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The intensity of the equatorial electrojet (EEJ) derived from the magnetic field measurements by the China Seismo-Electromagnetic Satellite (CSES) is analyzed for the low solar activity period of July 2018–April 2022. The CSES spacecraft flies in a Sun-synchronous orbit, providing the first continuous satellite observations of the afternoon EEJ at a fixed local time at 2 p.m. The EEJ intensities from CSES and concurrent observations from the Swarm satellite mission show a good correlation, supporting the reliability of the CSES EEJ data. Spectral analysis of the CSES data reveals the presence of three distinct oscillatory components in the day-to-day variation of the afternoon EEJ: (1) an eastward-propagating 2–3-day oscillation with zonal wavenumber 1, (2) a westward-propagating 5–6-day oscillation with zonal wavenumber 1, and (3) a zonally-symmetric 14–15-day oscillation. These oscillations result from upward-propagating waves in the atmosphere. That is, the first two can be attributed to the ultra-fast Kelvin wave and quasi-6-day wave, respectively, while the latter is likely due to the atmospheric lunar tide. The CSES EEJ data are also compared with lower thermospheric wind measurements by the Michelson Interferometer for Global High-Resolution Thermospheric Imaging (MIGHTI) onboard the Ionospheric Connection Explorer (ICON). The results suggest that the EEJ intensity correlates negatively with the equatorial eastward wind at 100–115 km, consistent with earlier studies. Contributions of different tidal wind components to longitudinal structures of the EEJ are evaluated. A four-peak structure during July–September can be largely explained by the eastward-propagating diurnal tide with zonal wavenumber 3 (DE3), while a two- or three-peak structure during December–January is mainly due to the combined effect of DE3 and the eastward-propagating diurnal tide with zonal wavenumber 2 (DE2). Furthermore, the CSES EEJ data are compared with the electron density measurements from the Langmuir probe onboard CSES. There is a positive correlation between the EEJ intensity and in-situ electron densities at ∼ 510 km from the same orbit, reflecting the plasma fountain effect. The correlation tends to be higher in the summer hemisphere, which may be due to the meridional wind in the thermosphere. [ABSTRACT FROM AUTHOR]

The statistics of day‐to‐day tidal variability within 35‐day running mean windows is obtained from Michelson Interferometer for Global High‐Resolution Thermospheric Imaging (MIGHTI)/Ionospheric Connection Explorer (ICON) observations in the 90–107 km height region for the year 2020. Temperature standard deviations for 18 diurnal and semidiurnal tidal components, and for four quasi‐stationary planetary waves are presented, as function of latitude, altitude, and day‐of‐year. Our results show that the day‐to‐day variability (DTDV) can be as large as 70% of the monthly mean amplitudes, thus providing a significant source of variability for the ionospheric E‐region dynamo and hence for the F‐region plasma. We further validate our results with COSMIC‐2 ionospheric observations and present an approach to extend the MIGHTI/ICON results to all latitudes using Hough Mode Extension fitting, to produce global tidal fields and their statistical DTDV that are suitable as lower boundary conditions for nudging and ensemble modeling of TIE‐GCM. In the future, this will likely help to establish a data‐driven perspective of space weather variability caused by the tidal weather of the lower atmosphere. Key Points: Statistics of the day‐to‐day variability (DTDV) of diurnal and semidiurnal tides in the ionospheric dynamo region is obtained from Michelson Interferometer for Global High‐Resolution Thermospheric Imaging/Ionospheric Connection ExplorerDTDV is validated with COSMIC‐2 ionospheric observationsStatistics is extended to all latitudes using Hough Mode Extensions [ABSTRACT FROM AUTHOR]

