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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. Plain Language Summary The winds in the mesosphere lower thermosphere (MLT) are strongly impacted by dramatic changes in the stratospheric winter polar regions associated with Sudden stratospheric warmings (SSWs). Models have shown that the climatological direction of the MLT north‐south and vertical circulation, characterized by equatorward flow near ∼100–120 km and poleward flow near ∼80–100 km, reverses following the onset of SSWs. Yet, the impacts and causes of these dynamical effects are not well established observationally due to the lack of comprehensive global measurements of the MLT region. This study evaluates the evolution of MLT winds and associated tidal and ultra‐fast Kevin wave variations during the January 2021 SSW using horizontal wind observations from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging instrument onboard the Ionospheric Connection Explorer to present observational evidence of a large MLT north‐south wind reversal due to the SSW and associated global‐scale wave influences. Key Points A prominent (∼30 m/s) reversal in the mesosphere lower thermosphere (MLT) mean meridional circulation during the January 2021 major sudden stratospheric warming (SSW) is observed in Michelson Interferometer for Global High‐resolution Thermospheric Imaging winds Strong (∼45 m/s) MLT westward flow enhancements and latitudinal (10S–40N) dependencies in the wind response occur as a result of the SSW Amplification in MLT SW2 and ultra‐fast Kevin wave1 zonal wind amplitudes are consistent with the observed westward flow enhancements

Meteor radar observations provide wind data ranging from 80 to 100 km altitude, while the Michaelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) onboard the Ionospheric Connection Explorer satellite offers wind data above 90 km altitude. This study aims to generate wind profiles in the mesosphere and lower thermosphere by combining the winds derived from meteor radar and MIGHTI observations over the Korean Peninsula from January 2020 to December 2021. The wind profiles derived from the two instruments are continuous at night, but they show discrepancies during the day. The atomic oxygen 557.7 nm (green line) emission intensity measured by MIGHTI peaks at approximately 100 km during the day and 94 km at night. The vertical gradient of the airglow volume emission rate is more pronounced during the day. These differences can cause day‐night differences in the MIGHTI wind retrieval accuracy, potentially leading to discrepancies during the day. Plain Language Summary This study aims to derive vertical wind profiles in the mesosphere and lower thermosphere (MLT) by integrating wind measurements from different techniques. Neutral winds in the MLT provide a means to study the activity of various atmospheric waves originating from the lower thermosphere and their propagation to the upper thermosphere. The Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) instrument onboard the Ionospheric Connection Explorer satellite provides wind measurements above 90 km altitude. A meteor radar in Korea provides wind data in the altitude range of 80–100 km. By combining the MIGHTI and meteor radar observations, we derive extended wind profiles in the MLT. While nighttime winds driven from two different techniques show good agreement, discrepancies exist in daytime winds. Vertical gradients in airglow intensity can affect the wind retrieval from MIGHTI's airglow measurement, and this factor can be one of the sources of daytime discrepancy. Key Points Vertical wind profiles above 80 km are derived from meteor radar and Ionospheric Connection Explorer (ICON) observations over Korea These observations produce continuous wind profiles at night, but discontinuity exists between two measurements during daytime Significant vertical variation in airglow intensity on the dayside can impact wind retrieval from ICON airglow observations

The high‐resolution Whole Atmosphere Community Climate Model with thermosphere/ionosphere extension (WACCM‐X) is used to study the impacts of gravity waves (GWs) on the thermospheric circulation and composition. The resolved GWs are found to propagate anisotropically with stronger eastward components at most altitudes. The dissipation of these waves in the thermosphere produces a net eastward forcing that reaches peak values between 200 and 250 km at mid‐high latitudes in both hemispheres. Consequently, the mean circulation is weakened in the winter hemisphere and enhanced in the summer, which in turn impacts the thermospheric composition. Most notably, the column integrated O/N2 in both hemispheres is reduced and agrees better with observations. The mean thermospheric GW forcing in the meridional direction has comparable amplitude and acts to modify the gradient‐wind relationship. Plain Language Summary Small‐scale waves originate from the lower atmosphere have been shown to propagate into the thermosphere. To study their effects a high‐resolution whole atmosphere model has been employed. Using this high‐resolution model, which can partially resolve the small‐scale waves, we can directly quantify the force exerted by these waves on the general circulation in the thermosphere. We found that such force is strong, and affects the thermospheric circulation in both winter and summer hemisphere. This consequently changes the distribution of important thermospheric species. One measure of the thermospheric composition is the ratio of atomic oxygen and molecular nitrogen, which is an indicator of the relative abundance of atomic and molecular species. This ratio has been grossly over‐estimated in previous modeling studies. It is reduced as a result of the circulation change, and is much better agreement with observations. Key Points Gravity waves (GWs) resolved by high‐resolution WACCM‐X displays anisotropic propagation GW forcing alters thermospheric circulation The circulation change leads to a much improved thermospheric O/N2

