Evolution of the geomagnetic daily variation at Tatuoca, Brazil, from 1957 to 2019: a transition from Sq to EEJ

Abstract:

The characterization of the solar quiet (Sq) and equatorial electrojet (EEJ) magnetic field signals at a certain longitudinal sector on the ground typically demands a latitudinal chain of magnetometer stations. However, in the Brazilian sector, it is possible to investigate the transition from the low-latitude domain (Sq) to the equatorial domain (EEJ) by the records from a single geomagnetic observatory: Tatuoca (TTB), which has been operating since 1957 in northern Brazil. This is possible due to the strong secular variation of geomagnetic inclination in Brazil (-0.4°/year, the strongest worldwide), which caused the magnetic equator to move over 1100 km northwards since 1957 and to pass through TTB in 2013. We recovered and processed TTB hourly mean values of the geomagnetic field horizontal component H from 1957 until 2019, allowing the investigation of long-term changes in the geomagnetic daily variation. The Sq-EEJ transition is clearly captured in TTB data. The H seasonal and day-to-day variability increased as the magnetic equator approached, but their amplitude normalized to the average daily variation remained at similar levels. Meanwhile, the amplitude of the lunar variation, normalized in the same way, increased from 5% to 12%. We also show how counter electrojet events vary with latitude at TTB. CHAMP and Swarm satellite data were used to better understand the effects of the EEJ currents on TTB data. In addition, simulations with the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) based on different geomagnetic main field configurations were used to estimate the contribution of the main field secular variation to the observed long-term changes in the geomagnetic daily variation at TTB.