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The lack of observational data for the AlO molecule in the mesosphere/lower thermosphere may be due to ablated aluminum reacting quickly to form other species. Previously proposed reaction pathways show that aluminum could be ablated in the atmosphere from meteoritic activity, but there currently exist very limited spectroscopic data on the intermediates in these reactions, limiting the possible detection of said molecules. As such, rovibrational spectroscopic data are computed herein using quartic force field methodology at four different levels of theory for the neutral intermediates AlCO3, OAlO2, and HOAlO2. Each molecule exhibits multiple vibrational modes with large vibrational transition intensities. For instance, the C–O stretch (ν1) in AlCO3 has a harmonic intensity of 536 km mol−1, the Al–O stretch (ν2) in OAlO2 has an intensity of 678 km mol−1, and the out-of-plane torsion (ν9) in HOAlO2 has an intensity of 158 km mol−1. All three molecules have exceptionally large dipole moments of 6.27, 4.21, and 5.04 D, respectively. These properties indicate that all three molecules are good candidates for potential atmospheric observation utilizing vibrational and/or rotational spectroscopic techniques. [ABSTRACT FROM AUTHOR]

• Twilight phenomena occurs in the thermosphere. • An equation has been derived to calculate diurnal atmospheric height using the twilight images analysis. • The diurnal atmosphere has estimated height ∼304 km above sea level. • Lower wind speeds and temperatures resulted in a higher thickness of diurnal atmosphere. • Higher relative humidity and dew point resulted in a higher thickness of diurnal atmosphere. • The first appearance of dawn twilight is at dip = ∼15°. An equation has been derived through which the height of the diurnal atmospheric layer (lower atmosphere) can be calculated using ground base twilight image analysis. This equation relies on the angular altitude of the twilight with the dip at that moment. Based on the images, it was found that true twilight of dawn occurs at a dip of 14.90° ± 0.17° as detected by the camera or ∼15° by considering the elevation above sea level (while it may be delayed to ∼14.7–14° with the naked eye), which is accompanied by an average twilight angular altitude of 2.66° ± 0.23°. From these values, the twilight height is estimated to be 303.80 ± 5.69 km above sea level. It is shown that the twilight region is the peak zone of the thermosphere. Its height also varies with time according to changes in various atmospheric parameters. Lower wind speeds, higher relative humidity, higher dew point, and lower temperatures resulted in a higher altitudinal angle of the atmosphere and consequently a greater diurnal atmospheric layer's thickness. Image analysis shows that the thermosphere always has a maximum value of light brightness during the night even before the appearance of twilight. Additionally, It defined the light source, shapes, and stages of twilights that detected by images. This implies that sky image analysis is an inexpensive method for understanding the properties and various parameters of the atmosphere. [ABSTRACT FROM AUTHOR]

Nitric Oxide (NO) significantly impacts energy distribution and chemical processes in the mesosphere and lower thermosphere (MLT). During geomagnetic storms, a substantial influx of energy in the thermosphere leads to an increase in NO infrared emissions. Accurately predicting the radiative flux of Nitric Oxide is crucial for understanding the thermospheric energy budget, particularly during extreme space weather events. With advancements in computational techniques, machine learning (ML) has become a highly effective tool for space weather forecasting. This effort becomes even more worthwhile considering the availability of two decades of continuous NO infrared emissions measurement by TIMED/SABER along with several other key thermospheric variables. We present the scheme of development of an ML-based predictive model for Nitric Oxide Infrared Radiative Flux (NOIRF). Various ML algorithms have been tested for better predictive ability, and an optimized model (NOEMLM) has been developed for the study of NOIRF. This model is able to extract the underlying relationships between the input features and effectively predict the NOIRF. The NOEMLM predictions have very good agreements with SABER observation during quiet time as well as geomagnetic storms. In comparison with the existing TIEGCM model, NOEMLM has very good performance, especially during extreme space weather conditions. The results of this study suggest that utilizing geomagnetic and space weather indices with ML/AI can serve as superior parameters for studying the upper atmosphere, as compared to focusing on specific species having complex chemical processes and associated uncertainties in constituents. ML techniques can effectively carry out the analysis with greater ease than traditional chemical studies. [ABSTRACT FROM AUTHOR]

A four-quadrant wind imaging interferometer is a new generation of wind imaging interferometer with the valuable features of being monolithic, compact, light, and insensitive to temporal variations in the source. Its applications include remote sensing of the wind field of the upper atmosphere and observing important dynamical processes in the mesosphere and lower thermosphere. In this paper, we describe a new phase step determination approach based on the Lissajous figure, which provides an efficient, accurate, and visual method for the characterization and calibration of this type of instrument. Using the data from wavelength or thermal fringe scanning, the phase steps, relative intensities, and instrument visibilities of four quadrants can be retrieved simultaneously. A general model for the four-quadrant wind imaging interferometer is described and the noise sensitivity of this method is analyzed. This approach was successfully implemented with four-quadrant wind imaging interferometer prototypes, and its feasibility was experimentally verified. [ABSTRACT FROM AUTHOR]

Geomagnetic storms can cause large variations in the ionosphere, but their impacts on the mesosphere and lower thermosphere (MLT) are not well understood. Based on the Total Electron Content (TEC) data and the meteor neutral winds data over Mohe (53.5°N, 122.3°E) and Beijing (40.3°N, 116.2°E), we analyze the tidal variations during six intense geomagnetic storms from 2016 to 2021. According to the six intense geomagnetic storms, we found that intense geomagnetic storms can lead to diurnal and semidiurnal tidal enhancements in TEC, while their influences on tidal variations in the MLT region are not always captured. Responses of tidal enhancement in the MLT region to the intense geomagnetic storms are more obvious at a lower latitude at Beijing, but the tidal amplitude changes are not proportional to the Dst indices. Some semidiurnal tides are significantly enhanced prior to the onset of geomagnetic storms, which needs to be statistically investigated in the future based on additional observations. [ABSTRACT FROM AUTHOR]

Using the SuperThermal Electron Transport (STET) model coupled with the Super‐thermal Proton Electron Atomic Hydrogen—tRansport in the Ionosphere and Thermosphere (SPEAH‐RIT) codes, we demonstrate that temporal variability of ionospheric conductance is defined by several time scales: the temporal variations of the magnetospheric source, the start time of electron precipitation, and the termination of the corresponding source. In these cases, the time scales are defined by dissipation of energetic electrons and effective recombination processes. The results presented in this paper were applied in the regions of pulsating aurora and polar arcs, demonstrating the fact that ionospheric conductance requires some time to form and decay. These time delays constitute an effective "inertia" in the conductance calculation which is not accounted for in many global models which assume an instantaneous connection between precipitation and conductance. Ionospheric conductance inertia influences the temporal variation in ionospheric and magnetospheric electric fields, and as a result, impacts magnetosphere‐ionosphere (MI) dynamics on a global scale. Plain Language Summary: We demonstrate in this manuscript that temporal variability of ionospheric conductance is defined by several temporal scales and has inertias to its development and decay. Ionospheric conductance inertia influences the temporal variation in ionospheric and magnetospheric electric fields, and as a result, the further reconfiguration of electron precipitation dynamics. The results presented in this paper were applied in the regions of pulsating aurora and polar arcs clearly demonstrate an effective "inertia" of ionospheric system to electron precipitation dynamics. Key Points: Ionospheric conductance is the system response to electron precipitation in the regions of pulsating and discrete aurorasIonospheric conductance carries the inertias of its development and decayConductance formation in pulsating aurora and discrete auroral arcs are defined by multiple temporal scales [ABSTRACT FROM AUTHOR]

Neutral thermospheric density is a crucial parameter for orbital tracking and upper atmosphere research. Here, we present a method to monitor global neutral thermospheric density testing simulations in various space environments with various satellite configurations and altitudes. The simulation results at 1° × 1° pixel resolution show that (1) within the range of 300–600 km orbital altitudes, the monitoring accuracy improves with the increase of the number of satellites, but this gain is significantly attenuated when the number of satellites increases to approximately 1,000; (2) the monitoring accuracy decreases with the increase of the altitude; and (3) the method presented in this study exhibits robustness under different space weather conditions. Furthermore, a differentiated satellite usage strategy (High Tightness and Low Looseness, HTLL) is developed to reduce the number of satellites used. Preliminary results indicate that the use of the HTLL method can reduce the number of satellites used from 2,592 to 1,044, whereas the maximum average relative deviation (ARD) does not exceed 1 %. The methods presented in this study have potential for future applications in the development of thermosphere research and satellite tracking. [ABSTRACT FROM AUTHOR]

The response of NO emission at 5.3 μm in the thermosphere to the geomagnetic storm on 24 April 2023 is analyzed using TIMED/SABER observations and TIEGCM simulations. Both the observations and the simulations indicate a significant enhancement in NO emission during the storm. Observations show two peaks around 50°S/N in the altitude–latitude distribution of NO emission and its relative variation. Additionally, the peak emission and enhancement are stronger on the nightside compared with the dayside. The peak altitude in the Northern Hemisphere is approximately 2–10 km higher than in the Southern Hemisphere; meanwhile, the peak altitude on the dayside is approximately 2–8 km higher than that on the nightside. Simulations reveal three peaks around 50°S, the equator, and 65°N, with peak altitudes at higher latitudes being slightly lower than those observed. In general, the altitude–latitude distribution structure of the relative variation in simulated NO emission matches observations, with two peaks around 50°S/N. TIEGCM simulations suggest that the increase in NO density and temperature during a geomagnetic storm can lead to an increase in NO emission at most altitudes and latitudes. Furthermore, the significant enhancement around 50°S/N is mainly attributed to the changes in NO density. [ABSTRACT FROM AUTHOR]

Storm-time ionospheric electrodynamics effects have been the subject of extensive studies. The solar wind/magnetosphere/ionosphere and thermosphere disturbance wind dynamos have long been identified as the main drivers of low latitude storm-time electrodynamics. Extensive detailed studies showed that climatology of low latitude disturbance electric fields and currents is in good agreement with results from global theoretical and numerical models. Over the last decade, however, numerous studies have highlighted that the response of low latitude electrodynamics to enhanced geomagnetic activity is significantly more complex than previously considered. It is now clear that the electrodynamic disturbance processes are affected by a larger number of solar wind and magnetospheric parameters and that they also have more significant spatial dependence. This is especially pronounced during and after large geomagnetic storms when multiple simultaneous disturbance processes are also active. In this work, we briefly review the main past experimental and modeling studies of low latitude disturbance electric fields, highlight new results, discuss outstanding questions, and present suggestions for future studies. [ABSTRACT FROM AUTHOR]

