaly (SAMA). The Hcomponent amplification can be explained by enhancements of the zonal electric field near the magnetic equator driven by F-region neutral winds and waves in the fast-mode of propagation during sunrise.

The 2021 Antarctic Total Eclipse: Ground Magnetometer and GNSS Wave Observations From the 40 Degree Magnetic Meridian

“…Recent work investigating the occurrence of PC3 waves at the dawnside terminator has shown evidence of what is termed the “sunrise effect” (Saka & Alperovich, 1993; Silva et al., 2020; Somsikov, 2011; Tanaka et al., 2004; Yagova et al., 2017). While the physical processes underlying the generation of these waves remains poorly understood, it is noteworthy that PC3 waves occur frequently at the dawn side terminator.…”

“…However, most observations of the “sunrise effect” have been at mid to low latitudes, where ionospheric currents behave quite differently than at the high latitudes studied herein. One of the proposed generation mechanisms for these waves involves the motion of neutral winds across a thermal conductivity gradient (Silva et al., 2020). It is therefore suggested that further observational study of eclipse related ULF waves include a thermospheric wind measurement in conjunction with ionospheric sounding.…”

The 2021 Antarctic Total Eclipse: Ground Magnetometer and GNSS Wave Observations from the 40 Degree Magnetic Meridian

“…Recent work investigating the occurrence of PC3 waves at the dawnside terminator has shown evidence of what is termed the "sunrise effect" (Saka & Alperovich, 1993;Somsikov, 2011;Tanaka et al, 2004;Yagova et al, 2017;Silva et al, 2020). While the physical processes underlying the generation of these waves remains poorly understood, it is noteworthy that PC3 waves occur frequently at the dawn side terminator.…”

“…However, most observations of the "sunrise effect" have been at mid to low latitudes, where ionospheric currents behave quite differently than at the high latitudes studied herein. One of the proposed generation mechanisms for these waves involves the motion of neutral winds -11-manuscript submitted to JGR: Space Physics across a thermal conductivity gradient (Silva et al, 2020). It is therefore suggested that further observational study of eclipse related ULF waves include a thermospheric wind measurement in conjunction with ionospheric sounding.…”

Latitudinal dependence of Pc3-4 amplitudes across the dip equator along the 210º Magnetic Meridian

“…Consequently, the source and propagation mechanisms of the equatorial and very low latitude pulsations are not fully understood, and they might be related to either upstream waves or mechanisms of pulsations at higher latitudes [Yumoto, 1986;Feng et al, 1995]. Some studies reported an attenuation in the amplitudes of Pc3 pulsation at the dip equator especially for short period Pc3 pulsations [Yumoto et al, 1985;Itonaga and Kitamura, 1993;Roy and Rao, 1998;Takla et al, 2011, Graziela et al, 2020, while other studies reported Pc3 amplitudes enhancement at the dip equator [Sarma and Sastry, 1995;Matsuoka et al, 1997;Saka et al, 2001;Zanandrea et al, 2004]. The dip equator is an imaginary line that roughly passes through the points where a magnetic needle has no dip.…”