Abstract We report the signature of solar forcing in the thermosphere using daytime temperature profiles from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) instrument on NASA's Ionospheric Connection (ICON) Explorer mission. We compare temperatures from 90 to 133 km altitude with solar extreme ultraviolet flux and also with results from a Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM‐ICON) forced with MIGHTI wind and temperature observations at the bottom boundary near 97 km. For moderate solar activity in late 2021 to early 2022, MIGHTI temperature oscillations correlate with the 27‐day solar rotation period (r = 0.84) at 133 km with an average amplitude of 38 K, more than twice the GCM amplitude for the same time period. At 110 km, average amplitudes are 8 K whereas the GCM shows insignificant correlation below 115 km (<0.3). These results reveal that solar forcing in the lower thermosphere is stronger than predicted.

Equatorial plasma bubbles (EPBs) pose significant challenges to trans-ionospheric radio communication and navigation systems. Because of their disruptive effects, evaluating and predicting them is key for the development of space systems. The influence of geomagnetic activity on EPBs remains unclear, partly due to limited low-latitude neutral wind measurements. Neutral winds drive electric fields and plasma transport, both of which are essential for EPB development. This study addresses this gap using a superposed epoch analysis of key parameters: geomagnetic indices, magnetic zonal and meridional neutral winds, and the index (an indicator of EPB occurrence derived from in situ plasma density measurements). Using data from the Ionospheric Connection Explorer satellite between 2020 and 2022, and focusing on 18:00-23:00 solar local time, our analysis reveals regional differences in EPB behavior. In South America, EPB development is suppressed during the recovery phase of geomagnetic storms, whereas this effect is not detected in other longitudinal sectors. We also observe a global westward shift in zonal winds near the peak of geomagnetic disturbances. These findings improve our understanding of EPB dynamics and highlight the regional variability in their response to geomagnetic forcing.

The prereversal enhancement (PRE) is a brief surge in upward plasma velocity in the evening equatorial ionosphere and a driver of equatorial spread-F. This study reports the first PRE climatology from Ionospheric Connection Explorer (ICON) data, exhibiting seasonal and longitudinal variability that is qualitatively consistent with results from two previous satellite missions. Previous missions, however, lacked the neutral wind observations to characterize their impact on the PRE. To quantitatively assess wind impacts, numerical experiments are performed with a standalone dynamo solver using winds from the TIEGCM-ICON, which is driven from below by observed tides. To quantify the impact of solar/magnetic geometry, such as the alignment between the solar terminator and the magnetic meridian, the model was first driven with seasonally and longitudinally averaged winds (which includes seasonally averaged zonal-mean winds and migrating tides). This reproduces the observed PRE variability with a correlation of 0.44. Incorporating longitudinally and seasonally varying wind patterns improves the correlation to 0.68. This suggests that climatological wind variability is an important driver of PRE variability, but future work is needed to account for the missing variability. Potential missing drivers include conductivity variability near the terminator and mesoscale wind features such as the solar terminator wave.

The lower to upper atmosphere vertical coupling via atmospheric solar tides is very variable and affects the dynamics, composition and electrodynamics of thermosphere-ionosphere (TI) system. In addition, complex solar wind forcing is always impacting the high latitude region and its effects can extend to the mid- and low latitude region. The Ionospheric Connection (ICON) explorer mission provides almost 3 years of data and an opportunity to examine the variation in the TI due to lower atmospheric and MI forcing. This is facilitated by the ICON Level4 product, the thermosphere-ionosphere-electrodynamics general circulation model (TIEGCM) driven by tides fitted to ICON observations via the Hough Mode Extension (HME) method. The effects of the upward propagating tides can be isolated by examining the difference between two TIEGCM simulations with and without tidal HME forcing at the model’s lower boundary, while the effects of solar and magnetospheric variability can be estimated by the difference to a simulation with constant solar and geomagnetic forcing.In this presentation we use over 2 years of TIEGCM simulations to evaluate the model by comparing primarily to ICON observations and examine the captured TI variations. A special focus in our comparison will be on the neutral wind and its two-way coupling to ion drift and plasma distribution. For specific time period we will delineate the contributions due to lower atmospheric tidal forcing from the one due to solar and magnetospheric forcing and quantify the separate effects on the neutral wind, ion drift, and plasma variation.

