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Comprehensive pattern of ionospheric troughs' location in winter for all local times, longitudes, high (HSA), and low (LSA) solar activity, in both hemispheres, is at first investigated. Statistical analysis based on a large dataset of Interkosmos‐19, CHAMP, and Kosmos‐900 satellites was performed for quiet geomagnetic conditions Kp = 1–3. Three troughs were considered: high‐latitude trough (HLT) located inside the auroral oval, main ionospheric trough (MIT) located equatorward from the auroral oval, that is, at subauroral latitudes, and mid‐latitude ring ionospheric trough (RIT). The main purpose was to study the formation of the troughs' diurnal pattern in different conditions. The main problem was to distinguish MIT from RIT and MIT from HLT. For this purpose, early morning hours (04–06 LT), late morning hours (07–10 LT), day, evening, and night conditions were examined. In the early morning sector, RIT was separated from MIT and eliminated from the dataset. In the late morning sector, MIT and HLT were first clearly divided, although only for HSA. During the day, in the Northern Hemisphere under all conditions, HLT is mainly observed, in the Southern Hemisphere at poorly lit longitudes only the daytime MIT, and at well‐lit longitudes only the HLT is observed. The division of MIT and HLT was carried out according to the (statistical) position of the equatorial boundary of the auroral oval precipitation. At night, longitudinal variations in the MIT position determine the asymmetry of the hemispheres. Thus, the occurrence and position of MIT and HLT depend on the hemisphere, longitude, and solar activity. Plain Language Summary: Dynamics of the ionization troughs determines the variation in a structure of the winter and equinoctial subauroral and auroral ionosphere. Unfortunately, many aspects of this dynamics are still poorly known, which does not allow creating an adequate model of the high‐latitude ionosphere. Such a model is necessary to describe the conditions for propagation of short radio waves. In the international reference model of the ionosphere IRI, the trough as a structure is absent; therefore, IRI poorly describes high latitudes. In this paper, for the first time, a comprehensive pattern of diurnal variations in the different troughs position in winter high‐latitude ionosphere is constructed. They are found to be highly dependent on hemisphere, longitude, and solar activity. The results of this work will be used to build an adequate model of the high‐latitude ionosphere, which is being now created in the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation of the Russian Academy of Sciences (IZMIRAN). Key Points: Comprehensive pattern of diurnal variations in winter ionospheric troughs' position in Northern and Southern hemispheres was obtainedStatistical analysis based on a large dataset of Interkosmos‐19, CHAMP, and Kosmos‐900 satellites was performedDiurnal variations in trough position depend strongly on solar activity, hemisphere, and longitude [ABSTRACT FROM AUTHOR]

The existence of an additional stratification in the daytime equatorial ionospheric F region (the F3 layer) was known since the 1940s. However, its characteristics and the underlying physical mechanism have been uncovered only recently. In this paper, we present and discuss the F3 layer characteristics observed by six ionosondes distributed over equatorial and low latitudes (−20° to +25° dip latitudes) in the Brazilian longitude sector during the strongest geomagnetic storm (DstMin = −223 nT) of solar cycle 24, the St. Patrick's Day storm of 17 March 2015. Two eastward prompt penetration electric field (PPEF) events, as seen in equatorial electrojet, occurred during the main phase of the storm on 17 March 2015, a strong one (~100 nT) at around ~1200 UT and a weak one (~50 nT) at around ~1725 UT. Local time variations in the F3 layer occurrence and ionospheric base height (h′F), peak height (hmF), and peak electron density (Nmax) are investigated. Notably, the F3 layer occurred at all six locations, more distinctly during the stronger PPEF event. The large latitudinal extend in the occurrence of the F3 layer in opposite hemispheres (−20° to +25° dip latitudes) covering the equatorial ionization anomaly crests observed for the first time is interpreted in terms of the combined effect of the super plasma fountain generated by the eastward PPEF and storm time equatorward neutral wind. Key Points: F3 layer occurred over a large latitudinal extent from 20°S to 25°N dip latitudes during 17 March 2015 storm‐induced PPEF phasesThe PPEF‐generated superfountain led to the F3 layer occurrence earlier at the equator and later at higher latitudes in both hemispheresF3 layer was more distinct and stronger and lasted longer in the Northern Hemisphere due to the storm time asymmetric equatorward neutral winds [ABSTRACT FROM AUTHOR]