The storm‐time temperature difference with respect to its quiet‐time expectation (ΔT) in the mesosphere and lower thermosphere were studied during the extreme storms on 2024 Mother's Day and 2003 Halloween Day. The storm‐time ΔT were determined by performing daily zonal running mean on the temperature profiles in the ascending and descending nodes separately. The storm‐time ΔT had peak values of ≥25 K and extended downward to ∼100 km globally. Above 105 km, the global mean ΔT had values of ≥20 K in the early morning and of ≥15 K in the late afternoon during storm‐time. At high latitudes, the storm‐time ΔT was larger in the late afternoon than in the early morning. This is opposite to that at middle and low latitudes. Adiabatic warming/cooling caused by the heating‐induced circulation changes outside of the auroral oval is likely responsible for the local time and latitude dependence of the storm‐time ΔT.

It is well known that Mesosphere and Lower Thermosphere (MLT) temperatures at both high and mid‐low latitudes respond significantly to geomagnetic storms. However, during major geomagnetic storms, the temperature response in the MLT equator is relatively weak and has received little attention. In 2024, two superstorms, one on Mother's Day and another in October, provided a rare opportunity to investigate the effects of superstorms on equatorial MLT temperatures. During superstorms, MLT equatorial temperature increase observed by TIMED/SABER can surpass 40 K and vary with local time, showing stronger responses during nighttime and the dawn sector compared to midafternoon and dusk. The equatorial MLT temperature increases surpassed those in low and even mid‐latitude regions. ΣO/N2 increase derived from TIMED/GUVI indicates that downward vertical flows result in the equatorial temperature anomaly increase. This study enhances our understanding of MLT dynamics under extreme space weather conditions, providing valuable insights for improved predictive models.

First characterization of year‐round Na layers from 75 to 150 km is enabled with 7 years (2011–2017) of high‐detection‐sensitivity lidar observations over Boulder (40.13°N, 105.24°W). Clear annual and semiannual oscillations (AO and SAO) are revealed in the nightly‐mean thermosphere‐ionosphere Na (TINa) (∼105–150 km) number density and volume mixing ratio with the summer maximum but spring equinox (March/April) minimum. Such stark contrast to the summer minimum in the main Na layers (∼75–105 km) supports the theory of TINa formed via TINa+ ion neutralization (TINa++e−→TINa+hν $\\text{TIN}{a}^{+}+{e}^{-}\\to \\text{TIN}a+h\\nu $). The SAO/AO amplitude ratio profiles (75–150 km) exhibit significant changes (∼0.06–2), linking TINa SAO to thermospheric density SAO and the minimal wave/eddy transport around midlatitude equinoxes which hinders TINa+ ion production and upward transport via reduced diffusion of the main Na layer. Stronger TINa in autumn than in spring equinox is explained by the maximal (minimal) meteoric influx occurring in September (April).

This study investigates the formation of a newly identified storm‐time ionospheric phenomenon, the bubble‐like ionospheric super‐depletion structure (BLISS) in F‐region plasma densities. The observed BLISS initiates from the magnetic dip equator in the post‐sunset region and expands westward and poleward, reaching midlatitudes of ∼40°, which is distinct from equatorial plasma bubbles. The mechanism of BLISS formation remains unclear. To systematically study the driving processes for this phenomenon, we employ the Multiscale Atmosphere Geospace Environment (MAGE) model that fully couples the magnetosphere, ionosphere and thermosphere models. Our MAGE simulation successfully reproduced the observed BLISS event during the 8 September 2017 storm. It shows that the depletion is caused by the vertical E × B transport as it expands westward and poleward toward midlatitudes. Numerical experiments demonstrate that sudden enhancements in southward solar wind Bz induce strong prompt eastward penetration electric fields, which are the primary cause of the BLISS and its expansion.