Changes in the thermospheric wind originating in storm-time transients in high-latitude Joule heating and ion circulation are effective in modifying conditions throughout Earth's upper atmosphere and ionosphere. Among the effects these drivers can produce are large-scale gravity waves (GWs), characterized by significant wind transients that propagate away from the auroral zone, driving transient ion motion during their 1-2 hr passage. Longer period changes in mean winds can develop over the following hours to days, depending on the duration and magnitude of the high latitude heating, and also extend globally. The effectiveness of these processes in modifying the mean density of the daytime ionosphere likely depends on the extent to which these disturbances reach the daytime equatorial region and downward into the E-region wind dynamo (below 180 km). A study of a month of observations made during the ICON mission reveals the variety of behaviors with both transient effects and longer-term changes in mean winds. The duration of auroral inputs, as opposed to the average input over time, is identified as important to the development of dynamo-modifying zonal disturbance winds. During geomagnetic disturbances, we find that the predictive capability of a general circulation model (TIEGCM) for meridional wind transport is good (R .8) while the storm-time zonal wind transport is harder to predict (R .5). This study is the first of its kind, measuring winds and storm responses continuously for a month in both the daytime E- and F- regions simultaneously with 97 min cadence.

Following the eruption of the Hunga Tonga-Hunga Ha'apai (hereafter called ‘Tonga’) volcano just before local sunset on 15 January 2022, satellite data reveals the formation of a large-scale plasma depletion surrounding the region. This depletion persisted for roughly 14 hours, until local sunrise resumed plasma production. By combining in-situ and remote satellite observations, we seek to characterize the depletion's magnitude, spatial scale, and temporal evolution in the hours following the eruption. We will compare this to observations of ionospheric holes following previous impulsive lower atmospheric events, such as the 2011 Tohoku earthquake. Finally, we will investigate the dominant mechanism for locally depleting the plasma following this event, considering field-aligned ion drag, cross B transport due to electric fields arising from dynamo or other effects, and changing recombination rates. We aim ultimately to better understand the coupling between the lower atmosphere and ionosphere/thermosphere system following impulsive events such as this eruption.

Every evening, the sunset removes the primary energy input to the upper atmosphere, causing rapid atmospheric cooling and generating disturbances called solar terminator waves (STWs). Although they theoretically occur every night, STWs remain poorly understood, partially because the rapidly changing atmospheric conditions near sunset make measurements challenging. This study examines neutral wind measurements from the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) on board NASA's Ionospheric Connection Explorer (ICON) observatory to uncover signatures of STWs. We report the north-south wind signatures of STWs and their altitude profile from 200 to 300 km, both of which have never been previously reported. We show that STWs are some of the largest amplitude dynamical features above 200 km near solstices, but are much weaker near equinoxes. By comparing our observations with the outputs of four different models, we find that STWs are likely generated directly or indirectly (from wave propagation) below 97 km. Future work is necessary to better understand how STWs are generated, how they vary on a daily basis, and the extent of their impacts on Earth's upper atmosphere.

AbstractTwo ∼2‐week Ultra‐Fast Kelvin Wave (UFKW) events centered on days 158(203) during 2021 are investigated using winds, temperatures, plasma drifts and electron densities (Ne) measured by the Ionospheric CONnections (ICON) mission. Eastward‐propagating longitudinal wave‐1 (s = −1) structures with periods 2.5–4.0d, thought to mainly reflect Ultra‐Fast Kelvin waves (UFKWs), reveal ±45 ms−1 zonal winds (U) at 100 km for both events. Height‐latitude structures of the 3.0(3.5)d‐period UFKWs are obtained for the first time for both temperature (T, 94–120 km) and U (94–280 km) between 12°S and 39°N latitude. Maximum values of 36(29) ms−1 for U and 12(15)K for T occur at 102(106) km altitude and within ±3° latitude. The U‐T peak height displacement remains unexplained. Vertical wavelengths are in the range 36–43 km for both U and T during both events. Concurrent with the E‐region dynamo winds, topside (580 km) F‐region field‐aligned (±20–40 ms−1), meridional (±5–10 ms−1) and vertical (±5–10 ms−1) drift and Ne (±20–40%) 2.5–4.0d s = −1 variations are also measured. These key elements of atmosphere‐ionosphere (A‐I) coupling, contemporaneously measured for the first time, are relevant to testing the internal consistency of A‐I models. The mean wind propagation environment of the UFKWs is also quantified, showing no appreciable effects on the UFKW structures, consistent with modeling and theory.