A strong westward wind pattern at altitudes of ∼140–250 km at high latitudes (∼65°) in the approximate longitude range of 100°E–200°E in the Southern Hemisphere, and from 200°E to 300°E in the Northern Hemisphere, known as a "wind wall," is investigated. Simulations from the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM‐X) and the Global Airglow (GLOW) model, along with Wind Imaging Interferometer (WINDII) observations, allowed the description of most of the wind wall features. The results indicate that the wind wall is linked to the well‐known high‐latitude wind convection pattern. This atmospheric circulation is significantly stronger during active geomagnetic conditions, where its dusk anticyclonic circulation cell is also enhanced, which results in the origin of the wind wall. The study also reveals the impacts of the wind wall on the vertical velocity and its consequences for the O(1S) green line emission on the lower thermosphere, as well as its influence on the atomic oxygen depletion at high latitudes, which is mainly a response to the complex high‐latitude wind convection pattern and its spatial and temporal variation. Key Points: Strong thermospheric westward winds at polar regions, known as a wind wall, are associated with the high‐latitude convection patternThe wind wall pattern has its origin in the active geomagnetic conditionsOver the lower thermosphere, the wind wall leads to perturbations on the vertical winds, the O(1S) emission rates, and the atomic oxygen [ABSTRACT FROM AUTHOR]

Hemispheric asymmetries of the Vertical Total Electron Content (VTEC) were observed during the first recovery phase of the geomagnetic storm on September 7-8, 2017. These asymmetries occurred at the mid latitudes at two different local times simultaneously: In the European-African sector (early morning), the storm time VTEC in the southern/northern hemisphere was higher/lower than the quiet time value, suggesting the southern/northern hemisphere entered the positive/negative phase (N-S+). In the East Asian-Australian sector (afternoon), the storm time VTEC change was positive in the northern hemisphere, but negative in the southern hemisphere (N+S-). The electron density profiles from digisondes demonstrated that the asymmetries appeared in the F region density as well. The plasma drifts data from digisondes, the column-integrated [O]/[N2] ratio from GUVI onboard the TIMED satellite, and the detrended VTEC were utilized to study the drivers of the asymmetries. Traveling Ionospheric Disturbance (TID) signatures were identified in the digisonde drift and detrended VTEC data before the appearance of the asymmetry. The magnitude of TIDs was larger in the hemisphere where the negative phase occurred later. The storm time [O]/[N2] ratio change was positive in Africa (S+) and negative in Europe (N-). However, the [O]/[N2] measurements were not available in the East Asian-Australian sector during the focused period. The hemispheric differences in the vertical drifts were also observed in both sectors. Therefore, the observed hemispheric asymmetries in both sectors are suggested to be due to the hemispheric asymmetries in the thermospheric composition change, vertical drift, and TID activity. [ABSTRACT FROM AUTHOR]

We present statistical investigation of the high‐latitude ionospheric current systems in the Northern Hemisphere (NH) and Southern Hemisphere (SH) during low (Kp < 2) and high (Kp ≥ 2) geomagnetic activity levels. Nearly 4 years of vector magnetic field measurements are analyzed from the two parallel flying Swarm A and C satellites using the spherical elementary current system method. The ionospheric horizontal and field‐aligned currents (FACs) for each auroral oval crossing are calculated. The distributions of the mean values of FACs as well as the horizontal curl‐free and divergence‐free currents in magnetic latitude and magnetic local time for each hemisphere and activity level are presented. To estimate the NH/SH current ratios for the two activity levels, we remove seasonal bias in the number of samples and in the Kp distribution by bootstrap resampling. This is done in such a manner that there are equal number of samples from each season in each Kp bin. We find that for the low activity level, the currents in the NH are stronger than in the SH by 12±4% for FAC, 9±2% for the horizontal curl‐free current, and 8±2% for the horizontal divergence‐free current. During the high activity level, the hemispheric differences are not statistically significant. This suggests that the local ionospheric conditions, such as magnetic field strength or daily variations in insolation, may be important and play a larger role during quiet than disturbed periods. This issue must be studied further. Key Points: FACs and horizontal currents from Swarm data are analyzed statistically for both Northern (NH) and Southern Hemispheres (SH) using SECS methodWhen seasonal bias is removed from the data, interhemispheric asymmetry is found for low but not for high magnetic activity caseIn low magnetic activity case both FACs and horizontal currents are about 10% more intense in the NH than in the SH [ABSTRACT FROM AUTHOR]