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• Studied mesospheric high-frequency gravity waves (GW) over Tirunelveli. • Unusual high-frequency GW activity in meridional wind is observed in June 2019. • Perturbation ellipse method indicates the GW direction in the north–south plane. • Anticorrelation of GW activity with OLR reveals tropospheric convection as a source. • Presence of dynamic instability is inferred at mesosphere indicating GW dissipation. The recent upgrade of Medium-Frequency (MF) wind radar at Tirunelveli (8.7⁰N, 77.8⁰E) has improved the height and time resolution of the wind measurements, which are utilized in the present study to examine high frequency (in periods 20–60 min) gravity waves (GW) in the mesosphere and lower thermosphere (MLT) region. We observe, in addition to the dominant wave activity during equinox months, dominant episodes of high-frequency GW activities in the meridional winds during the times 31 May–4 June and 25–27 June 2019. Using the perturbation ellipse method, we infer the direction of propagation of the GW and it is found to be in the north–south plane. The GW activity in the MLT region exhibits anti-correlation with NOAA outgoing longwave radiation (OLR) and positive correlation with rainfall rates indicating the latent heat release due to tropical convection as the possible source of the GW. Besides, the presence of dynamical instability is inferred from the calculations using the radar wind and the spaceborne SABER (Sounding of Atmosphere using Broadband Emission Radiometry) temperature data suggesting a possible causality of convectively generated GWs dissipation. [ABSTRACT FROM AUTHOR]

Joule heating and radiative cooling usually play key roles in high‐latitude thermospheric temperature changes during geomagnetic storms. In the mesosphere and lower thermosphere (MLT), however, the causes of storm‐time temperature changes at high latitudes are still elusive. Here, we elucidate the nature and mechanisms of MLT temperature variations at high latitudes during the 10 September 2005 storm by diagnostically analyzing the MLT thermodynamics in the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIMEGCM) simulations. In the storm's initial and main phases, the MLT temperature decreases at 0:00 local time (LT)−12:00 LT, but increases in the 12:00 LT–24:00 LT sector at high latitudes. Afterward, the temperature decrease disappears and temperature increase occurs at all local times in the high latitudes. Adiabatic heating/cooling and vertical advection associated with vertical winds are the main drivers of high‐latitude temperature changes in the entire altitude range of the MLT region. However, around the auroral oval and above ∼100 km, the Joule heating rate is comparable to the heating caused by vertical advection and adiabatic heating/cooling associated with vertical winds and becomes one of the major contributors to total heating in the high‐latitude MLT region. The effects of Joule heating can penetrate down to ∼95 km. Horizontal advection also plays a key role in storm‐time MLT temperature changes inside the polar cap and becomes larger than the adiabatic heating/cooling above ∼105 km. Plain Language Summary: In previous works, Joule heating and radiative cooling were proposed to be the most important processes to determine the temperature changes in the thermosphere during storms. However, we found that Joule heating is important near the auroral oval above 95 km. Joule heating changes the temperature and then the pressure gradient drives horizontal wind changes in the thermosphere. The divergent and convergent winds associated with horizontal wind changes cause vertical winds, which are transmitted to the MLT region. The MLT downward vertical winds bring warm air and upwelling winds bring cold air if the temperature profile increases with altitude. The vertical winds cause adiabatic heating and cooling by convergence and divergence. The two physical mechanisms dominate MLT total heating and then change MLT temperature. The MLT temperature increase and decrease can be observed at 12:00–24:00 and 0:00–12:00 LT, respectively. Of course, the effect of Joule heating is an important heating term near the auroral oval, but it is a minor contributor to adiabatic heating/cooling and vertical advection below 100 km. The horizontal advection has an additional effect on the temperature changes in the polar cap. The rest of the other physical processes can be negligible, including radiative cooling. Key Points: Mesosphere and lower thermosphere (MLT) temperature increases at 12:00–24:00 LT and decreases at 0:00–12:00 LT at high latitudes in the early phases of the stormAdiabatic heating/cooling and vertical advection are the dominant drivers of high‐latitude MLT temperature changesJoule heating and horizontal advection also play key roles in MLT temperature changes near the auroral oval and inside the polar cap [ABSTRACT FROM AUTHOR]