Both composition and temperature play a crucial role in determining the NO radiative cooling in lower thermosphere as observed by TIMED/SABER. In this work, we present a detailed investigation of seasonal variation in thermospheric NO radiative cooling. We have carried forward the investigation of Li et al. (2018) regarding the variations in local nighttime peak NO radiative cooling and its altitude during solar maximum and solar minimum conditions. By analyzing latitudinal changes over quiet times for each month in year 2018, it is evident that both the investigated parameters exhibit summer-winter variability. The qualitative contribution of different species (i.e., NO, and O), and temperatures in determining the vertical profile of NO radiative cooling for different latitudes is investigated by utilizing the NRLMSISE-00 estimated parameters, and SNOE observed NO density. The temperature, NO density, and compositional variations due to asymmetrical solar heating and associated summer-to-winter circulation in both the hemispheres during solar minimum conditions seem to be the dominating factor in controlling the NO radiative cooling during different seasons. The altitudes at which maximum cooling by NO occurs exhibits an inverse correlation with the amount of radiative cooling. The region of enhanced NO densities (polar and summer hemispheric low-mid latitude regions) have larger NO radiative cooling with lower peak altitudes in comparison to other regions (equatorial to winter hemispheric low-mid latitude regions), where NO radiative cooling is low with higher peak altitude values. [ABSTRACT FROM AUTHOR]

The response of the thermosphere ionosphere system to an X1.3 class solar flare is studied using observations of the total electron content (TEC) and the Global Ionosphere Thermosphere Model (GITM) simulations. The solar flare erupted from the active region AR12975 on 30 March 2022. Owing to the absence of accompanying severe geomagnetic activity, it was possible to isolate the effects of the flare on the upper atmosphere. TEC data are processed for Continental USA (CONUS), employing filtering and binning techniques to create 2D variation maps. The spectral content of the TEC variations is analyzed using a wavelet coherence method. The immediate response of the solar flare exhibited broad similarities, while notable differences were observed during the recovery period between the East and West sides of the CONUS. GITM is used to explore the East–West asymmetry of the key T‐I parameters. Simulation results reveal that the coinciding interplanetary magnetic field southward turning had a greater influence on these parameters compared to the solar flare, while their nonlinear interaction introduced complex variations. Additional investigation reveals gravity wave damping also contributes to the asymmetric solar flare response. Plain Language Summary: This study examines how a solar flare, which erupted on 30 March 2022, affected the Earth's upper atmosphere. We used satellite data to measure changes in the amount of charged particles in the atmosphere above the United States (specifically the area known as CONUS) and ran computer simulations to better understand these changes. We found that while the immediate reaction of the atmosphere to the flare was similar across the CONUS, differences emerged between the East and West during the recovery phase. The simulations showed that these differences were influenced by local time differences between the East and West, as well as by variations in the Earth's magnetic field geometry above these regions. Additionally, the study identified that the damping of gravity waves also played a role in these regional differences. Overall, the research highlights how complex interactions between various factors can lead to different responses to solar events in different parts of the CONUS. Key Points: Spectral content and temporal variation of total electron content perturbations associated with an X1.3 Solar Flare are analyzedThe thermosphere‐ionosphere exhibits a longitudinal flare response during the recovery phaseThe nonlinear interaction of the solar flare with the southward turning of the interplanetary magnetic field Bz ${\mathrm{B}}_{z}$ introduces significant variability [ABSTRACT FROM AUTHOR]

The Iterative Driver Estimation and Assimilation (IDEA) data assimilation technique was used with the Whole Atmosphere Model (WAM) to improve neutral density specification in the upper thermosphere. Two different neutral density data sources were examined to enhance the capability of simulating the global thermospheric state. The first were accelerometer estimates of neutral density from the Challenging Mini‐Satellite Payload (CHAMP) satellite. The second were neutral density estimates from the Global Ultraviolet Imager (GUVI) limb‐scan airglow observations aboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. Due to the intensity of the November 2003 storm, two changes were necessary in WAM. The first was allowing the Kp geomagnetic index to exceed 9 and the second was changing the relationship between Kp and the solar wind parameters used to drive the model. With these changes, results show that IDEA effectively captures the thermospheric neutral density at the CHAMP satellite altitude and follows the time‐dependence through the November 2003 storm period. Furthermore, a cross‐comparison was conducted with the GUVI dayside limb scan measurements. GUVI neutral densities within 270–320 km show the closest agreement with WAM when CHAMP data was assimilated by IDEA. We speculate on the potential for observations from GUVI at 300 km to be used as a data source in the IDEA‐WAM simulations. These simulations demonstrate the utility of the IDEA data assimilation technique with physical models and that using either accelerometer observations or ultraviolet airglow limb measurement during extreme storm periods could be used. Plain Language Summary: Strong variations in the neutral density of the thermosphere heavily impact the position of low Earth orbit objects as they respond to radiative inputs from the Sun in the extreme ultraviolet (UV) wavelength range, energetic particle precipitation in the auroral regions, and global‐scale electrical currents generated during geomagnetic storms. In this study, the Iterative Driver Estimation and Assimilation (IDEA) data assimilation technique was used with the Whole Atmosphere Model (WAM) to improve neutral density specification in the upper thermosphere. The Challenging Mini‐Satellite Payload (CHAMP) satellite and the Global Ultraviolet Imager (GUVI) limb scan UV airglow observations aboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite were examined to enhance the capability of simulating the global thermospheric state. Results show that IDEA effectively captures the thermospheric neutral density at the CHAMP satellite altitude throughout the November 2003 storm period. Furthermore, GUVI limb scan measurements on the dayside were cross‐compared with IDEA densities. We speculate on the potential for observations from GUVI during the daytime to be used as a data source in the IDEA‐WAM simulations. This study demonstrates data assimilative physical models are capable of improving neutral density specification for better orbit determination and prediction of low Earth orbiting satellites. Key Points: Iterative Driver Estimation and Assimilation (IDEA) data assimilation system greatly improves the Whole Atmosphere Model (WAM) thermospheric neutral density nowcasting capabilityDifferent locations of observed and modeled high‐latitude density hole structures lead to discrepancy between Challenging Mini‐Satellite Payload and the model outputIDEA output is validated against Global Ultraviolet Imager data, indicating a potential use case for limb‐scan measurements in the WAM neutral density forecast [ABSTRACT FROM AUTHOR]

The effects of Martian dust storms are not only limited to lower atmospheric regime, but the increased dust storm activity could also affect the vertical structure of the constituents in the thermosphere. To this end, this paper investigates the changes in the vertical mixing of neutral and ionic species densities in the thermosphere before and during a regional (2016) and a global (2018) dust storm event; using Neutral Gas and Ion Mass Spectrometer (NGIMS)/ Mars Atmosphere and Volatile Evolution (MAVEN) observations. Care has been taken to keep a restricted solar zenith angle variation (25º) to avoid the effects of changes in solar illumination on the distribution of thermospheric species densities. Contrasting differences in the vertical distribution of neutral (CO2, CO, O, N2, Ar, He) and ionic (CO2+, O+, O2+, N2+, CO+, Ar+) atmospheric species before and during the regional and global dust storm events are noticed. [ABSTRACT FROM AUTHOR]

Based on TIMED/SABER temperature observations and the Real‐Time Multivariate Madden‐Julian Oscillation (MJO) Index, for the first time, we discussed the daily resolution non‐migrating DE3 temperature tide response to the MJO phenomenon during MJO phases between ±45° latitude from 2003 to 2012 in the Mesosphere and Lower Thermosphere (MLT) region. We found: (a) The DE3 tide significantly depends on different MJO phases in the four seasons above 100 km altitude. The DE3 tidal phase exhibits varying responses to the different MJO phases. (b) The DE3 tide response to the MJO phenomenon is stronger at the solstices than at the equinoxes, and the response at the June solstices is stronger. (c) The symmetric component of the DE3 response to the MJO phenomenon is stronger than the asymmetric component. In summary, the daily resolved DE3 tide response to the MJO phenomenon, especially the DE3 tidal phase dominates certain seasonal characteristics and symmetry in the MLT region. Plain Language Summary: The MJO phenomenon is one of the dominant modes of intra‐seasonal variability in the troposphere in the tropical regions, and some seasonal atmospheric characteristics of the MLT region thought to be generated from the tropospheric MJO phenomenon. Based on TIMED/SABER temperature observations and the RMM Index of the troposphere, we quantified the daily resolution MLT tide response to the MJO phenomenon for the first time. We discuss the 30‐90‐day period DE3 tide response to the MJO phenomenon during phases 1–8 in different seasons between ±45° latitudes from 2003 to 2012 at the altitude of 70–108 km. We found: (a) The DE3 tide, including its amplitude and phase, shows a significant dependence on the MJO phase in the four seasons above 100 km altitude. (b) The DE3 tide affected by the MJO phenomenon are stronger at the solstices than at the equinoxes, and the June solstices exhibits the strongest MJO response. Moreover, there are some differences between the northern and southern hemispheres. (c) The symmetric component of the DE3 tide response to the MJO phenomenon is stronger than the asymmetric component in the four seasons. In summary, the daily resolved DE3 tide response to the MJO phenomenon, especially during the different MJO phase, dominates certain seasonal characteristics and symmetry in the MLT region. Key Points: The daily resolved DE3 temperature tidal component response to the Madden‐Julian Oscillation (MJO) phenomenon is presented in the Mesosphere and Lower Thermosphere regionThe DE3 tidal component response to the MJO phenomenon during different MJO phases dominates certain seasonal characteristicsThe symmetric component of the DE3 tide response to the MJO phenomenon is stronger than the asymmetric component [ABSTRACT FROM AUTHOR]

Effects of Hurricane Grace in August 2021 are studied in the thermosphere and ionosphere, using data from the COSMIC‐2, ICON, and GOLD satellites. Significant impacts on electron density, thermospheric winds, and temperature are observed after the onset of the hurricane, compared to the pre‐hurricane phase. Comparison of the observations during the hurricane with the ones during a non‐hurricane year clearly provides further evidence for substantial hurricane‐induced thermospheric and ionospheric changes. We reveal an enhancement in electron density during the hurricane's rapid intensification and pronounced changes in thermospheric winds. Additionally, the low‐latitude thermosphere exhibits considerable warming of up to 70 K around 150 km during this period. These changes highlight the long‐range vertical coupling mechanisms between hurricanes and the upper atmosphere, and provide valuable insights into the profound influence of meteorological events on upper atmospheric dynamics, emphasizing the need for further exploration. Plain Language Summary: Our research investigates how hurricanes, specifically Hurricane Grace in 2021, affect Earth's upper atmosphere. The hurricane was able to influence layers of the atmosphere that are critical for satellite communication and GPS. We used data from different satellites to study changes in the number of free electrons per unit volume of air (electron density), neutral winds, and temperature. We found that during the intensification of the hurricane, electron density increased in the ionosphere, which is the layer of Earth's atmosphere containing a high concentration of ions and free electrons. We also observed substantial changes in the winds high up in the atmosphere and found that a region of the upper atmosphere around 150 km warmed significantly. This research helps us understand how weather events on the Earth's surface can impact conditions hundreds of kilometers above the surface in Earth's atmosphere. Key Points: 2021 Hurricane Grace's impact on electron density, thermospheric winds, and temperature are analyzed using COSMIC‐2, ICON, and GOLDIonospheric electron density showed notable variations, predominantly increasing with the hurricane's intensificationSignificant thermospheric warming and circulation changes are observed [ABSTRACT FROM AUTHOR]