Ionospheric day‐to‐day variability is ubiquitous, even under undisturbed geomagnetic and solar conditions. In this paper, quiet‐time day‐to‐day variability of equatorial vertical E × B drift is investigated using observations from ROCSAT‐1 satellite and the Whole Atmosphere Community Climate Model with thermosphere and ionosphere eXtension (WACCM‐X) v2.1 simulations. Both observations and model simulations illustrate that the day‐to‐day variability reaches the maximum at dawn, and the variability of dawn drift is largest around June solstice at ~90–180°W. However, there are significant challenges to reproduce the observed magnitude of the variability and the longitude distributions at other seasons. Using a standalone electro‐dynamo model, we find that the day‐to‐day variability of neutral winds in the E‐region (≤~130 km) is the primary driver of the day‐to‐day variability of dawn drift. Ionospheric conductivity modulates the drift variability responses to the E‐region wind variability, thereby determining its strength as well as its seasonal and longitudinal variations. Further, the day‐to‐day variability of dawn drift induced by individual tidal components of winds in June are examined: DW1, SW2, D0, and SW1 are the most important contributors. Plain Language Summary: The ionosphere is different from one day to the next, even under geomagnetic and solar quiet condition. The vertical E × B drift at the geomagnetic equator is a key parameter that influences the state of the ionosphere and atmosphere. In this paper, we study the quiet‐time day‐to‐day variability of the equatorial vertical E × B drift by ROCSAT‐1 observations and the Whole Atmosphere Community Climate Model with thermosphere and ionosphere eXtension (WACCM‐X) v2.1 simulations. Both observations and WACCM‐X show that day‐to‐day variability is large at dawn and dusk and it also changes with season and longitude. To better study the variability, we carried out numerical experiments with a new standalone electro‐dynamo model. We found that wind variability below ~130 km is the main contributor, and winds above ~130 km plays a secondary role. The ionospheric conductivity mediates the drift variability response to the wind variability and thus affects its strength and seasonal and longitudinal variations. Further, we examine the variability of dawn drifts generated by different tidal components in June and find that DW1, SW2, D0, and SW1 are the most important ones. This work emphasizes the importance of lower atmospheric variability in studying and predicting the day‐to‐day variations of ionosphere and space environment. Key Points: We quantify the quiet‐time day‐to‐day variability of equatorial vertical drift as a function of local time, longitude, and seasonBoth observations and models show the largest variability occurring at ~ LT 06, which is mainly driven by the variability of E‐region windsThe variability of dawn drift induced by tidal components are examined, with DW1, SW2, D0, and SW1 being the most important contributors [ABSTRACT FROM AUTHOR]

Abstract The Swarm satellites observed small-amplitude (0.1–5 nT) magnetic fluctuations perpendicular to the geomagnetic field. These so-called magnetic ripples (MRs) have a period of around a few tens of seconds along the satellite orbit in the topside ionosphere at middle and low latitudes. They are spatial structures from small-scale field-aligned currents. We investigated the following three characteristics of the MRs. First, we used Swarm observations to confirm their basic characteristics obtained from the Challenging Minisatellite Payload satellite. That is, the global distribution of the average MR amplitudes has clear geographic, seasonal, and local time dependence that is highly correlated with ionospheric conductivities. Second, we found that the average amplitudes of the MRs derived from the Swarm-B satellite, which flies at a ~50 km higher altitude, are slightly smaller than those of the Swarm-A and Swarm-C. This difference suggests that the location of the origin of MRs is below ~460 km altitude, i.e., not in the magnetosphere. Last, to provide evidence of correlation between the MRs and meteorological phenomena, we performed statistical and event analyses with typhoon track data, which are a source of acoustic and gravity waves. The data from 54 typhoons during the period from November 26, 2013, to July 31, 2016, were used for statistical analysis. The results show that the average amplitudes of the MRs during typhoon activity on the dawn, dusk, and night sides are larger than those during non-typhoon conditions. Event analyses indicate amplitude enhancements of the MRs around typhoons, and the latitude of the enhancement migrated with the typhoon. These analyses indicate that typhoon activity is correlated with MR activity and that cumulus convection activity other than typhoons may also affect MR amplitudes. Graphical abstract .