Leveraging the unique perspective enabled by Global‐scale Observations of the Limb and Disk, we examined the characteristics of equinox transitions in the thermospheric column integrated ratio of atomic oxygen to molecular nitrogen (O/N2) in the Northern Hemisphere. We found that the timing of the O/N2 equinox transition from winter to summer or vice versa exhibits a progression with latitude, particularly, near spring equinox. The O/N2 equinox transition is far slower during spring compared to fall, leading to a remarkable seasonal asymmetry. Ionospheric Connection Explorer observed a prominent asymmetry in the summer‐to‐winter circulation in the middle to upper thermosphere, implying that the inter‐hemispheric circulation plays a crucial role in the O/N2 equinox transition. Additionally, since the wave‐driven meridional circulation in the lower thermosphere displays a seasonal asymmetry between the northward‐to‐southward and southward‐to‐northward transitions, we would anticipate that the O/N2 equinox transition is also influenced by the lower atmospheric forcing. Plain Language Summary Thermospheric equinox transition refers to the most dynamic portion of the seasonal variation, which occurs near equinox and is featured by switching from either winter state to summer state or vice versa. Despite considerable advance being made in characterizing and understanding thermospheric seasonal variations over the past few decades, the rapid transitional process around equinox remains poorly understood. Very recently, NASA's Global‐scale Observations of the Limb and Disk and Ionospheric Connection Explorer missions provided a brief chance of simultaneous observations of composition and neutral winds in the thermosphere. Leveraging two missions offers an unparallel opportunity to delineate the equinox transition and to unveil the governing physical processes. Our data analysis reveals the complex nature of the equinox transitions, which is a combined effect of both the summer‐to‐winter interhemispheric circulation and the lower thermospheric wave‐driven circulation. Key Points We examine the thermospheric equinox transition in the Northern Hemisphere with Global‐scale Observations of the Limb and Disk O/N2 and Ionospheric Connection Explorer winds Equinox transition exhibits an evident progression in latitude and a seasonal asymmetry between spring and fall Thermospheric summer‐to‐winter circulation and wave‐driven circulation regulate the O/N2 equinox transitions

We present simulation results of the vertical structure of Large Scale Traveling Ionospheric Disturbances (LSTIDs) during synthetic geomagnetic storms. These data are produced using a one‐way coupled SAMI3/Global Ionosphere Thermosphere Model (GITM) model, where GITM provides thermospheric information to SAMI3 (SAMI3 is Another Model of the Ionosphere), producing LSTIDs. We show simulation results which demonstrate that the traveling atmospheric disturbances (TADs) generated in GITM extend to the topside ionosphere in SAMI3 as LSTIDs. The speed and wavelength (600–700 m/s and 10º–20° latitude) are consistent with LSTID observations in storms of similar magnitudes. We demonstrate the LSTIDs reach altitudes beyond the topside ionosphere with amplitudes of <5% over background which will facilitate the use of plasma measurements from the topside ionosphere to supplement measurements from Global Navigation Satellite System in the study of Traveling Ionospheric Disturbances (TIDs). Additionally, we demonstrate the dependence of the characteristics of these TADs and TIDs on longitude. Plain Language Summary Large Scale Traveling Ionospheric Disturbances are a type of wave that occurs in the ionosphere, a layer of the atmosphere dominated by plasma where the motions of particles are highly subject to the magnetic field, during geomagnetic storms. We utilize two models of Earth's atmosphere and ionosphere to show how these waves behave and show that their location, timing, and speed is dependent on various storm characteristics, timing, and location. We also show that a high‐altitude satellite measuring plasma density in the ionosphere should be able to detect the characteristics of these waves. Key Points We demonstrate that traveling ionospheric disturbances can be produced in simulations of the ionosphere‐thermosphere system We show that these traveling ionospheric disturbances extend to the topside ionosphere in simulations

The response of Nitric Oxide (NO) 5.3 µm infrared radiative emission to the 2024 Mother's Day storm is investigated by utilizing Sounding of Atmosphere using Broadband Emission Radiometry (SABER) observations onboard the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics (TIMED) satellite. Although NO emission displayed a drastic enhancement, it recorded the highest nighttime emission of the SABER era. The intensified field‐aligned currents from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) measurements showed maximum equatorward penetration in the dusk/night sector, reaching below the magnetic latitude of 40° in both hemispheres. TIEGCM simulation demonstrated the strongest Joule heating rate, along with enhanced thermospheric density, NO, atomic oxygen, and temperature structures that sustained for more than 24 hr. The investigation reveals that the strongest nighttime NO emission can be attributed to the enhanced Joule heating rate, driven by stronger equatorial expansion of field‐aligned currents, along with an unprecedented enhancement in thermospheric temperature and composition.