The wind dynamo in the ionosphere leads to differential motion of ions and electrons, which in turn sets up electric fields and currents. Observations show that daytime lower thermospheric horizontal winds have large vertical gradients. Numerical modeling conducted approximately 50 years ago demonstrated that the zonal wind shears in the similar to 130-180 km altitude range can generate off-equatorial relative minima (dips) in the daytime height-integrated eastward current density, appearing as westward sidebands north and south of the equatorial electrojet (EEJ). This study observationally confirms this connection for the first time by combining Ionospheric CONnection explorer zonal wind profiles and Swarm latitudinal zonal currents. We demonstrate observationally that the magnitude of the EEJ sideband current is proportional to the strength of westward turning winds with altitude in the Pedersen conductivity dominated region. Additional numerical experiments explain the importance of wind shear in different altitude regions in generating the sideband current. This study contributes to the better understanding of the neutral wind effect on the local current generation. The winds in the E-region ionosphere push the plasma in the presence of Earth's magnetic field, causing ions and electrons to move separately, producing electric current. The low-latitude ionospheric current system consists of an intense eastward current at the magnetic equator (called Equatorial Electro-Jet (EEJ)) and off-equatorial reduced eastward or relative westward currents (EEJ sideband currents) in both hemispheres. Modeling studies have shown that the altitudinal gradient of the zonal wind is related to the strength of the EEJ sideband currents. However, observational studies to validate these results have been missing to this date. This study utilizes simultaneous observations from Ionospheric CONnection explorer and Swarm satellites to provide insights on the connection between low-latitude winds and currents, which will improve our understanding of the causes of daytime ionospheric variability. First observational evidence of effect of zonal wind gradients at similar to 130-180 km on low-latitude Equatorial Electro-Jet sideband currents The altitude variation of the zonal wind gradient modulates the latitudinal variation of EEJ sideband currents With increasing westward wind shear, the strength of low-latitude Equatorial Electro-Jet sideband current increases

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.

AbstractNumerical forecasts of plasma convective instability in the postsunset equatorial ionosphere are made based on data from the Ionospheric Connections Explorer satellite (ICON) following the method outlined in a previous study. Data are selected from pairs of successive orbits. Data from the first orbit in the pair are used to initialize and force a numerical forecast simulation, and data from the second orbit are used to validate the results 104 min later. Data from the IVM plasma density and drifts instrument and the MIGHTI red‐line thermospheric winds instrument are used to force the forecast model. Thirteen (16) data set pairs from August (October), 2022, are considered. Forecasts produced one false negative in August and another false negative in October. Possible causes of forecast discrepancies are evaluated including the failure to initialize the numerical simulations with electron density profiles measured concurrently. Volume emission 135.6‐nm OI profiles from the Far Ultraviolet (FUV) instrument on ICON are considered in the evaluation.

Large vertical shears in the E-region neutral zonal winds can lead to ion convergences and contribute to plasma irregularities, but climatological studies of vertical shears of horizontal winds in a global scale are lacking due to the limitations of data coverage. The Ionospheric Connection Explorer (ICON) Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) has provided neutral wind observations with an unprecedented spatial coverage. In this study, the climatology of dayside E-region neutral wind shears has been examined using 2-years' data (2020-2021). Specifically, the study focuses on large wind shears with a magnitude larger than 20 m/s/km, since large wind shears are more likely to cause significant perturbation in the ionosphere-thermosphere (I-T) system. The results show that the probability of occurrence of large shears is strongly dependent on the altitude, with the vertical profile varying with shear direction, latitude, season, and local time. In general, below 110 km altitude, large negative shears of the eastward wind are most likely to happen during summer at 8-10 LT in 25(degrees)N-40(degrees)N latitudes, showing a high probability across nearly all longitudes. Meanwhile, large positive shears tend to occur in 10(degrees)S-10(degrees)N latitudes, with peak probabilities exhibiting roughly consistent longitudinal structures across 8-10 LT in all seasons. The discrepancies between positive and negative large shear distributions underlie different global tidal influences. The large-shear occurrence probabilities above 110 km are generally small, except in latitudes above 25(degrees)N during the winter for positive shears.

