Effects of Ionospheric Conductivity and Ground Conductance on Geomagnetically Induced Currents During Geomagnetic Storms: Case Studies at Low-Latitude and Equatorial Regions

Abstract

Geomagnetic field variations recorded by fluxgate magnetometers are used to evaluate geomagnetically induced currents (GICs) under the equatorial electrojet (EEJ) and South Atlantic Magnetic Anomaly (SAMA) during two geomagnetic storms in March and June 2015. Geomagnetic stations with information about the underground electrical conductivity structure and that can be approximated by unidimensional (1-D) models for calculation of the geoelectric field are selected. GICs levels are estimated using a realistic local power grid model located in the central region of Brazil, artificially moved to the sites where the geomagnetic measurements are available. GIC magnitudes are not large during the storms, and a maximum amplitude of 3.8 A was estimated at an equatorial station positioned over high resistivity underground during the main phase of the June storm. Effects of the ionospheric currents over the measurement sites and of the conductivity distribution beneath these sites are also evaluated. It is observed that both (EEJ) and (SAMA) increase the GICs amplitudes, with the greatest effects associated with daytime (EEJ) currents. A cutoff frequency was identified for both the (EEJ) and (SAMA) so that signals with frequencies lower than 2 mHz (periods longer than 500 s) are not amplified by the ionospheric currents. In relation to the underlying conductivity structure, GIC magnitude is affected by variation of the surface impedance and the sampling rate of the geomagnetic field. In our case, differences in amplitude of the surface impedance at the Nyquist frequency control the relative effects of the ground conductance between the stations.