The moderate geomagnetic storm on 1 December 2023 is interesting that the aurora is unexpectedly observed in Beijing. During this storm, the behaviors of the equatorial electrojet (EEJ) might deserve to be explored. When the polarization of the daytime electric field changes westward, the direction of EEJ turns westward as well, termed the counter electrojet (CEJ). Using the total electron content from Global Navigation Satellite System, observed EEJ from Tatuoca station and low‐orbit Swarm satellites, and the corresponding simulations from the Thermosphere‐Ionosphere Electrodynamic General Circulation Model, the polarization oscillations of EEJ responses (ΔEEJ) during the geomagnetic storm on 1 December 2023 are explored. A significant polarity oscillation is established in the Swarm‐observed ΔEEJ at 9.6 and 12.8 local time (LT). A similar oscillation is also found at 15 LT. The strong polarity oscillations of ΔEEJ are dominated by the electric field, with ignorable effects from the ionospheric Cowling conductivity. The correlation coefficient between ΔEEJ and electric field changes (ΔE) is larger than 0.87, confirming the roles of the electric field. In the pre‐noon sector, the quasi‐oscillations of ΔEEJ are controlled by both the neutral winds and the prompt penetration electric field (PPEF). At noon, ΔEEJ is primarily driven by the PPEF, with minor effects from neutral winds. At afternoon, ΔEEJ is attributed to the PPEF, and the effects of neutral winds are ignorable. Key Points: A strong polarity oscillation of ΔEEJ is established at 9.6, 12.8, and 15 local time (LT)The polarity oscillations in ΔEEJ are dominated by the electric field, with a correlation coefficient larger than 0.87The effects from both prompt penetration electric field and neutral winds have LT dependences [ABSTRACT FROM AUTHOR]

Global Ionosphere Thermosphere Model (GITM) results have been compared with measurements from Global‐scale observations of the limb and disk (GOLD) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER). For the first time, GOLD‐derived exospheric temperature and column‐integrated O/N2 ${\mathrm{O}/\mathrm{N}}_{2}$ ratio measurements have been used to validate GITM model results. We examine two geomagnetic storm events for which we drive GITM with space weather conditions to understand how well the model reproduces the thermospheric responses to geomagnetic activity. In this paper, a recently developed auroral model, the Feature Tracking of Aurora (FTA) model, has been employed to calculate auroral electron precipitation in GITM (GITM w/FTA), and results are compared with the OVATION prime (OP) driven GITM model (GITM w/OP). GITM w/FTA simulated temperature, neutral density, and nitric oxide (NO) density are generally higher compared to the GITM w/OP model. During the geomagnetic storm, the GITM model and GOLD‐derived exospheric temperature agree between ∼0° ${\sim} 0{}^{\circ}$ and 5° $5{}^{\circ}$N latitude in the equatorial region. GOLD measurements show strong O/N2 ${\mathrm{O}/\mathrm{N}}_{2}$ peaks on either side of the equator during the geomagnetic storm period, which is also observed in our model results. The NO cooling peaks estimated by GITM models are ∼ ${\sim} $20 km lower than SABER observations during geomagnetic storms. GITM w/FTA better matches SABER observations than GITM w/OP except at low latitudes during storm time. Our model‐model/data comparisons show that improved auroral models are needed to better capture the thermospheric variations during geomagnetic storms. Key Points: Global Ionosphere Thermosphere Model was run with the OVATION Prime and the Feature Tracking of Aurora models, and results compared with satellite observations of exospheric temperature, O/N2 ${\mathrm{O}/\mathrm{N}}_{2}$, and NO emissionOur model results show strong O/N2 ${\mathrm{O}/\mathrm{N}}_{2}$ peaks at ±30° $\pm 30{}^{\circ}$ latitudes during storm time, which is consistent with Global‐scale observations of the limb and disk observationsGITM w/FTA model reproduces the SABER NO emission more accurately at high latitudes during geomagnetic storms [ABSTRACT FROM AUTHOR]

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 E×B $\mathbf{E}\times \mathbf{B}$ 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. Plain Language Summary: 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. Key Points: Pre‐midnight topside plasma bubbles are inhibited over eastern South America during the recovery phase of a geomagnetic stormZonal wind measurements show evidence of wind‐driven electric fields inhibiting plasma bubble formationAlthough theoretical predictions also anticipate changes in the meridional wind, they are not observed [ABSTRACT FROM AUTHOR]

Using observations from dual‐spacecraft (dual‐SC, Swarm A and C) and simulations from the Thermosphere‐Ionosphere Electrodynamics General Circulation Model (TIEGCM), this work investigates the ionospheric radial current (IRC) in response to subauroral polarization streams (SAPS) during the geomagnetic storm on 23 April 2023. At noon, a radially inward disturbance IRC (ΔIRC) emerges in response to SAPS, leading to a further intensification of inward IRC. The model simulation indicates that the dynamo current prevails over the polarization current, serving as the primary driver of the inward ΔIRC. Additionally, the significantly enhanced Pedersen conductivity potentially amplifies the inward ΔIRC. At dusk, ΔIRC exhibit a slight outward trend before 21 UT and a pronounced inward trend around 21–24 UT. This temporal variation is attributed to the equatorward propagation of SAPS‐induced thermospheric winds within 3–4 hr. The noontime inward electric field at the dip equator arises from both equatorward winds at low‐mid latitudes and local eastward winds. However, at dusk, the inward polarization electric field primarily stems from local disturbed eastward winds. Key Points: SAPS induce an inward ΔIRC at noon and triggers a slight outward ΔIRC followed by a substantial inward ΔIRC at duskThe interplay between neutral wind dynamo current and polarization current determines the direction of ΔIRCThe local eastward wind contributes to the inward electric field around noon and dusk [ABSTRACT FROM AUTHOR]

A comprehensive modeling study has been conducted to investigate space weather effects at Mars during the 10 September 2017 solar flare, utilizing an integrated framework that combines the global magnetohydrodynamic (MHD) model and Mars Global Ionosphere‐Thermosphere Model (MGITM). This is the first time the thermosphere‐ionosphere‐magnetosphere system is self‐consistently simulated under realistic, time‐varying conditions. Our simulations align well with observations from the Mars Atmosphere and Volatile EvolutioN (MAVEN). Recognizing that complexities due to highly disturbed upstream conditions and rotating crustal fields obscure solar flare effects in orbit‐to‐orbit comparisons, we perform controlled simulations of nonflare and flare cases and exploit their contrast to quantify spatiotemporal variations in flare impact. Our results highlight pronounced and rapid dayside ionospheric perturbations, contrasting with weaker and delayed nightside responses. Notably, in the topside ionosphere, O2+ ${\mathrm{O}}_{2}^{+}$ and CO2+ ${\mathrm{O}}_{2}^{+}$ densities increase primarily on the dayside below ∼ ${\sim} $300 km altitude, peaking with an increase of 20%–30%. The O+ ${\mathrm{O}}^{+}$ density shows a more significant increase of up to ∼ ${\sim} $50%, extending into the magnetosphere and nightside via plasma transport, increasing its total loss rate by 14%. We observe distinct altitude‐dependent patterns in dayside electron density enhancements in percent, characterized by a weakening with altitude and a rapid decay below ∼ ${\sim} $150 km in line with the flare development, and a gradual intensification between ∼ ${\sim} $150–300 km due to plasma transport and flare‐induced atmospheric upwelling. Earlier Mars Global Surveyor observations were limited to the low‐altitude pattern due to atmospheric expansion and missed the higher altitude variations observed by MAVEN. Plain Language Summary: This study investigates the impact of a powerful X8.2‐class solar flare on 10 September 2017 at Mars. Using an integrated modeling framework that combines two state‐of‐the‐art global models, our simulations provide insights into distinct altitude‐dependent patterns of flare‐induced ionospheric density enhancements, pronounced day‐night asymmetry, and a moderate increase in ion escape. Key findings include rapid variations in ionospheric density enhancements at lower altitudes, with a gradual intensification at higher altitudes due to plasma transport and atmospheric upwelling. These findings contribute to a better understanding of how solar flares impact the space environment of unmagnetized or weakly‐magnetized planets like Mars. Key Points: The solar flare causes pronounced and rapid dayside ionospheric perturbations and weaker and delayed effects on the nightsideO2+ and CO2+ densities increase by 20%–30% above 150 km on the dayside, while O+ increases by up to 50% and over larger spacePercentage increase of dayside e− densities decreases with altitude and decays fast below ∼150 km, and rises and decays slowly above that [ABSTRACT FROM AUTHOR]

The Chilean Observation Network De MeteOr Radars (CONDOR) commenced deployment in June 2019 and became fully operational in February 2020. It is a multi-static meteor radar system consisting of three ~1º latitudinally separated stations. The main (central) station is located at the Andes Lidar Observatory (ALO, 30.25º S, 70.74º W) and is used for both transmission and reception. The two remote sites are located to the north and south and are used for reception only. The southern station is located at the Southern Cross Observatory (SCO, 31.20º S, 71.00º W) and the northern station is located at Las Campanas Observatory (LCO, 29.02º S, 70.69º W). The successful deployment and maintenance of CONDOR provide 24/7 measurements of horizontal winds in the mesosphere and lower thermosphere (MLT), and permit the retrieval of spatially resolved horizontal winds, vertical winds, and temperatures. This is possible because of the high meteor detection rates. Over 30,000 quality controlled underdense meteor echoes are detected at the ALO each day and in total ~88,000 events are detected each day over the three sites. In this paper, we present the system configuration of the CONDOR and discuss the validation and initial results of its data products. The motivations of deploying the CONDOR system also include the combination of results with other co-located ground-based instruments at the ALO, which provide uniquely cross-validated and cross-scale observations of the MLT dynamics with multiple scientific goals. [ABSTRACT FROM AUTHOR]

Atmospheric migrating diurnal tide (DW1) is one of the prominent variabilities in the mesosphere and lower thermosphere (MLT). The existence of the solar cycle dependence of DW1 is debated, and there exist different and even opposite findings at different latitudes. In this paper, the solar cycle dependence of temperature DW1 in the equatorial mesosphere and lower thermosphere (MLT) is investigated using temperature global observations from TIMED/SABER spanning 22 years (2002–2023). The results show that (a) the solar cycle dependence of temperature DW1 is seen very clearly at the equator. The maximum correlation coefficient between DW1 and the F10.7 index occurs at 87km, with 0.72; the second maximum coefficient occurs at 99 km, with 0.62. The coefficient could reach 0.87 at 87 km and 0.67 at 99 km after dropping the years influenced by the Stratosphere Quasi-biennial oscillation (SQBO) disruption event. (b) DW1 shows a lag response to the solar cycle at the equator. DW1 amplitudes show a 1-year lag to the F10.7 index at 87 km and a 2-year lag to the F10.7 index at 99 km. [ABSTRACT FROM AUTHOR]

