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Abstract

The equatorial electrojet (EEJ) is an electric current that flows eastwards in the ionospheric E-region, along themagnetic equator. Occasionally, it reverses during morning and afternoon hours, leading to periods of westwardcurrent that are known as counter electrojet (CEJ) events. The EEJ/CEJ magnetic signal can be isolated fromother large-scale variations by using a pair of stations from the same longitude sector, one equatorial and other oflow-latitude, and taking the difference between their H components.Here, we present the first analysis of CEJ climatology and CEJ dependence on solar flux and lunar phasefor the Brazilian sector, based on an extensive ground-based data set for the years 2008 to 2017 from thegeomagnetic observatory Tatuoca (1.2◦S, 48.5◦W), and we compare it to the results found for Huancayoobservatory(12.0◦S, 75.3◦W) in the Peruvian sector.We found a predominance of morning CEJ events for both sectors. The afternoon CEJ occurrence rate inthe Brazilian sector is twice as high as in the Peruvian sector. The afternoon CEJ occurrence rate strongly dependson season, with maximum rates occurring during the northern-hemisphere summer for the Brazilian sector andduring the northern-hemisphere winter for the Peruvian sector. Significant discrepancies between the two sectorsare also found for morning CEJ rates during the northern-hemisphere summer. These longitudinal differences arein agreement with a CEJ climatology derived from contemporary Swarm satellite data and can be attributed in partto the well-known longitudinal wave-4 structure in the background EEJ strength that results from nonmigratingsolar tides and stationary planetary waves. Simulations with the Thermosphere-Ionosphere-ElectrodynamicsGeneral Circulation Model show that the remaining longitudinal variability of CEJ rates during northern summercan be explained by the effect of migrating tides in the presence of the varying geomagnetic field in the SouthAtlantic Anomaly.