The Swarm satellites observed small-amplitude (0.1-5 nT) magnetic fluctuations perpendicular to the geomagnetic field. These so-called magnetic ripples (MRs) have a period of around a few tens of seconds along the satellite orbit in the topside ionosphere at middle and low latitudes. They are spatial structures from small-scale field-aligned currents. We investigated the following three characteristics of the MRs. First, we used Swarm observations to confirm their basic characteristics obtained from the Challenging Minisatellite Payload satellite. That is, the global distribution of the average MR amplitudes has clear geographic, seasonal, and local time dependence that is highly correlated with ionospheric conductivities. Second, we found that the average amplitudes of the MRs derived from the Swarm-B satellite, which flies at a ~50 km higher altitude, are slightly smaller than those of the Swarm-A and Swarm-C. This difference suggests that the location of the origin of MRs is below ~460 km altitude, i.e., not in the magnetosphere. Last, to provide evidence of correlation between the MRs and meteorological phenomena, we performed statistical and event analyses with typhoon track data, which are a source of acoustic and gravity waves. The data from 54 typhoons during the period from November 26, 2013, to July 31, 2016, were used for statistical analysis. The results show that the average amplitudes of the MRs during typhoon activity on the dawn, dusk, and night sides are larger than those during non-typhoon conditions. Event analyses indicate amplitude enhancements of the MRs around typhoons, and the latitude of the enhancement migrated with the typhoon. These analyses indicate that typhoon activity is correlated with MR activity and that cumulus convection activity other than typhoons may also affect MR amplitudes. [ABSTRACT FROM AUTHOR]

Abstract: Recently the University of Michigan''s Global Ionosphere–Thermosphere Model (GITM) has been equipped with new potential dynamo solver. GITM explicitly solves the full three-dimensional dynamo equations of the thermosphere and ionosphere self-consistently with realistic forcing. Unlike the other coupled global ionospheric models, GITM relaxes the hydrostatic assumption. It includes the IGRF magnetic field with the modified APEX coordinate system, set for the date of the simulation. The model runs are initiated with MSIS model for the neutral atmosphere and with IRI model values for the ionospheric charged particles, but eventually computes these quantities self consistently. The model is also updated to include lower boundary tidal forcing. During geomagnetic quiet conditions, the poleward boundary of the ionospheric dynamo in the model is fixed at ±70° geomagnetic latitude. Two-dimensional electrostatic potential patterns are constructed assuming the equipotential field lines. The model is run for 10 days to ensure quasi-equilibrium state. The GITM simulations reproduce the stronger dynamo processes near dusk during solar maximum. The seasonal dependence is investigated during spring equinox, June and December solstices. GITM results indicate a weaker evening pre-reversal enhancement (PRE) during winter and a stronger PRE during equinoctial months. It is found that the time of PRE occurrence in the Philippine sector is independent of the solar activity and season. Also the seasonal effects are prominent near dawn and dusk sectors. The morning reversal of the vertical drifts occurs late during March and the evening reversals are late during month of December. The effects of different lower boundary tidal inputs have been studied, which reveal the significance of E-region wind dynamo in determining F-region plasma drifts. The model is capable of capturing the salient features of the low to mid-latitude ionosphere, and should be a helpful tool for the investigation of the upper atmosphere and ionosphere. [Copyright &y& Elsevier]

The geomagnetic fields, which play important roles in the ionospheric dynamo, can greatly affect the global distribution of ionospheric electric fields, currents and other ionospheric electrodynamics phenomena. In the study of ionospheric electrodynamics phenomena, such as the longitudinal variations of ionospheric electric fields, the non-dipolar component of the geomagnetic fields must be taken into account. In this paper, we deduce a theoretical electric field model for ionospheric dynamo at mid- and low-latitude which adopt a modified magnetic apex coordinates system. In the new electric field model, the geomagnetic fields can be calculated from either the IGRF model or the dipole field model, and the neutral winds and conductivities are calculated based on empirical models. Then the dynamo equation for the electric potential is finally solved in terms of the line-by-line iteration method, and the ionospheric electric fields and currents are derived from the calculated potential. Our model can reproduce the main features of the ionospheric electrodynamics processes, so it will be a useful tool for the investigation of the upper atmosphere and ionosphere. [ABSTRACT FROM AUTHOR]