Our solar system's path has recently been shown to potentially intersect dense interstellar clouds 2 and 7 million years ago: the Local Lynx of Cold Cloud and the edge of the Local Bubble. These clouds compressed the heliosphere, directly exposing Earth to the interstellar medium. Previous studies that examined climate effects of these encounters argued for an induced ice age due to the formation of global noctilucent clouds (NLCs). Here, we revisit such studies with a modern 2D atmospheric chemistry model using parameters of global heliospheric magnetohydrodynamic models as input. We show that NLCs remain confined to polar latitudes and short seasonal lifetimes during these dense cloud crossings lasting ∼105 years. Polar mesospheric ozone becomes significantly depleted, but the total ozone column broadly increases. Furthermore, we show that the densest NLCs lessen the amount of sunlight reaching the surface instantaneously by up to 7% while halving outgoing longwave radiation. Plain Language Summary: As the Solar System moves through the interstellar medium, it encounters different astrophysical environments. By tracing back the path of the Sun, two possible crossings of dense interstellar clouds 2 and 7 million years ago have been identified. These clouds are dense enough to compress the solar wind to inside of Earth's orbit, exposing Earth's atmosphere to interstellar gas. Previous studies that explored terrestrial climate changes due to these event argued for a global cooling effect that could trigger an ice age. In this study, we revisit this topic with a modern computational atmospheric chemistry model. We find that high‐altitude water significantly enhances the density and coverage of noctilucent clouds (NLCs) near the mesopause. In contrast with previous studies, this effect is neither permanent nor global, though some denser NLCs may still block up to 7% of sunlight from reaching Earth's surface. Furthermore, HOx compounds greatly deplete mesospheric ozone. We find for the first time that this mesospheric ozone decrease allows for a stratospheric ozone increase, resulting in an increase in the total ozone column. In order to assess the complete global climate response to these events, a more complete 3D model is required. Key Points: As a result of colliding with interstellar clouds 2 and 7 million years ago, Earth's upper atmosphere received an abundance of hydrogenBy converting interstellar hydrogen to water in the lower thermosphere, thick noctilucent clouds would have formedHOx compounds could have depleted mesospheric ozone by up to 99%, though the total ozone column generally increases [ABSTRACT FROM AUTHOR]

​​​​​​​At the altitudes of the mesosphere–lower thermosphere, OH and HO 2 play a significant role in many physicochemical processes. Thus, monitoring their spatiotemporal evolution, together with other chemically active trace gases, is one of the most important problems for this atmosphere region, in which direct measurements are difficult. This paper studies the nighttime OH and HO 2 chemical equilibria using the 3D chemical transport modeling within the general approach, which includes the identification of the main sources and sinks in the equilibrium space–time areas and the derivation of analytical criteria for equilibrium validity. The presented analysis shows that there are extended areas where nighttime HO 2 and OH are close to their local equilibrium concentrations, determined mainly by the reaction between HOx and Ox components among themselves and with H 2 O 2 , N, NO, NO 2 , and CO. In the upper mesosphere–lower thermosphere, the equilibrium expressions can be shortened so that they include the HOx – Ox chemistry only. These expressions describe the HO 2 and OH equilibria from the top down to some boundaries, the altitude positions of which vary in the interval between 72–73 and 85 km and depend essentially on season and latitude. The developed analytical criteria reproduce the main features of these boundaries well almost everywhere. Due to weak sensitivity to uncertainties of reaction rates and other parameters, the criteria can be regarded as a robust instrument for HO 2 and OH equilibrium validation. The obtained results allow us to extend previously proposed methods for the retrieval of poorly measured components from measurement data and to develop new approaches. [ABSTRACT FROM AUTHOR]

We performed a series of numerical experiments to study the changes in atmospheric and ionospheric parameters caused by the propagation and dissipation of atmospheric waves from the meteorological source in the troposphere under various geomagnetic conditions. The simulation was carried out using the high-resolution regional atmospheric model AtmoSym and the Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP). Acoustic and internal gravity waves from a meteorological source were included in the large-scale model by additional source to the heat balance equation without using parameterization. A multi-model investigation showed that the propagation of waves from the troposphere leads to: a local heating of the thermosphere and a decrease in n[O]/n[N 2 ] ratio over the wave generation region; the formation of a dipole-like spatial structure of TEC disturbances with positive and negative values in the vicinity of the source of atmospheric waves. It is shown that the most important mechanism determining the dipole-like spatial structure of TEC disturbances under the influence of atmospheric waves is disturbances of the meridional component of the thermospheric wind. Thermospheric disturbances associated with geomagnetic activity have insignificant effect on the TEC response to the propagation and dissipation of atmospheric waves from the meteorological source in the troposphere. [ABSTRACT FROM AUTHOR]

Based on two meteor radars in mid‐latitudes of China, the mid‐latitude lower thermospheric neutral wind responses to the 2015 St. Patrick's Day great storm are investigated. The AE and PCN indices presented the similar quasi‐5‐hour oscillations during the storm. Interestingly, the analogous and close‐correlated storm‐time quasi‐5‐hour oscillations were also observed in both the meridional wind differences at 90–102 km derived from meteor radars. The meridional wind disturbances in the lower thermosphere also showed the extension toward the lower latitudes. It has been found that the enhanced equatorward wind disturbances at 250 km estimated by the Horizontal Wind Model‐14 and Fabry‐Perot Interferometer (FPI) emerged accordingly with the increases of AE and PCN with a time delay. And the enhancements of equatorward (poleward) wind disturbances at 250 km were accompanied by the increments of equatorward (poleward) wind disturbances at 94 km with a time lag of a few hours. It is thus suggested that the multiple intensified Joule heating events with quasi‐5‐hour time intervals were triggered by the successive substorm expansions during the storm. Then the Joule heating events led to the vertical wind and temperature disturbances in the mid‐latitude lower thermosphere via disturbing the thermospheric meridional circulation, which consequently induced the quasi‐5‐hour meridional wind disturbances therein. Plain Language Summary: The neutral temperature and global circulation in the middle and upper atmosphere can be strongly disturbed by the solar and geomagnetic activity. However, the effects of geomagnetic activity on the neutral winds in the mesosphere and lower thermosphere region at middle latitudes have not been fully understood. Using the neutral wind measurements from two meteor radars located at latitudes of 53.5°N and 40.3°N around 120°E meridian in China, we found that the meridional winds at different latitudes showed the prominent and similar disturbance patterns during the 2015 St. Patrick's Day geomagnetic storm. There exhibited the obvious quasi‐5‐hour oscillations in the meridional wind differences at 90–102 km derived from meteor radars. And such wind oscillations appeared to be analogous and close‐correlated to the quasi‐5‐hour oscillations in the AE and PCN indices. The possible contributors and physical processes responsible for such wind disturbances in the mid‐latitude lower thermosphere are discussed. This study is helpful to understand the influences of geomagnetic activity on the neutral winds in the mid‐latitude lower thermosphere region as well as the possible dynamic processes therein. Key Points: Storm‐time quasi‐5‐hour oscillations emerged in both the AE/PCN indices and the meridional winds in the mid‐latitude lower thermosphereThe increments of equatorward/poleward winds at 94 km exhibited the close correspondence and correlation to those at 250 km with a time lagThe successive Joule heating events led to the periodic meridional wind disturbances via disturbing thermospheric meridional circulation [ABSTRACT FROM AUTHOR]

A negative ionospheric storm occurred over the North American sector on 4 November 2021. By the integration of hemispheric power (HP), Ionospheric Connection Explorer (ICON), Global‐scale Observations of the Limb and Disk and global navigation satellite system total electron content (TEC) observations, a complete observation chain is obtained for the first time. At 07:11 UT, prominent energy was input into high latitudes. From 11:51 to 15:49 UT, strong southwestward neutral wind was observed by ICON over the south of North America, followed by significant ∑O/N2 depletion (60%) and temperature enhancement (∼300 K at ∼150 km). TEC depletion started at 12:00 UT in the northeastern North America and expanded to most of the United States. The TEC reduction showed the latitudinally tilted structure similar to that of ∑O/N2 depletion and temperature enhancement, which was shaped by the southwestward neutral wind. The thermosphere‐ionosphere‐electrodynamics general circulation model (TIEGCM) reproduced the observations qualitatively and was utilized to investigate the details of the negative storm generation and evolution processes. Strong equatorward wind and ∑O/N2 depletion reached the North America at midnight, much earlier than the TEC depletion, which began at sunrise. At 90°W, the time delay between TEC and ∑O/N2 depletions increased from ∼3.3 hr at 30°N to ∼11 hr at 60°N. Several factors contributed to the time delay including the time difference between midnight and sunrise, ∑O/N2 transport direction, westward wind and the variation of solar elevation angle with latitude. Plain Language Summary: During geomagnetic storms, the increase/decrease of ionospheric plasma density during geomagnetic storms relative to geomagnetic calm period is called positive/negative ionospheric storms. Due to the lack of observations of thermospheric properties, there are very few studies on negative storms over middle latitudes using direct neutral wind and temperature observations. This paper investigates the negative storm over the North American sector on 4 November 2021. A complete observation chain of the negative storm generating processes is presented by using hemispherical power index, neutral wind, neutral temperature and composition, and total electron content (TEC) measurements. Strong southward and westward neutral wind were observed by Ionospheric Connection Explorer satellite over middle and low latitudes. Global‐scale Observations of the Limb and Disk satellite also recorded prominent thermospheric composition changes and temperature enhancement. Subsequently, TEC depletion occurred was consistent with that of neutral temperature and composition variations. A numerical model is employed to uncover the details of the processes. Thermospheric composition changes occurred between 40° and 60°N firstly at 06:00 UT in the North America sector while TEC depletion appeared at 12:00 UT. There were four factors contributed to the time lag between TEC and composition changes including the time difference between midnight and sunrise, composition transport, westward wind and solar elevation angle. Key Points: During a negative storm, strong southwestward neutral winds and prominent ∑O/N2 depletion were observed by Ionospheric Connection Explorer and Global‐scale Observations of the Limb and Disk, respectivelyA complete observation chain of the negative ionospheric storm generation at middle latitudes is first established for the first timeThe time delay between TEC and ∑O/N2 depletions increased from 3.3 hr at 30°N to ∼11 hr at 60°N [ABSTRACT FROM AUTHOR]