The mesospheric inversion layer (MIL) phenomenon is a temperature enhancement (10–50 K) in a vertical layer (∼10 km) lasting several days and spanning thousands of kilometers within the mesosphere. As MILs govern the mesospheric variability, their study is crucial for a better understanding of the middle‐atmosphere global circulation. MIL phenomenon is also important for applications in aeronautics as perturbations in the mesosphere are significant issues for the safe reentry of rockets, space shuttles, or missiles. However, the description of this phenomenon remains incomplete, since no observations of MIL's effects on winds exist, hampering an understanding of the mechanisms responsible for their formation. This study investigates simultaneous wind‐temperature observations in the altitude range of 30–90 km during MIL events. Strong winds deceleration occurred in the same altitude range as the temperature inversion, confirming the role of gravity waves in MIL's formation mechanisms. Plain Language Summary: Atmospheric waves propagate from the lower to upper layers, transferring their energy throughout the atmosphere. The mesosphere (50–90 km) is subject to these energy transfers, causing unexpected temperature increases (10–50 K) over a vertical layer (∼10 km). These deviations are called mesospheric inversion layers (MILs). Though largely observed in temperature profiles, the MIL phenomenon remains misunderstood, as MIL's impacts on the wind in the middle atmosphere remain unknown. In this study, we first reported simultaneous wind‐temperature observations between 30 and 90 km during MIL events. We observed a strong wind deceleration in the same altitude range where the temperature increases. This result argues in favor of the role of gravity waves in MIL's formation mechanisms. Key Points: First simultaneous wind and temperature observations in the altitude range 30–90 km during mesospheric inversion layer eventsAccording to these new observations, there is a strong wind deceleration occurring at the same altitude that the temperature inversionThese results argue in favor of the mesospheric inversion layer's formation mechanism involving gravity wave dissipation [ABSTRACT FROM AUTHOR]

This article reviews main progress in investigations of long-term trends in the mesosphere, thermosphere, and ionosphere over the period 2018–2022. Overall this progress may be considered significant. The research was most active in the area of trends in the mesosphere and lower thermosphere (MLT). Contradictions on CO 2 concentration trends in the MLT region have been solved; in the mesosphere trends do not differ statistically from trends near the surface. The results of temperature trends in the MLT region are generally consistent with older results but are developed and detailed further. Trends in temperatures might significantly vary with local time and height in the whole height range of 30–110 km. Observational data indicate different wind trends in the MLT region up to the sign of the trend in different geographic regions, which is supported by model simulations. Changes in semidiurnal tide were found to differ according to altitude and latitude. Water vapor concentration was found to be the main driver of positive trends in brightness and occurrence frequency of noctilucent clouds (NLCs), whereas cooling through mesospheric shrinking is responsible for a slight decrease in NLC heights. The research activity in the thermosphere was substantially lower. The negative trend of thermospheric density continues without any evidence of a clear dependence on solar activity, which results in an increasing concentration of dangerous space debris. Significant progress was reached in long-term trends in the E-region ionosphere, namely in foE (critical frequency of E region, corresponding to its maximum electron density). These trends were found to depend principally on local time up to their sign; this dependence is strong at European high midlatitudes but much less pronounced at European low midlatitudes. In the ionospheric F2 region very long data series (starting at 1947) of foF2 (critical frequency of F2 region, corresponding to the maximum electron density in the ionosphere) revealed very weak but statistically significant negative trends. First results of long-term trends were reported for the topside ionosphere electron densities (near 840 km), the equatorial plasma bubbles, and the polar mesospheric summer echoes. The most important driver of trends in the upper atmosphere is the increasing concentration of CO 2 , but other drivers also play a role. The most studied one was the effect of the secular change in the Earth's magnetic field. The results of extensive modeling reveal the dominance of secular magnetic change in trends in foF2 and its height (hmF2), total electron content, and electron temperature in the sector of about 50 ∘ S–20 ∘ N, 60 ∘ W–20 ∘ E. However, its effect is locally both positive and negative, so in the global average this effect is negligible. The first global simulation with WACCM-X (Whole Atmosphere Community Climate Model eXtended) for changes in temperature excited by anthropogenic trace gases simultaneously from the surface to the base of the exosphere provides results generally consistent with observational patterns of trends. Simulation of ionospheric trends over the whole Holocene (9455 BCE–2015) was reported for the first time. Various problems of long-term-trend calculations are also discussed. There are still various challenges in the further development of our understanding of long-term trends in the upper atmosphere. The key problem is the long-term trends in dynamics, particularly in activity of atmospheric waves, which affect all layers of the upper atmosphere. At present we only know that these trends might be regionally different, even opposite. [ABSTRACT FROM AUTHOR]

