Evaluation of dB/dt amplitudes and sources over the Brazilian region during geomagnetic storms in the 2021–2022 biennium

Abstract

The rate of change of the geomagnetic field ($dB/dt$) observed at Earth’s surface has been used as a proxy of geomagnetically induced currents, which are known to be hazardous for grounded technological systems such as high-voltage power grid systems. The $dB/dt$ rates have been well characterized in the high latitude region ($\gtrsim 60°$), but much less information is available for the low latitudes to date. To overcome this limitation, we have examined $dB/dt$ rates on the $H$ component of the geomagnetic field as measured by an array of stations operated by the EMBRACE Magnetometer Network in Brazil. The main focus is to characterize $dB/dt$ occurrence at very low latitudes ($\lesssim 22°$) of the Brazilian peculiar territory, whose magnetic measurements are influenced by the equatorial electrojet and the presence of the South Atlantic Magnetic Anomaly (SAMA). The period investigated is from 2021 to 2022, over the ascending phase of the solar cycle 25. The statistical analysis demonstrates that $dB/dt$ peak magnitudes are generally below 0.5 nT/s during magnetic storms, and exhibit a dependence on the solar cycle for the station near the center of the SAMA. However, we obtain for a particular case in 2021 that $dB/dt$ reached 1.35 nT/s in magnitude at the magnetic equator during daytime, which is a significant value even for higher latitudes. A case study of the induced geoelectric fields during this event shows that the conductivity structure beneath two compared sites plays a major role in the amplitude of such fields than $dB/dt$ amplitude. Also, the MLT distributions of $dB/dt$ indicate that ultra-low frequency waves can be a major source of these signals in lower latitudes.