Abstract: Three-dimensional structures of the ionospheric dynamo currents are examined using the neutral winds in a general circulation model of the middle atmosphere at Kyushu University. A quasi-three-dimensional ionospheric dynamo model is constructed assuming an infinite parallel conductivity in the ionosphere. This model is able to simulate both the equatorial electrojet and the global Sq current system successfully. The simulated results reveal that the equatorial electrojet is confined in quite narrow latitudes around the equator accompanied with meridional current circulations and satisfies a non-divergent structure mainly within the E region. A vertically stratified double layered structure is seen in the east–west current density near the focus latitude of the global Sq current system. It is shown that the stratified structure mainly consists of the east–west Hall current associated with the eastward wind of zonal wavenumbers 1 and 2 in the lower altitudes and the westward wind of zonal wavenumber 2 in the upper altitudes. The day-to-day variation of the neutral winds can significantly vary the induced ionospheric dynamo current system, which is recognized as changes of the focus latitude and/or the maximum value of the equatorial electrojet. [Copyright &y& Elsevier]

Abstract: The thermospheric neutral wind and ionospheric layer conductance at latitudes higher than 45° (provided from empirical models), as well as the conductivities caused by solar irradiation (deduced from model parameters: neutral temperature, concentration and electron density) are used to calculate the distribution of the electrostatic potential, current and electric field between geomagnetic latitude ±72° and 0–24h MLT in terms of the ionospheric current continuity equation, on June- and December-solstices during strong storms. The driving electric field across the polar cap and the region 2 field-aligned current, caused by the magnetospheric coupling, are taken into account in the calculation. The results represent the complexities and varieties about the coupling between disturbances of the magnetosphere, the thermospheric wind and the distribution of ionospheric conductance. The penetration of disturbed currents from the auroral zone to mid- and low-latitude area is often asymmetrical between the southern and northern hemispheres. Both the intensification of the eastward equatorial electrojet on the day side at the beginning of a storm, and later, the appearance of the westward equatorjet after noon, can be found frequently during storm periods. In the moderate high latitude areas the existence of a eastward electric field in the midnight-dawn region is an expected mechanism, which can be used to explain phenomena found many times in our previous works, due to its E × B upper-ward drift effect. [Copyright &y& Elsevier]

The spatial and frequency characteristics of the equatorial enhancement for several period bands from minutes range to Sq harmonics are established using geomagnetic data from upgraded/new networks that were operational during the International Equatorial Electrojet Year along the Indian and Brazilian sectors. In addition to the regular Sq harmonics and well-developed train of Pc5 pulsations, the periodic components in the period range of 20–160 min are isolated using the data adaptive singular spectrum analysis technique. Equatorial enhancement factor ranges between 3 and 5, both for magnetic fields associated with ionospheric dynamo or the processes that have their primary origin in magnetosphere. Examination of the spatial behavior of variations from equatorial to polar region revealed that magnetic fluctuations in equatorial region have bi-modal origin; related to the ionospheric dynamo and magnetospheric process, the latter resulting from almost instantaneous transmission of polar electric field to equatorial region. When these two sources operate independently, the magnetic fluctuations in equatorial belt in relation to that at low latitudes are enhanced roughly by a factor of 4±1 due to enhanced Cowling conductivity. However, when the two mechanisms work in an interactive coupled manner, they may account for the variability of the equatorial enhancement seen in individual cases. [Copyright &y& Elsevier]

A theoretical model for ionospheric electric fields at mid- and low-latitudes is developed. In the geomagnetic dipolar coordinate system, we deduce a partial differential equation of electric potential from the ionospheric dynamo theory. The deduced equation is taken as the fundamental equation for the present model. The principal parameters for the model input, the thermospheric neutral winds, the densities and temperatures of electrons, ions and neutral atoms and molecules, are obtained from the empirical models HWM93, IRI90 and MSISE90, respectively. In terms of the relaxation iteration method, the partial equations for electric potential are solved successfully, and then, the ionospheric electric potential, fields, and currents are derived. Our model can reproduce the main features of the ionospheric electrodynamical processes such as the equatorial electric jet (EEJ), so it will be a useful tool for studying the upper atmosphere and ionosphere. [ABSTRACT FROM AUTHOR]