Equatorial Counter Electrojet Longitudinal and Seasonal Variability in the American Sector

Abstract

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

Counter‐Electrojet Occurrence as Observed From C/NOFS Satellite and Ground‐Based Magnetometer Data Over the African and American Sectors

“…The interpretation of this difference partly lies in the fact that the EEJ/CEJ derived from magnetometer data is a representation of electric current system in the ionospheric E region (90–110 km) which is basically over 300 km below the altitude of the C/NOFS satellite. In general, we have observed that CEJ occurrences are more prevalent in the local morning and afternoon/evening, a result that agrees with existing literature (e.g., Alex & Mukherjee, ; Chandrasekhar et al, ; Rabiu et al, ; Soares et al, ; Venkatesh et al, ). The C/NOFS satellite data found significant CEJ occurrence in the afternoon hours compared to magnetometer observations over both American and African sectors.…”

“…In fact, their monthly statistics based on magnetometer data showed that morning CEJs were a common feature up to 100% over AAE apart from months of June, November and December in 2009. Seasonally, we have shown that higher differences in CEJ occurrence rate between the African and American sectors for both C/NOFS satellite and magnetometer observations is in the Southern Hemisphere winter (June–August), a finding similar to results reported in Soares et al (), and attributed to the dominance of wave‐4 pattern. This is also consistent with the most recent comprehensive climatological analysis based on CHAMP satellite data which showed that CEJ occurrence rate peaks around July–August (Zhou et al, ).…”

“…Morning CEJ occurrence in ΔH is therefore dominant for both PUER (69.2 • W longitude) and ALTA (56.1 • W longitude). For the Brazilian sector where ALTA lies, our results are consistent with findings in Venkatesh et al (2015) and Soares et al (2018). Statistically, C/NOFS data are in agreement with magnetometer observations in showing predominantly morning CEJ occurrence with about 80% and 48% for PUER during 2008-2010 and 2011-2014, respectively, and 50% for ALTA in 2011-2014 during 0700-1000 LT.…”

“…Zhou et al (2018) showed that CEJ occurence rate reduces to about 4% at noon. Differences in CEJ occurrence rate between SWARM satellite and magnetometer observations have been recently reported (Soares et al, 2018), but their "qualitative agreement" was emphasized. Morning CEJ occurrence in ΔH is therefore dominant for both PUER (69.2 • W longitude) and ALTA (56.1 • W longitude).…”

“…Sometimes, the direction of the current reverses to westward, a phenomenon called counter electrojet (CEJ) due to a number of physical mechanisms including (but not limited to) ionospheric variability during stratospheric warming periods (Siddiqui et al, ; Stening et al, ; Vineeth et al, ), westward prompt penetrating electric field leading to ionospheric disturbed dynamo (Kikuchi et al, ; Yizengaw et al, ), and vertical upward winds uplifting ions thereby cancelling the vertical polarization electric field (Raghavarao & Anandarao, ). Therefore, the complex variability of CEJ is influenced/modulated by variations in local time, longitude (mainly related to migrating and nonmigrating tides), seasonal dependence, lunar cycles, magnetic activity, and solar activity (Marriott et al, ; Mayaud, ; Rabiu et al, ; Rastogi, ; Singh et al, ; Soares et al, ; Zhou et al, ). Since the CEJ's first detection (Gouin, ), various studies have investigated the CEJ occurrence based mainly on magnetometers deployed in equatorial and/or low‐latitude regions (e.g., Alex & Mukherjee, ; Rastogi, , and references therein).…”