Increasing carbon dioxide concentrations in the mesosphere and lower thermosphere are increasing radiative cooling in the upper atmosphere, leading to thermospheric contraction and decreased neutral mass densities at fixed altitudes. Previous studies of the historic neutral density trend have shown a dependence upon solar activity, with larger F10.7 values resulting in lower neutral density reductions. To investigate the impact on the future thermosphere, the Whole Atmosphere Community Climate Model with ionosphere and thermosphere extension has been used to simulate the thermosphere under increasing carbon dioxide concentrations and varying solar activity conditions. These neutral density reductions have then been mapped onto the Shared Socioeconomic Pathways published by the Intergovernmental Panel on Climate Change. The neutral density reductions can also be used as a scaling factor, allowing commonly used empirical models to account for CO2 trends. Under the "best case" SSP1‐2.6 scenario, neutral densities reductions at 400 km altitude peak (when CO2 = 474 ppm) at a reduction of 13%–30% (under high and low solar activity respectively) compared to the year 2000. Higher CO2 concentrations lead to greater density reductions, with the largest modeled concentration of 890 ppm resulting in a 50%–77% reduction at 400 km, under high and low solar activity respectively. Plain Language Summary: Carbon dioxide (CO2) concentrations are increasing throughout the atmosphere, not just at ground level. While this results in global warming in the lower atmosphere, the much less dense upper atmosphere does not trap the radiated heat, resulting in cooling of the upper atmosphere. As the upper atmosphere cools, it contracts, reducing the atmospheric density at a fixed altitude. Satellites traveling in low Earth orbit, such as the International Space Station at 400 km altitude, experience atmospheric drag, slowly reducing their altitude until they "re‐enter" and burn up in the lower, denser atmosphere. Reducing neutral densities will increase satellite orbital lifetimes as they experience less drag. The upper atmosphere has been simulated under increasing CO2 concentrations and solar activity conditions. This has also been linked to potential future CO2 concentration scenarios. Scaling factors have been created allowing simpler, faster models to account for CO2 density reductions. Under a best‐case scenario (SSP1‐2.6) where CO2 concentrations peak in around the year 2065 and then decline, densities at 400 km are 13%–30% lower compared to the year 2000 at the CO2 peak concentration, and then recover as CO2 reduces. However, densities continue to reduce if CO2 concentrations keep rising. Key Points: Whole Atmosphere Community Climate Model with ionosphere and thermosphere extension has been used to model future thermospheric density reductions under increasing carbon dioxide concentrations and solar activityThe reductions in density have been mapped onto the Shared Socioeconomic Pathways to show future scenarios while accounting for solar cyclesDensities at 400 km are 13%–30% lower under high and low solar activity respectively in the SSP1‐2.6 scenario when CO2 peaks at 474 ppm [ABSTRACT FROM AUTHOR]

In this study, we investigate the impacts of the 17 March 2013 strong geomagnetic storm on the wholly coupled Solar Wind‐Magnetosphere‐Ionosphere‐Thermosphere system‐of‐systems. Obtained from multipoint observations, our new results show (1) the solar‐wind Alfven waves propagating antisunward in the sheath region and (2) oscillating solar wind interplanetary magnetic field (IMF) and electric (E) field (IEF EY) that powered (3) rigorous dayside and nightside flux transfer events (FTEs) when (4) the nightside‐reconnection‐related short circuiting led to fast‐time Subauroral Ion Drifts (SAID) and Subauroral Polarization Streams (SAPS) E field development across the inner‐magnetosphere plasmapause where the solar‐wind Alfven waves (4) transitioned into kinetic Alfven waves (5) fueling the hot zone. Also, the antisunward solar‐wind Alfven waves (6) drove enhanced large‐scale region‐1 field‐aligned currents creating (7) undershielding conditions (8) allowing the dawn‐to‐dusk convection E field's earthward penetration, and (9) generated increased solar‐wind kinetic energy, which became deposited (10) to the ionosphere increasing the ionospheric electron temperature (by the downward flowing suprathermal electron fluxes) and (11) to the thermosphere oscillating the neutral winds and increasing the neutral temperature, and finally leading to (12) the development of bright stable auroral red (SAR) arcs in (13) the enhanced SAID/SAPS flow channels (FCs) developed during FTEs, (14) demonstrated with FC‐2 and FC‐3 events, in the enhanced polar convection that (15) the Rice Convection Model could reproduce. Finally, we conclude the antisunward‐propagating large‐amplitude solar‐wind Alfven waves' ultimate significant role in creating the favorable conditions for the various phenomena documented with the new observational results (1–14). Plain Language Summary: We investigate the wholly coupled Solar Wind‐Magnetosphere‐Ionosphere‐Thermosphere (SW‐M‐I‐T) system‐of‐systems operational during the 17 March 2013 strong geomagnetic storm. Results show the development of two significant phenomena in the coupled SW‐M system: (a) the region 1 field‐aligned currents and (b) the antisunward solar‐wind Alfven waves. While (a) created undershielding conditions allowing the dawn‐to‐dusk electric field (Edd) earthward propagation, (b) drove (B field, E field, wind) oscillations and generated kinetic energy (up to ∼400 TW) that became deposited (up to ∼5 TW) into the magnetosphere efficiently (up to ∼2.5%). In the coupled M‐I system, the kinetic energy drove reconnection events while the Edd convected the plasma in a two‐cell pattern. During nightside reconnection, the earthward injected ring current (RC) protons led to the development of the hot zone and conjugate large‐scale E fields near/across the plasmapause, while the downward heat flux channeled into the ionosphere elevated the electron temperature (Te). In the coupled I‐T system, the elevated Te increased the thermospheric neutral temperature (Tn) and both (Te and Tn) ignited the stable auroral red arc while the thermospheric neutral winds became oscillatory because of the antisunward solar‐wind Alfven waves that acted as the ultimate driver of the wholly coupled SW‐M‐I‐T system‐of‐systems operational. Key Points: Solar‐wind Alfven waves led to SAID/SAPS development via reconnection and transitioned into kinetic Alfven waves fueling the hot zoneEnhanced R1 FACs led to undershielding permitting the dawn‐to‐dusk convection E field's earthward penetration (to L ≈ 6 RE) on the nightsideOscillating thermospheric neutral winds and neutral temperature increase led to the development of bright SAR arcs [ABSTRACT FROM AUTHOR]

We analyze the gravity waves (GWs) from the ground to the thermosphere during 11–14 January 2016 using the nudged HI Altitude Mechanistic general Circulation Model. We find that the entrance, core and exit regions of the polar vortex jet are important for generating primary GWs and amplifying GWs from below. These primary GWs dissipate in the upper stratosphere/lower mesosphere and deposit momentum there; the atmosphere responds by generating secondary GWs. This process is repeated, resulting in medium to large‐scale higher‐order, thermospheric GWs. We find that the amplitudes of the secondary/higher‐order GWs from sources below the polar vortex jet are exponentially magnified. The higher‐order, thermospheric GWs have concentric ring, arc‐like and planar structures, and spread out latitudinally to 10 − 90°N. Those GWs with the largest amplitudes propagate against the background wind. Some of the higher‐order GWs generated over Europe propagate over the Arctic region then southward over the US to ∼15–20°N daily at ∼14 − 24 UT (∼9 − 16 LT) due to the favorable background wind. These GWs have horizontal wavelengths λH ∼ 200 − 2,200 km, horizontal phase speeds cH ∼ 165 − 260 m/s, and periods τr ∼ 0.3 − 2.4 hr. Such GWs could be misidentified as being generated by auroral activity. The large‐scale, higher‐order GWs are generated in the lower thermosphere and propagate southwestward daily across the northern mid‐thermosphere at ∼8–16 LT with λH ∼ 3,000 km and cH ∼ 650 m/s. We compare the simulated GWs with those observed by AIRS, VIIRS/DNB, lidar and meteor radars and find reasonable to good agreement. Thus the polar vortex jet is important for facilitating the global generation of medium to large‐scale, higher‐order thermospheric GWs via multi‐step vertical coupling. Plain Language Summary: Gravity waves (GWs) are perturbations in the Earth's atmosphere created by various processes. When a GW breaks, it imparts momentum to the atmosphere, which in turn can become unbalanced and generate secondary GWs. The same process happens at higher altitudes where the secondary GWs break, thereby generating higher‐order GWs. We simulate the primary, secondary and higher‐order GWs on 11–14 January 2016 using a GW‐resolving whole atmosphere model. We find that the entrance, core and exit regions of the jet that encircles the polar vortex generate primary GWs. In addition, the polar vortex jet magnifies the amplitudes of higher‐order GWs from sources below the jet, such as mountain waves. The resulting higher‐order GWs have horizontal wavelengths of hundreds to thousands of km with concentric ring, arc‐like and planar structures in the thermosphere. We compare the simulated GWs with AIRS, VIIRS/DNB, lidar, and meteor radar observations and find reasonable to good agreement. Key Points: The vertical shear of the horizontal wind in the entrance, core and exit regions of the polar vortex jet generate GWs in the stratosphereHigher‐order GWs generated over Europe and Asia propagate over the Arctic then southward over the CONUS at ∼9 − 16 LT due to favorable windsLarge‐scale GWs with λH ∼ 3,000 km and cH ∼ 650 m/s propagate southwestward across the northern thermosphere at ∼8–16 LT [ABSTRACT FROM AUTHOR]

The climate of the upper atmosphere, including the mesosphere, thermosphere and ionosphere, is changing. As data records are much more limited than in the lower atmosphere and solar variability becomes increasingly dominant at higher altitudes, accurate trend detection and attribution is not straightforward. Nonetheless, observations reliably indicate that, on average, the mesosphere has been cooling, the density in the thermosphere has been decreasing, and ionospheric layers have been shifting down. These global mean changes can be largely attributed to the increase in CO 2 concentration, which causes cooling and thermal contraction in the middle and upper atmosphere. The decline in thermosphere density is particularly relevant from a practical viewpoint, as this reduces atmospheric drag and thereby increases orbital lifetimes and the build-up of space debris. Long-term changes in the ionosphere can have further practical implications and are not only driven by the increase in CO 2 concentration, but also by changes in the Earth's magnetic field. The empirical models that are mostly used to inform applications in industry on the state of the upper atmosphere, as well as being widely used in science, do not yet properly account for long-term trends in the mesosphere, thermosphere and ionosphere. This is problematic when long-term future projections are needed or models rely strongly on older data. This review provides an overview of the main evidence of long-term trends observed in the mesosphere, thermosphere and ionosphere, together with the latest insights on what causes these trends. It is hoped that this may serve as a starting point to include long-term trends in (semi-) empirical models to benefit all users of these models. We also offer some thoughts on how this could be approached. [ABSTRACT FROM AUTHOR]

In this study, we continue a series of works devoted to modeling and studying the sensitivity of global atmospheric dynamic processes to variations in solar emissions during the 11-year solar activity (SA) cycle. We focus on studying the response of meridional atmospheric circulation in the middle atmosphere to changes in thermosphere characteristics with changes in SA. For this purpose, numerical simulations of the general atmospheric circulation were carried out using the nonlinear numerical circulation model of the middle and upper atmosphere MUAM. The main mechanism of the influence of thermospheric disturbances on the underlying layers is assumed to be a change in the conditions of propagation and reflection of planetary waves (PWs) due to changes in SA. Changes in temperature, zonal and meridional wind are shown to localize along waveguides, demonstrating the significant role of PWs in transmitting thermospheric disturbances, caused by changes in SA, to the middle atmosphere. The magnitude of changes in meridional circulation can reach 10 % in the northern stratosphere between SA maxima and minima.