Abstract First results are presented from the conjugate maneuvers performed by NASA's Ionospheric Connection Explorer (ICON) spacecraft. During each several‐minute maneuver, ICON crosses the magnetic equator, measuring the plasma drift at the ∼600‐km apex of a magnetic field line and the neutral wind profiles (∼90–300 km altitude) along both ends of that field line. The analysis utilizes 149 pairs of maneuvers separated by ∼24 hr but at nearly the same location and local time. Principal component regression reveals that 39 ± 7% and 24 ± 9% of the day‐to‐day variance in the daytime vertical and zonal drift, respectively, is attributable to conjugate neutral winds. The remaining variance is likely driven by external potentials from non‐conjugate winds and geomagnetic activity (median Kp 2−). Zonal winds at 100–113 km and >120 km altitude are the primary drivers of conjugate vertical and zonal drift variance, respectively. These observations can test vertical‐coupling mechanisms in whole‐atmosphere models.

This study uses satellite measurements of plasma densities and thermospheric winds to analyze the effects of the November 2021 geomagnetic storm on ionospheric pre-midnight topside plasma bubbles over South America. Using observations from the Ionospheric Connection Explorer (ICON) and the Global-scale Observations of the Limb and Disk (GOLD) satellites, we find that pre-midnight topside plasma bubbles were inhibited over eastern South America during the recovery phase of the storm. This is particularly notable because of the otherwise high occurrence rate of plasma bubbles at these longitudes during this season. This inhibition coincided with the recovery phase of the geomagnetic storm, marked by a northward turning of the z-component of the interplanetary magnetic field (IMFBz) and quiet-time values of the SuperMAG Auroral Electrojet Index (SME). We observed a westward turning of the zonal wind before the bubble inhibition, so we conclude the inhibition of topside plasma bubbles is likely related to a westward disturbance dynamo electric field (DDEF) causing a downward ExB $\mathbf{E}\times \mathbf{B}$ drift and suppress the growth of the instability responsible for bubble development. Contrary to theoretical predictions, we do not observe notable changes to the meridional wind during the event. These results provide new insights into the ionosphere-thermosphere system's response to geomagnetic storms and highlight the role of wind patterns in inhibiting ionospheric irregularities, contributing to better predictive models for these phenomena. Geomagnetic storms negatively affect communication and navigation systems by producing unpredictable changes in ionospheric densities. Plasma bubbles are irregularities in the ionosphere that occur mainly in the equatorial region and may be affected by geomagnetic storms. Studying plasma bubbles is crucial because they can significantly reduce the performance of communication and navigation systems, leading to signal loss or degradation. We conducted a study on how a geomagnetic storm in November 2021 affected the occurrence of pre-midnight plasma bubbles over South America. Using data from two satellites (ICON and GOLD), we observed that these plasma bubbles disappeared during the storm's recovery phase. Our analysis suggests that this inhibition is likely linked to changes in wind patterns in the ionosphere. Specifically, a westward wind created conditions that prevented the generation of plasma bubbles. This study offers new insights into how winds during geomagnetic storms affect the ionosphere and helps improve predictions for communication and navigation systems affected by these disruptions. Pre-midnight topside plasma bubbles are inhibited over eastern South America during the recovery phase of a geomagnetic storm Zonal wind measurements show evidence of wind-driven electric fields inhibiting plasma bubble formation Although theoretical predictions also anticipate changes in the meridional wind, they are not observed

The spectral line profile of the atomic oxygen O 1 D 2 — 3 P 2 transition near 6300 Å in the airglow has been used for more than 50 years to extract neutral wind and temperature information from the F‐region ionosphere. A new spectral model and recent samples of this airglow emission in the presence of the nearby lambda‐doubled OH Meinel (9‐3) P 2 (2.5) emission lines underscores earlier cautions that OH can significantly distort the OI line center position and line width observed using a single‐etalon Fabry‐Perot interferometer (FPI). The consequence of these profile distortions in terms of the emission profile line width and Doppler position is a strong function of the selected etalon plate spacing. Single‐etalon Fabry‐Perot interferometers placed in the field for thermospheric measurements have widely varying etalon spacings, so that systematic wind biases caused by the OH line positions differ between instruments, complicating comparisons between sites. Based on the best current determinations of the OH and O 1 D line positions, the ideal gap for a single‐etalon FPI wind measurements places the OH emissions in the wings of the O 1 D spectral line profile. Optical systems that can accommodate prefilters with square passbands less than ∼3 Å in the optical beam can effectively block the OH contamination. When that is not possible, a method to fit for OH contamination and remove it in the spectral background of an active Fabry‐Perot system is evaluated.

