Effect of a huge crustal conductivity anomaly on the H-component of geomagnetic variations recorded in central South America

Abstract

We describe here an analysis of the H-component of the geomagnetic field recorded in several temporary stations operating simultaneously in the central-eastern region of Brazil during nighttime pulsation events in 1994 and the sudden commencement of the St. Patrick's Day magnetic storm in 2015. A significant amplification in the amplitude of the geomagnetic variations is consistently observed in one of these stations. Magnetovariational analysis indicates that the amplification factor is period dependent with maximum amplitude around 100 s. Integrated magnetotelluric (MT) and geomagnetic depth soundings (GDS) have shown that this station is positioned just over a huge 1200-kmlong crustal conductor (estimated bulk conductivity greater than 1 S/m). We propose that the anomalous signature of the geomagnetic field at this station is due to the high reflection coefficient of the incident electromagnetic wave at the interface with the very good conductor and by skin effects damping the electromagnetic wave in the conducting layers overlying the conductor. There are some indication from the GDS data that the conductor extends southward beneath the sediments of the Pantanal Basin. In this region is being planned the installation of a new geomagnetic observatory, but its preliminary data suggest anomalous geomagnetic variations. We understand that a detailed MT survey must be carried out around the chosen observatory site to evaluate the possible influence of induced currents on the local geomagnetic field.

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

“…All stations were positioned well inland, far away from any ocean or mountain range, but a recent study has shown an extensive anomaly in the underground conductivity beneath the CUI site (Padilha et al, 2017). An amplitude enhancement up to tens of percent is observed on the ground geomagnetic variations recorded at this station.…”

“…The clock of the data recording system was automatically calibrated by global radio signals (LF OMEGA; Saka et al, 1996), which kept the time accuracy within 100 ms during the data acquisition. The stations operated simultaneously from September to November 1994 and their data were previously used in several studies to investigate different characteristics of geomagnetic variations at very low and equatorial latitudes (e.g., Shinohara et al, 1998;Padilha et al, 2003Padilha et al, , 2017Zanandrea et al, 2004;Rastogi et al, 2008). Figure 1 shows a map of South America with the location of the six geomagnetic stations.…”

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

“…This shows that the PRM station is much closer to the central region of the SAMA. In this region, azimuthally drifting energetic particles trapped in the Earth's Van Allen internal radiation belt approach the Earth's surface due to the low values of geomagnetic field intensity, interacting with the dense atmosphere and producing enhanced ionization at ionospheric E layer heights (see, for example, Paulikas, 1975, for a review).…”

The Embrace Magnetometer Network for South America: First Scientific Results

“…With respect to EM induction studies, Padilha et al () published results on the effect of a huge crustal conductivity anomaly on the H component of geomagnetic variations recorded in central South America. These effects directly affect the geomagnetic records of our CBA station.…”

“…Basic description of the model Sutcliffe (1999) Empirical model developed for providing a geomagnetic daily variation over the Southern African region as a function of season, sunspot number, and degree of geomagnetic activity, based on artificial neural networks Alken and Maus (2007) Empirical model of the equatorial electrojet magnetic signature as a function of longitude, local time, season, solar flux, and lunar local time, derived from CHAMP, Ørsted, and SAC-C satellites Unnikrishnan (2012) Empirical model for providing a geomagnetic daily variation over the Alibag observatory, India, for solar quiet conditions, based on artificial neural networks Ouadfeul et al (2015) Empirical model for providing a geomagnetic daily variation over the German Wingst observatory, Germany, as function of degree of geomagnetic activity, based on artificial neural networks Jayapal et al (2016) Empirical model developed for the horizontal component of the Earth's magnetic field over Thiruvananthapuram, India, as a function of the solar cycle, seasonal, and degree of geomagnetic activity, based on Fourier analyzes Radio Science 10.1002/2018RS006540 (Rostoker, 1972). Therefore, we decided to investigate the use of equation (2) for computing on the range of the component over the 3 h period at each magnetic station (N = 1) instead of using the H variation (or either X and Y components) given by equation (1).…”