The equatorial electrojet occasionally reverses during morning and afternoon hours, leading to periods of westward current in the ionospheric E region that are known as counter electrojet (CEJ) events. We present the first analysis of CEJ climatology and CEJ dependence on solar flux and lunar phase for the Brazilian sector, based on an extensive ground-based data set for the years 2008 to 2017 from the geomagnetic observatory Tatuoca (1.2°S, 48.5°W), and we compare it to the results found for Huancayo (12.0°S, 75.3°W) observatory in the Peruvian sector. We found a predominance of morning CEJ events for both sectors. The afternoon CEJ occurrence rate in the Brazilian sector is twice as high as in the Peruvian sector. The afternoon CEJ occurrence rate strongly depends on season, with maximum rates occurring during the northern-hemisphere summer for the Brazilian sector and during the northern-hemisphere winter for the Peruvian sector. Significant discrepancies between the two sectors are also found for morning CEJ rates during the northern-hemisphere summer. These longitudinal differences are in agreement with a CEJ climatology derived from contemporary Swarm satellite data and can be attributed in part to the well-known longitudinal wave-4 structure in the background equatorial electrojet strength that results from nonmigrating solar tides and stationary planetary waves. Simulations with the Thermosphere-Ionosphere-Electrodynamics General Circulation Model show that the remaining longitudinal variability in CEJ during northern summer can be explained by the effect of migrating tides in the presence of the varying geomagnetic field in the South Atlantic Anomaly.The quiet-time CEJ is mainly related to changes in the atmospheric tides that dominate the global wind system at ionospheric heights (Gurubaran, 2002;Hanuise et al., 1983), and it is mostly observed during a few hours in the morning (MCEJ) or afternoon (ACEJ) periods. Under disturbed conditions, other mechanisms play a role in addition to the tidal variability, such as the prompt penetration of polar electric field into equatorial SOARES ET AL. 9906

Geophysical events such as earthquakes, volcanoes and tsunamis can cause atmospheric waves such as acoustic waves and gravity waves (Yeh & Liu, 1974). Acoustic waves have frequencies higher than the acoustic cutoff frequency (∼3.2 mHz at the stratopause), while gravity waves have frequencies lower than the Brunt-Väisälä frequency (∼2.7 mHz at the stratopause). They can propagate away from the source, transferring energy and momentum into the middle and upper atmosphere. As the waves propagate to higher altitudes, they grow in amplitude due to decreasing atmospheric density. Yeh and Liu (1974) estimated that a seismic wave with vertical ground displacement of 5 mm could lead to an acoustic wave whose vertical wind velocity reaches 30 m/s at an altitude of 150 km. Such a large perturbation of the neutral atmosphere would have a significant impact on the dynamics and electrodynamics of the ionosphere. Indeed, ionospheric disturbances associated with acoustic and gravity waves have been reported following strong earthquakes and other geophysical events for many decades (see reviews by e.g., Astafyeva, 2019;Meng et al., 2019).Atmospheric oscillations with frequencies near the acoustic cutoff frequency are frequently observed after eruption events (Kanamori et al., 1994). Modeling studies have shown that those oscillations can be explained by acoustic waves trapped between the ground and thermosphere (e.g.

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This paper examines the response of the upper atmosphere to equatorial Kelvin waves with a period of ∼3 days, also known as ultrafast Kelvin waves (UFKWs). The whole atmosphere model Ground‐to‐topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) is used to simulate the UFKW events in the late summer of 2010 and 2011 as well as in the boreal winter of 2012/2013. When the lower layers of the model below 30‐km altitude are constrained with meteorological data, GAIA is able to reproduce salient features of the UFKW in the mesosphere and lower thermosphere as observed by the Aura Microwave Limb Sounder. The model also reproduces ionospheric response, as validated through comparisons with total electron content data from the Gravity field and steady‐state Ocean Circulation Explorer satellite as well as with earlier observations. Model results suggest that the UFKW produces eastward‐propagating ∼3‐day variations with zonal wavenumber 1 in the equatorial zonal electric field and F region plasma density. Model results also suggest that for a ground observer, identifying ionospheric signatures of the UFKW is a challenge because of ∼3‐day variations due to other sources. This issue can be overcome by combining ground‐based measurements from different longitudes. As a demonstration, we analyze ground‐based magnetometer data from equatorial stations during the 2011 event. It is shown that wavelet spectra of the magnetic data at different longitudes are only in partial agreement, with or without a ∼3‐day peak, but a spectrum analysis based on multipoint observations reveals the presence of the UFKW.