After days of intense solar activity, active region AR3664 launched seven CMEs toward Earth producing an extreme G5 geomagnetic storm commencing at 17:05 UT on 10 May 2024. The storm impacted power grids, disrupted precision navigational systems used by farming equipment, and generated aurora seen around the globe. The storm produced remarkable effects on composition, temperature, and dynamics in the Earth's thermosphere that were observed by NASA's Global‐scale Observations of the Limb and Disk (GOLD) mission and are reported here for the first time. We use synoptic disk images of ΣO/N2 and neutral temperature (at ∼160 km) measured by GOLD to directly link dynamics resulting from the storm with dramatic changes in thermospheric composition and temperature. We observe a heretofore unseen spatial morphology simultaneously in ΣO/N2, neutral temperature, and total electron content. Equator‐to‐pole temperature differences reach 400 K with high latitude peak neutral temperatures near 160 km exceeding 1400 K. Plain Language Summary: On Saturday 10 May 2024, the sun launched a wave of energized plasma toward the Earth. A large disturbance in the Earth's magnetic field associated with the solar wind resulted in an extreme geomagnetic storm. The storm impacted power grids, disrupted navigational systems used by farming equipment, and produced aurora seen around the globe. The storm produced remarkable effects in the Earth's upper atmosphere that were observed by NASA's Global‐scale Observations of the Limb and Disk (GOLD) mission. In this letter, we use images measured by GOLD to directly link atmospheric dynamics resulting from the May 10–12 superstorm with dramatic changes in composition, temperature, and global circulation in the Earth's upper atmosphere. We observe previously unseen structure in the upper atmosphere associated with equator‐to‐pole temperature differences exceeding 400 K. Peak neutral temperatures near 160 km exceed 1400 K at high latitudes. Key Points: GOLD disk images of ΣO/N2 and neutral temperature link storm time dynamics with changes in thermospheric composition and temperatureWe observe a previously unseen spatial morphology in ΣO/N2, neutral temperature, and total electron contentPeak equator‐to‐pole temperature differences exceed 400 K but relax to pre‐storm conditions well before ΣO/N2 [ABSTRACT FROM AUTHOR]

Different meteor radars at low latitudes observed abnormally strong westward mesospheric winds around the March Equinox of 2023, that is, during the first phase of the Mesospheric Semiannual Oscillation. This event was the strongest of at least the last decade (2014–2023). The westward winds reached −80 m/s at 82 km of altitude in late March, and decreased with increasing altitude and latitude. A considerable increase in the diurnal tide amplitude was also observed. The Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension constrained to meteorological reanalysis up to ∼50 km does not capture the observed low‐latitude behavior. Additionally, these strong mesospheric winds developed during the westerly phase of the Quasi‐Biennial Oscillation, in accordance with the filtering mechanism of gravity waves in the stratosphere proposed in previous works. Finally, analysis of SABER temperatures strongly suggests that the breaking of the migrating diurnal tide may be the main driver of these strong winds. Plain Language Summary: Around the March Equinox of 2023, abnormally strong westward winds were observed in the low latitude region at altitudes between 80 and 100 km. This event was the strongest in at least the last decade. The westward winds reached a maximum amplitude of 80 m/s at 82 km of altitude during late March, and decreased with increasing altitude and latitude. A considerable increase in the amplitude of the diurnal tide was also observed. Simulations based on a whole atmosphere global circulation model constrained to meteorological reanalysis up to ∼50 km do not capture the observed behavior. Results based on specular meteor radar and satellite measurements suggest that the strong westward winds were driven by two main factors: the filtering mechanism of eastward‐propagating gravity waves in the stratosphere and the breaking of the diurnal tide at about 85 km of altitude. Key Points: Strong mesospheric westward winds during March equinox 2023 are observed globally at low latitudesThe westward winds reached a peak of −80 m/s at 82 km, the largest in the last ten years, accompanied by an enhancement of the diurnal tideThe breaking of the DW1 plays a role in generating these strong winds, in addition to the filtering of gravity waves in the stratosphere [ABSTRACT FROM AUTHOR]

Earth experienced the strongest geomagnetic storm in 20 years over 10–13 May 2024. The Ap and Dst geomagnetic indices were 273 and −291.94 nT on 11 May. The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics satellite observed significant enhancement in thermospheric infrared emission at 15, 5.3, 4.3, and 1.27 μm. On 11 May the daily global power radiated by nitric oxide (NO) at 5.3 μm was 1.41 TW and by carbon dioxide (CO2) at 15 μm was 1.35 TW. These are the largest single day power values observed by SABER in 22 years and the first time the daily power radiated by NO exceeded that of CO2. The total infrared power (above background) radiated during the storm was 2.64 TW (2.28 × 1017 J). Significant enhancement in limb radiance observed at 4.3 μm (to 250 km tangent height) is likely indicative of NO + formation during the storm. Plain Language Summary: The layer of Earth's atmosphere above 100 km is referred to as the "thermosphere" and can be thought of as the boundary between the space environment above it and the atmosphere below it. In early May 2024, an active sunspot ejected a significant amount of charged particles (protons and electrons) within a strong magnetic field toward Earth. This "coronal mass ejection" or CME encountered Earth's high atmosphere 10–13 May, causing a significant "geomagnetic storm" that significantly heated the thermosphere and modified its chemical composition. The effects of this storm were observed by the SABER instrument which is flying on the NASA Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics satellite. Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observes infrared light emitted by carbon dioxide, nitric oxide, molecular oxygen, and ionized nitric oxide produced during the storm. The 10–13 May storm caused the global average infrared radiation levels to increase by nearly a factor of 10 at some wavelengths in 1 day. The storm and the infrared radiation levels dissipated back to pre‐storm levels over the next 2 days. This storm was the third strongest in terms the power (2.64 trillion watts) radiated by the thermosphere in the past 22 years observed by SABER. The observed enhancements in infrared radiation at multiple wavelengths offer the opportunity to more fully understand how the Sun interacts with and influences Earth's highest atmosphere. Key Points: The daily Ap (273 nT) and Dst (−291.94 nT) indexes on 11 May 2024 were the largest since the launch of the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics (TIMED) satellite in December 2001The power radiated 11 May by NO and CO2 exceeded 1 TW each and the NO power (1.41 TW) exceeded the CO2 power (1.35 TW) for the first timeTotal storm radiated power, above background, is 2.64 TW (2.28 × 1017 J), making it the third strongest storm during the TIMED era [ABSTRACT FROM AUTHOR]

The long‐standing "energy crisis" at the giant planets refers to the anomalous heating of planetary thermospheres compared to the available energy from solar irradiance. The coupling between planetary magnetospheres and their upper atmospheres is thought to address these crises, though the sources and pathways of energy transport have not been fully explored at each system. In particular, the total available energy from the upstream solar wind at each planet has not been comprehensively quantified. Here we apply recently developed models of energy conversion by magnetic reconnection and the Kelvin‐Helmholtz instability to each of the Giant Planets, providing estimates of the average external energy inputs for each system between 1985 and 2020. We find that external energy associated with solar‐wind‐magnetospheric coupling significantly exceeds that from solar extreme ultraviolet photons. While internal energy sources are known to dominate at Jupiter and Saturn, external sources may be significant at Uranus and Neptune. Plain Language Summary: The upper atmospheres of Jupiter, Saturn, Uranus, and Neptune are all hotter than would be predicted by their exposure to sunlight alone. The space plasma environment around each planet provides an additional reservoir of energy that can be used to provide this heating. Here we quantify the average energy input to each planetary system due to variation of the photons and plasmas emitted from the Sun. We find that the interaction between a planetary magnetic field and the solar interplanetary magnetic field provides a strong source of external energy at each of the planets, on average significantly more than that of light from the Sun. Key Points: Solar wind‐magnetospheric‐coupling provides higher energy input than solar extreme ultraviolet at the giant planetsSolar activity drives solar‐wind‐magnetospheric coupling much more strongly than planetary seasonThe solar wind may drive the energy input to the upper atmospheres at Uranus and Neptune [ABSTRACT FROM AUTHOR]

The equations governing the neutral wind velocities and temperature, which depict the oscillations of atmospheric tides, incorporate various elements of the atmosphere. These factors comprise background wind, temperature profiles, and the atmosphere's composition, including ozone, oxygen, carbon dioxide, water, etc. The current work describes the method of solution of the atmospheric tidal model and investigates the effects of background wind and dissipation processes on the migrating semidiurnal tides during the March equinox. Thermal heating due to water vapour absorption in the troposphere, ozone in the stratosphere and oxygen absorption in the thermosphere is considered in this model. Furthermore, the model takes into account the various dissipation processes (i.e. ion drag and Hall coefficients, Newtonian cooling, divergence of momentum and heat fluxes due to molecular and eddy diffusion) and background wind. The semidiurnal tidal features obtained from the present model match well with the Global Scale Wave Model (GSWM-98). The bar charts are presented to quantify the effects of different parameters on horizontal winds for altitudes below 100 km and above 100 km, separately. It reveals that the background wind plays a vital role in deciding the semidiurnal tidal amplitudes of zonal and meridional winds. Momentum and heat flux coefficients also have a significant influence on semidiurnal tides. At higher altitudes (>100 km), the ion drag reduces the amplitude of semidiurnal tides considerably. [ABSTRACT FROM AUTHOR]

The effect of a meteorological storm in October 2018 in the Baltic Sea on the state of the mesosphere and lower thermosphere is investigated. The wave activity of internal gravity waves from TIMED/SABER satellite data is analyzed, and the effects of the meteorological storm at heights of 80–100 km are determined. A method based on mode decomposition from SABER data is adapted to calculate the gravity wave potential energy density (GWPED) and to isolate the temperature perturbations caused by their propagation at lower thermospheric heights. Wavelet analysis of the temperature perturbations revealed two ranges of vertical wavelengths, 5–8 km and 14–18 km. In the area of a meteorological storm, the amplitude of internal gravity waves with vertical wavelengths of 5–8 km increases, and the area of their maximum expands and shifts upward to heights of ~90 km, while on meteorologically quiet days these waves are observed at heights of 65–70 km and with smaller amplitudes. Above the area of a meteorological storm at heights of 90–100 km, the values of the gravity wave potential energy density increase significantly compared to quiet days before and after the storm, and the spatial extent of the perturbation area increases. [ABSTRACT FROM AUTHOR]

The results of the "Terminator" space experiment on board the International Space Station are presented. Images of Earth's atmosphere are obtained in the near IR spectral range with the limb geometry of observations under a full moon. The calculated vertical profiles of volume emission/scattering rate point that the aerosol layer occurs within the height region of 80–100 km in Earth's atmosphere. It is proposed that this layer is meteoric in origin. Estimates show that the size spectrum of aerosol particles lies within the 1–100 nm range. [ABSTRACT FROM AUTHOR]

