The secular variation of the core field is generally characterized by smooth variations, sometimes interrupted by abrupt changes, named geomagnetic jerks. The origin of these events, observed and investigated for over three decades, is still not fully understood. Many fundamental features of geomagnetic jerks have been the subject of debate, including their origin internal or external to the Earth, their occurrence dates, their duration and their global or regional character. Specific tools have been developed to detect them in geomagnetic field or secular variation time series. Recently, their investigation has been advanced by the availability of a decade of high-quality satellite measurements. Moreover, advances in the modelling of the core field and its variations have brought new perspectives on the fluid motion at the top of the core, and opened new avenues in our search for the origin of M. Mandea ( ) Helmholtz-Zentrum

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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

[1] Geomagnetic jerks are rapid time variations of the magnetic field at the Earth's surface that are thought to be of primarily internal origin. Jerks are relevant for studies of the Earth interior: they likely give information on core dynamics and possibly on mantle electrical conductivity. In such studies a precise determination of the jerk occurrence time and its error bar at each observatory is required. We analyze the most well-known global jerks (1969, 1978, and 1991) and a possible local jerk in 1999, considering all three components of the magnetic field (X, Y, and Z). Different data sets are investigated: annual means, 12 month running averages of observatory monthly means in rotated geomagnetic dipole coordinates, and data representing the core field contribution synthesized from the CM4 time-dependent field model. The secular variation in each component of the field around the time of a jerk was modeled by two straight line segments, using both least squares and 1-norm methods. The 1969The , 1978, and 1991 jerks were globally detected, while the 1999 event was only locally identified. Using this simple method enables us to calculate error bars in the jerk occurrence times and to quantify their nonsimultaneous behavior. We find that our error bars are not, in general, symmetric about the mean occurrence time and that the mean errors on the X and Z components of 1.7 years and 1.5 years are larger than that of 1.1 years on the Y component. Generally, the error bars were found to be larger in the Southern Hemisphere observatories. Our results are necessary prerequisites for further studies of the inverse problem that attempt to determine mantle electrical conductivity from variations in jerk occurrence times.