Thermospheric wind measurements above 200 km show a prominent migrating wave associated with the evening solar terminator.• The first observations of solar terminator wave altitude profiles reveal > 200 km vertical wavelengths above 200 km.• Comparison with numerical models suggests a lower atmospheric origin and the potentially significant role of gravity waves.

The sub-auroral region is located immediately equatorward from the auroral oval, where important magnetosphere-ionosphere-thermosphere dynamical processes take place.Historically, low-Earth orbit satellites as well as ground-based imagers and radars have provided important information about the region.However, in recent years it has become increasingly clear that there are several unexplored aspects of the region that remain elusive in part due to instrumental and observational limitations.Furthermore, the most commonly used large-scale "state of the art" models and theoretical frameworks of the region rely on a quasi steady-state approach which does not accurately represent the rich sub-auroral electrodynamics underway.Recent ground-based and in-situ measurements have revealed new and compelling dynamics underway in the sub-auroral region and reinvigorated the community's interest there.Optical signatures with unusual spectrographic properties, such as SAR arcs, STEVE, and the picket fence, have been associated with extreme and unusual sub-auroral plasma conditions, such as large ion flow velocities (~5-10 km/s) and extreme electron temperatures (>6000 K).Additionally, recent observations have revealed that the transformation of the sub-auroral region into these extreme conditions occurs within minutes.These new measurements pose a significant challenge to our existing theories and available instrumentation, and demonstrate the necessity of new development and deployment of in-situ and remote measurements of the sub-auroral ionosphere.This white paper highlights some of the open questions in the sub-auroral region that have arisen since the previous decadal survey.It provides recommendations on how science advances can be achieved to help close these open questions, and how new discoveries can be made possible.In short: 1.Establishing new and long-term support for unifying citizen and "traditional" scientists is required to sustain observations and discovery in sub-auroral science.2.NASA's Geospace Dynamics Constellation (GDC) mission is required to address and close many outstanding sub-auroral science questions and should be carried-out without delay. 3.New and long-term support for ground-based observations infrastructure is needed to better understand the nature and dynamics of the sub-auroral ionosphere.Existing infrastructure is too sparse in geographic distribution in the sub-auroral region within the United States. 4.In situ measurements of the lower-thermosphere in the sub-auroral region by rockets, lowaltitude satellites, and/or some other techniques are required to conclusively identify the source, energetic pathways, and mechanism of the extreme conditions reported at sub-auroral latitudes.

Abstract Sudden stratospheric warmings (SSWs) are large‐scale phenomena characterized by dramatic dynamic disruptions in the stratospheric winter polar regions. Previous studies, especially those employing whole atmosphere models, indicate that SSWs have strong impacts on the circulation of the mesosphere lower thermosphere (MLT) and drive a reversal in the mean meridional circulation (MMC) near 90–125 km altitude. However, the robustness of these effects and the roles of SSW‐induced changes in global‐scale wave activity to drive the reversal have been difficult to observe simultaneously. This work employs horizontal lower thermospheric (∼93–106 km altitude) winds near 10°S‐40°N latitude from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging instrument onboard the Ionospheric Connection Explorer (ICON) to present observational evidence of a prominent MLT MMC reversal associated with the January 2021 major SSW event and to demonstrate connections to semidiurnal tidal activity and possible associations with a ∼3‐day ultra‐fast Kevin wave.

Abstract Measurements from the Ionospheric Connections Explorer satellite (ICON) form the basis of direct numerical forecast simulations of plasma convective instability in the postsunset equatorial F region ionosphere. ICON data are selected and used to initialize and force the simulations and then to test the results one orbit later when the satellite revisits the same longitude. Data from the IVM plasma density and drifts instrument and the MIGHTI red‐line thermospheric winds instrument are used to force the simulation. Data from IVM are also used to test for irregularities (electrically polarized plasma depletions). Fourteen datasets from late March 2022, were examined. The simulations correctly predicted the occurrence or non‐occurrence of irregularities 12 times while producing one false positive and one false negative. This demonstrates that the important telltales of instability are present in the ICON state variables and that the important mechanisms for irregularity formation are captured by the simulation code. Possible refinements to the forecast strategy are discussed.