Low Earth orbit (LEO) radio occultation|radio occultations (RO) constellations can provide global electron density profiles (EDPs) to better specify and forecast the ionosphere‐thermosphere (I‐T) system. To inform future RO constellation design, this study uses comprehensive Observing System Simulation Experiments (OSSEs) to assess the ionospheric specification impact of assimilating synthetic EDPs into a coupled I‐T model. These OSSEs use 10 different sets of RO constellation configurations containing 6 or 12 LEO satellites with base orbit parameter combinations of 520 or 800 km altitude, and 24° or 72° inclination. The OSSEs are performed using the Ensemble Adjustment Kalman Filter implemented in the data assimilation (DA) Research Testbed and the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM). A different I‐T model is used for the nature run, the Whole Atmosphere Model‐Ionosphere Plasmasphere Electrodynamics (WAM‐IPE), to simulate the period of interest is the St. Patrick's Day storm on March 13–18, 2015. Errors from models and EDP retrieval are realistically accounted for in this study through distinct I‐T models and by retrieving synthetic EDPs through an extension Abel inversion algorithm. OSSE assessment, using multiple metrics, finds that greater EDP spatial coverage leading to improved specification at altitudes 300 km and above, with the 520 km altitude constellations performing best due to yielding the highest observation counts. A potential performance limit is suggested with two 6‐satellite constellations. Lastly, close examination of Abel inversion error impacts highlights major EDP limitations at altitudes below 200 km and dayside equatorial regions with large horizontal gradients and low electron density magnitudes. Plain Language Summary: The upper atmosphere, the region above 100 km altitude, is strongly influenced by space weather events that can negatively impact ground and space‐based technologies. These technologies include communication and navigation systems impacted by radio wave propagation through high altitudes plasma, a region called the ionosphere. Developing observing systems that provide global monitoring of the ionosphere is a critical need for understanding and forecasting space weather changes, such as radio occultations (RO) that provide plasma observations using global positioning radio signals. In this study, we evaluate these hypothetical RO observing systems in simulated experiments using data assimilation (DA), an approach that integrates synthetic observations into a physics‐based model. We find that increased observational coverage corresponds to better estimated plasma states, and that lower orbit altitude constellations yield higher observation counts. This study comprehensively incorporates model and observation errors to more realistically represent real‐world conditions. One limitation of RO data is highlighted in regions near the equator and at lower altitudes (below 250 km) where there is a breakdown in assumptions for observation retrieval. This study illustrates the clear operational benefits of these plasma observations, informing the future observing system design and aiding their use for space weather forecasting. Key Points: OSSE study assessing hypothetical radio occultation (RO) constellations, the first to comprehensively account for forecast model and Abel inversion errorsThe RO constellation with low‐ and high‐inclination orbits at 520 km altitude performs the best with the highest observation countsUncharacterized Abel inversion errors and poorly retrieved low plasma density limit assimilation impact on the equatorial ionosphere [ABSTRACT FROM AUTHOR]

In this work, we present a metastable helium lidar system for the measurements of metastable helium He(23S) density in the thermosphere and lower exosphere. This lidar system consists of a high power 1,083 nm pulsed laser, a 1 m aperture laser beam expander, six 1 m aperture receiving telescopes and a superconducting nanowire single‐photon detector (SNSPD). This system realizes metastable helium density detection up to 1,000 km. Daily rapid variation of metastable helium density within several hours was measured with a height resolution of 50 km for the first time. It demonstrates the capability of ground‐based lidar for continuous height‐resolved detection of the atmospheric metastable helium in the height range of 200–1,000 km. This is a promising tool to help study the coupling of neutral and ionized atmosphere in this height range and further providing observation basis for space weather prediction. Plain Language Summary: Many different kinds of lidar have been well developed for tens of years and can detect the atmosphere up to 180 km by different principles (such as aerosol scattering, Rayleigh scattering, resonance fluorescence scattering and so on). However, in the higher height range of 200–1,000 km, there are almost no suitable methods to monitoring the atmosphere continuously from the ground, except for one choice: metastable helium lidar. In this work, we built a powerful 1,083 nm pulsed laser and emit it vertically in to the atmosphere, the metastable helium atoms resonance with the laser and emits fluorescence photons. The scattered photons from the very high atmosphere are collected and focused by the large aperture telescope on the ground and detected by the photoelectric detector. Building such a lidar has many difficulties that have never been encountered by other conventional lidars, such as the large signal attenuation by long distance. This powerful lidar is built successfully in 2023 and make observations for several months. The first high quality initial data of the observed metastable helium density profile in 200–1,000 km is shown in this paper. This observation will continuously provide useful observation for the study of space physics and space weather forecasting. Key Points: A metastable helium lidar system for the measurements of metastable helium He(23S) density in 200–1,000 km has been developedDaily rapid variation of metastable helium density within several hours was measured with a height resolution of 50 km for the first timeThis is a promising tool to help providing observation basis for space weather prediction [ABSTRACT FROM AUTHOR]

Utilizing multistatic specular meteor radar (MSMR) observations, this study delves into global aspects of wind perturbations in the mesosphere and lower thermosphere (MLT) from the unprecedented 2022 eruption of the Hunga Tonga‐Hunga Ha'apai (HTHH) submarine volcano. The combination of MSMR observations from different viewing angles over South America and Europe, and the decomposition of the horizontal wind in components along and transversal to the HTHH eruption's epicenter direction allow an unambiguous detection and identification of MLT perturbations related to the eruption. The performance of this decomposition is evaluated using Whole Atmosphere Community Climate Model with thermosphere/ionosphere extension (WACCM‐X) simulations of the event. The approach shows that indeed the HTHH eruption signals are clearly identified, and other signals can be easily discarded. The winds in this decomposition display dominant Eastward soliton‐like perturbations observed as far as 25,000 km from HTHH, and propagating at 242 m/s. A weaker perturbation observed only over Europe propagates faster (but slower than 300 m/s) in the Westward direction. These results suggest that we might be observing the so‐called Pekeris mode, also consistent with the L1 pseudomode, reproduced by WACCM‐X simulations at MLT altitudes. They also rule out the previous hypothesis connecting the observations in South America to the Tsunami associated with the eruption because these perturbations are observed over Europe as well. Despite the progress, the L0 pseudomode in the MLT reproduced by WACCM‐X remains elusive to observations. Key Points: A decomposition of the horizontal wind in longitudinal and transverse components, allows clear identification of Hunga Tonga‐Hunga Ha'apai eruption signaturesEastward soliton‐like mesosphere and lower thermosphere horizontal wind perturbations are observed propagating at 242 m/s, up to at least 25,500 km from the epicenterWeaker westward propagating perturbations are observed at phase speeds in between the expected L0 and L1 speeds, but up to 17,000 km [ABSTRACT FROM AUTHOR]

Measurements of Ar and N2 densities at 160–250 km altitude from the Mars Atmosphere and Volatile Evolution (MAVEN) Neutral Gas and Ion Mass Spectrometer (NGIMS) during February 2015–February 2023 are analyzed to provide a comprehensive analysis of their diurnal‐ and zonal‐mean (DZM) structures, and ZM (solar‐synchronous) diurnal (DW1) and semidiurnal (SW2) tides. After applying a solar flux trend correction, multi‐year binning and averaging with respect to longitude, local solar time (LST), latitude and Ls at each height results in the first full global picture of these components of the ZM thermosphere for a single climatological Mars year. The following new observational insights into Mars thermosphere are obtained: The DZM N2 latitude versus Ls (latvsLs) structures contain a prominent latitudinally‐symmetric annual component (∼±25%–35%) due to the eccentricity of Mars orbit around the Sun, and an antisymmetric component (∼±30%–45%) below about 190 km that is seasonally‐symmetric and thus consistent with the tilt of Mars rotation axis. Aperiodic deviations from these symmetries increase with height and are tentatively attributed to dissipation of waves originating in the lower atmosphere. DW1 and SW2 maximize around 200–220 km altitude, suggesting existence of an unknown dissipation mechanism at higher altitudes. The DZM, DW1 and SW2 components of Ar generally exceed those of N2 by factors of 1.4–2.5. The scale heights of Ar and N2 between 205 and 245 km are also employed to derive DZM exosphere temperatures, which reflect aperiodic ∼±15K deviations from the annual‐mean in the latvsLs frame. Key Points: DZM of N2 and Ar densities (160–250 km) at Mars contain periodic latitude‐Ls variabilities due to Mars eccentric orbit and its axial tiltAperiodic latitude‐Ls variabilities in DZM N2 and Ar are potentially attributable to dissipation of waves from the lower atmosphereDZM exosphere temperatures derived from N2 and Ar scale heights (205–245 km) are aperiodic in the latitude‐Ls frame and are of order ±15K [ABSTRACT FROM AUTHOR]

The migrating diurnal tide (DW1) is derived by fitting Hough Mode Extensions to the TIMED/TIDI near‐global wind measurements within the mesosphere and lower thermosphere between 85 and 100 km from 2004 to 2014. The tidal amplitude peaks around the equinoxes with a large interannual variability of up to 50%. The correlation coefficients between the tidal amplitude variability and the solar cycle as represented by F10.7, stratospheric Quasi‐Biennial Oscillation (QBO), and El Niño‐Southern Oscillation (ENSO) are calculated every 10 days revealing seasonal dependencies. The interannual variability is positively correlated with QBO from spring to fall, maximizing around the equinoxes; anti‐correlated to the solar cycle in early winter; and anti‐correlated to ENSO in early winter and slightly in March. Multivariate linear regressions are performed to quantitatively analyze the linear relationships between the DW1 amplitude and those factors. The fittings perform best with the QBO at 30 and 50 hPa both being considered. The contribution of QBO peaks around January and October may be related to the polar vortex modulated by QBO in the northern and southern hemispheres, respectively. The correlation between the DW1 amplitude and ENSO is negative with time lags <∼5 months during early winter and spring. Key Points: Correlation coefficient is positive for Quasi‐Biennial Oscillation (QBO) from spring to fall; negative for solar cycle in early winter; and negative for El Niño‐Southern Oscillation in early winter and MarchQBO30 and QBO50 contribution peaks may be related to the modulated polar vortex in the northern and southern hemispheres, respectivelyThe DW1 amplitude and the Oceanic Niño Index are negatively correlated with time lags <∼5 months during early winter and spring [ABSTRACT FROM AUTHOR]

In this study, we use multi‐instrument observations (all‐sky imager (ASI), global navigation satellite system (GPS) receivers, digisonde) to study the interaction of nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) on 13 November 2018. The most attractive aspect of this event is that the interaction appeared between two dark bands both propagated southwestward. The airglow observations show that the latter band moved faster and caught up with the former, and these two bands merged into a new one. The propagating characteristics and morphology of the MSTIDs changed during the interaction process. The simulations from the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM) suggested that the ionospheric background zonal winds and electron density distributions could play essential roles in the interaction of the MSTIDs. Moreover, the merging process might be associated with the electrostatic reconnection. Plain Language Summary: This study shows an interesting medium‐scale traveling ionospheric disturbance (MSTID) event. Two MSTID bands encountered during the propagation, the latter dark band caught up with the former band, and then, the two bands merged into one. In contrast, these dark bands kept propagating southwestward. The Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM) simulations obtain the wind and electron density data, which provide a possible explanation that the changing of ionospheric zonal winds and ionospheric electron density might result in the interaction of these dark bands. Moreover, the merging process might be connected with the electrostatic reconnection, which could be influenced by E × B plasma flows in opposite directions. Key Points: Two bands propagated southwestward, the latter band moved faster and caught up with the former, they interacted and merged into oneThe chasing behavior showed the velocity differences, possibly related to the ionospheric zonal wind and electron density distributionThe merging process of the two bands might be connected with electrostatic reconnection [ABSTRACT FROM AUTHOR]

Upward propagating waves of lower atmospheric origin play an important role in coupling terrestrial weather with space weather on daily to inter‐annual timescales. Quantifying their short‐term (<30 days) variability is a difficult challenge because simultaneous observations at multiple local times are needed to sample diurnal cycles. This study demonstrates and validates a short‐term estimation method of the DE3 non‐migrating tide at the equator and then applies the technique to three independent data sets: MIGHTI, SABER, and TIDI. We find that daily DE3 estimates from SABER, MIGHTI, and TIDI at equator agree well with correlation coefficients ranging between 0.76 and 0.85. The daily DE3 amplitude variability is typically ∼7 m/s in zonal winds and ∼3 K in temperature. We also find that daily MLT variations and F‐region ionospheric DE3 from COSMIC‐2 Global Ionospheric Specification (GIS) show a correlation of 0.55–0.65, suggesting that not all ionospheric variability can be attributed to the E‐region dynamo; however, increasing correlation with increasing time‐scale suggests that lower atmospheric variability has pronounced impact on the ionosphere on intra‐seasonal scales. We find that the MLT and the F‐region ionosphere exhibit strong coherent intra‐seasonal oscillations (residual amplitudes upto 50%–60%); their coherency with the MJO in 2020 suggests a possible modulation of the upward propagating DE3 tide related to this major tropical tropospheric weather pattern. In addition, we find stratospheric QBO signatures in the MLT DE3 on inter‐annual scales. This study offers fresh observational insights into the pivotal role of tropospheric weather in shaping variability in the coupled thermosphere‐ionosphere system. Plain Language Summary: A growing body of research unequivocally demonstrates the important role upward propagating waves from the lower atmosphere play in shaping the meteorology of the mesosphere, thermosphere and ionosphere from daily to inter‐annual time scales. Understanding these variations is crucial for space weather studies and applications, including radio wave propagation, satellite communication, and space orbital debris. Among these oscillations, the diurnal eastward propagating tide with zonal wave number 3 (DE3) holds particular importance. This oscillation, driven by expansive tropospheric weather systems, can attain substantial amplitudes in the upper atmosphere. The primary objective of this study is to demonstrate and validate a methodology facilitating the short‐term estimation of DE3 to understand the tidal weather of the upper atmosphere. Our approach starts by using a physics‐based model of the upper atmosphere to test and validate this new daily DE3 retrieval method. Next, we apply our tidal estimation method to extensive satellite‐based measurements of winds, temperatures, and ionospheric electron density. Lastly, we analyze the results to provide fresh observational insights into the pivotal role of tropospheric weather on Earth's near space environment from daily to inter‐annual time scales. Key Points: Demonstrated and validated a short‐term DE3 tidal estimation technique at the equatorDaily DE3 tidal amplitude variability at the equator is typically ∼7 m/s in zonal winds and ∼3 K in temperatureStrong intra‐seasonal variations in MLT region DE3 and F‐region ionosphere electron densities possibly due to the MJO are observed [ABSTRACT FROM AUTHOR]

This study focuses on the poorly known effect of polar cap patches (PCPs) on the ion‐neutral coupling in the F‐region. The PCPs were identified by total electron content measurements from the Global Navigation Satellite System (GNSS) and the ionospheric parameters from the Defense Meteorological Satellite Program spacecraft. The EISCAT incoherent scatter radars on Svalbard and at Tromsø, Norway observed that PCPs entered the nightside auroral oval from the polar cap and became plasma blobs. The ionospheric convection further transported the plasma blobs to the duskside. Simultaneously, long‐lasting strong upper thermospheric winds were detected in the duskside auroral oval by a Fabry‐Perot Interferometer (FPI) at Tromsø and in the polar cap by the Gravity Recovery and Climate Experiment satellite. Using EISCAT ion velocities and plasma parameters as well as FPI winds, the ion drag acting on neutrals and the time constant for the ion drag could be estimated. Due to the arrival of PCPs/blobs and the accompanied increase in the F‐region electron densities, the ion drag is enhanced between about 220 and 500 km altitudes. At the F peak altitudes near 300 km, the median ion drag acceleration affecting neutrals more than doubled and the associated median e‐folding time decreased from 4.4 to 2 hr. The strong neutral wind was found to be driven primarily by the ion drag force due to large‐scale ionospheric convection. Our results provide a new insight into ionosphere‐thermosphere coupling in the presence of PCPs/blobs. Plain Language Summary: This study investigates how the evolution of the polar cap patches (PCPs) affects the upper layer of the Earth's atmosphere, termed the thermosphere. PCPs are dense patches of charged particles that move from the dayside to the nightside of the high‐latitude ionosphere through the polar cap region. Using the measurements by multiple ground‐based instruments and satellites, this study found that PCPs can enhance the formation of strong upper thermospheric winds. The winds are primarily driven by the ion drag force due to the interactions between charged particles and neutral gases. The results show that because of the arrival of PCPs, which increase the F‐region electron densities in the auroral oval, the ion drag acceleration acting on the neutrals can more than double and the related time constant of the ion drag can be halved. Key Points: Transportation of polar cap patches (PCPs) and their development in the nightside auroral oval was observed by multiple instrumentsVery strong, long‐lasting westward upper thermospheric wind in the duskside oval was associated with large‐scale ionospheric convectionHigh electron density produced by PCPs increases the ion drag force that drives the upper thermospheric wind [ABSTRACT FROM AUTHOR]

Twenty‐two years (2002–2023) of infrared radiative cooling rate data derived from the SABER instrument on the NASA TIMED satellite are presented. Global daily and global annual infrared power (Watts, W) emitted by nitric oxide (NO) and carbon dioxide (CO2) illustrate the variability of the geospace environment on timescales from days to decades. The 11‐year solar cycle (SC) is evident in the global power data and in vertical profiles of infrared cooling rates (nW/m3). The global annual power radiated by NO and CO2 are larger in 2023 than at any time since 2003 and 2002, respectively. The to‐date peak in NO infrared power in SC 25 is larger than in SC 24, is comparable to SC 20, but is less than in SCs 18–19 and 21–23. Two geomagnetic storms in 2023 radiated more than 1 TW and are in the top 10 strongest storms observed by SABER. Plain Language Summary: SABER is an instrument on the NASA TIMED satellite, launched in December 2001, and is still operating nominally in 2024. SABER measures the amount of infrared energy emitted by the nitric oxide (NO) molecule and the carbon dioxide (CO2) molecule from Earth's thermosphere, the region of atmosphere above 100 km altitude. These measurements are used to derive the amount of power (Watts) radiated globally by these two molecules on daily to annual timescales. The infrared power data is then analyzed to determine its variability in the lower thermosphere from daily to decadal time scales. Of particular interest is the influence of the well‐known 11‐year cycle of solar activity which is readily evident in the infrared power data. SABER also observes the effects of geomagnetic storms, identifying two strong storms in 2023 that each produced over 1 TW of infrared power. Key Points: SABER on TIMED has observed infrared radiation from the thermosphere since January 2002, for 22 years, the equivalent of two solar cyclesGlobal annual infrared power radiated by NO and CO2 are larger in 2023 than any time since 2003 and 2002, respectivelyTwo geomagnetic storms in 2023 exceeded 1 TW of radiated energy, above background, and are in the top 10 strongest storms observed [ABSTRACT FROM AUTHOR]

Time series of radiative cooling of the upper mesosphere and lower thermosphere (UMLT) by carbon dioxide (CO2) are examined for evidence of trends over 20 years. Radiative cooling rates in K day−1 provided by the SABER instrument are converted to time series of infrared power radiated from three distinct layers between 0.1 hPa and 0.0001 hPa (65–105 km). Linear regression against time and a predictor for solar variability provides estimates of the trend in exiting longwave radiation (ELR) from these layers. Trends in ELR are not significantly different from zero at 95% or 99% confidence in each layer. These results demonstrate energy conservation in the UMLT on decadal time scales and show that the UMLT continues to radiate the same amount of energy it receives despite cooling and contracting over two decades. These results are enabled by the long‐term stability of the SABER instrument calibration. Plain Language Summary: The Earth's upper mesosphere and lower thermosphere (UMLT) is the region between approximately 65 and 105 km in altitude. Infrared radiation emitted by CO2 is a fundamental component of the energy budget of the UMLT. Carbon dioxide (CO2) is increasing in this region. The amount of infrared energy emitted by CO2, over time, must balance the amount of energy from sunlight that is absorbed in the UMLT. Observations from orbiting satellites over the past two decades have shown that the temperature in the UMLT is decreasing due to the increasing CO2. A decreasing temperature implies a decrease in infrared energy emitted by an object. Energy conservation, however, requires that the amount of infrared energy radiated from the UMLT balance the amount of energy received, regardless of the temperature. In this paper we show that there is no significant long‐term change with time (i.e., zero trend) in the infrared energy emitted from the UMLT even though the UMLT temperature has been decreasing for the last 20 years (and longer) due to increasing CO2. This result confirms that energy is conserved in a cooling and contracting UMLT. Key Points: Time series of infrared power (W) radiated by CO2 from the upper mesosphere & lower thermosphere (UMLT) are developedTrends in radiated power are not different from zero at 95% or 99% confidence in the three layers examined between 0.1 and 0.0001 hPaIncreasing CO2 reduces the UMLT temperature while radiating the same amount of energy over time, thus conserving energy [ABSTRACT FROM AUTHOR]

Winds in the nighttime upper thermosphere are often observed to mimic the ionospheric plasma convection at polar latitudes, and whether the same is true for the daytime winds remains unclear. The dayside sector is subject to large temperature gradient set up by solar irradiance and it also contains the cusp, which is a hotspot of Poynting flux and a region with the strongest soft particle precipitation. We examine daytime winds using a Scanning Doppler Imager (SDI) located at the South Pole, and investigate their distribution under steadily positive and negative IMF By conditions. The results show that daytime winds exhibit significant differences from the plasma convection. Under negative IMF By conditions, winds flow in the same direction as the plasma zonally, but have a meridional component that is strongest in the auroral zone. As a result, winds are more poleward-directed than the plasma convection within the auroral zone, and more westward-directed in the polar cap. Under positive IMF By conditions, winds can flow zonally against the plasma in certain regions. For instance, they flow westward in the polar cap despite the eastward plasma convection there, forming a large angle relative to the plasma convection. The results indicate that ion drag may not be the most dominant force for daytime winds. Although the importance of various forcing terms cannot be resolved with the utilized dataset, we speculate that the pressure gradient force in the presence of cusp heating serves as one important contributor. [ABSTRACT FROM AUTHOR]