To better extract the tides represented in the European meteorological analysis ERA5, an analysis of the histograms of the diurnal and semi-diurnal modes as a function of longitudes was performed. This analysis revealed that modes with different characteristics appeared regionally along a single longitude. Retrieved migrating tides were compared with a tidal model showing global agreement below 60 km and twice the amplitude in meteorological analyses at mid-latitude. Non-migrating tidal modes have been identified along the tropical band. They logically appear above the convective zones, probably due to water vapor excess. Their characteristics are different from migrating components. This preliminary study has shown that it is necessary to develop additional observations allowing for more frequent sampling to retrieve migrating and non-migrating tides that can only be achieved with satellite constellations from space. [ABSTRACT FROM AUTHOR]

The results of temperature measurements in the lower thermosphere at altitudes of 90–130 km by the method of resonant scattering of radio waves on artificial periodic inhomogeneities (APIs) of the ionospheric plasma are presented. These inhomogeneities are created when the ionosphere is exposed to powerful HF radio emission. The temperature profile was obtained from measurements of the relaxation time of the API scattered signal. The data processes and the method of the temperature determination are given in detail. The height and temporal resolutions of the API technique are of the order of 1 km and 15 s, respectively, making it possible to study both fast and slow processes in the lower thermosphere. Large temperature variability at altitudes of 90–130 km during the day and from day to day, due to the propagation of atmospheric waves, has been confirmed. The temporal variations of the atmospheric parameters take place with periods from 15 min to some hours. There are often height profiles of the temperature with the wave-like variations and with the vertical scale of about 4–10 km. The irregular temperature profiles were observed above 100 km. [ABSTRACT FROM AUTHOR]

It has been found in experiments at the SURA mid-latitude heating facility that the modification of the ionospheric F2 layer by powerful HF radio waves gives rise to artificial injection of energetic electrons from the Earth's radiation belt into the atmosphere. The spectral, energy, and spatial characteristics of such an injection are presented in the paper. It is significant that the energetic electrons excite the atoms and molecules of the atmosphere to Rydberg energy levels, followed by the transition of the excited atoms and molecules to lower energy states, accompanied by the radiation of the microwave electromagnetic emissions. It has been shown that the artificial injection of energetic electrons can be considered as an independent powerful source of generation of secondary artificial turbulence, the effect of which manifests itself at ionospheric and mesospheric heights both near the heating facility and at a large distance from it up to a thousand or more kilometers. Examples of such generation are given. [ABSTRACT FROM AUTHOR]

The International Global Navigation Satellite Systems Service (IGS) ionospheric total electron content (TEC) data are used to study the periodic perturbation in the ionosphere during the 2019 Antarctic sudden stratospheric warming (SSW) event, a rare Southern Hemisphere minor SSW event in the last 40 years. A 14.5 day periodic signal with a zonal wavenumber of 0 is observed in the mesosphere and the lower thermosphere (MLT) region and the ionosphere during this SSW period, which could be related to the lunar tide. The 14.5 day periodic disturbance in the IGS TEC exhibits local time dependence and latitudinal variation, with the maximum amplitude appearing between 1000 and 1600 LT in the equatorial ionization anomaly (EIA) crest regions. Additionally, the 14.5 day periodic oscillation shows an obvious longitudinal variability, with the weakest amplitude appearing in the longitudinal region of 30° W–60° E. [ABSTRACT FROM AUTHOR]

Ground-based passive measurements of downwelling atmospheric radiation at ~110.836 GHz allow extracting the spectra of ozone self-radiation (rotational transition J = 61,5–60,6) coming from the low stratosphere–mesosphere and retrieving vertical profiles of ozone concentration at these altitudes. There is a notable (several hundred kHz) ambiguity in the determination of the resonance frequency of this important ozone line. We carried out long-term ground-based measurements of atmospheric microwave radiation in this range using upgraded apparatus with high technical accuracy and spectral resolution (~12 kHz). The obtained brightness temperature spectra allowed us to determine the frequency of this ozone line to be 110,835.909 ± 0.016 MHz. We verified that the Doppler frequency shift by horizontal wind as well as the variations of the tropospheric absorption had little effect on the obtained result. The found value was 131 ± 16 kHz less than that measured in the laboratory and differed from modern model calculations. At the same time, it was close to the results of early semiempirical calculations made more than 40 years ago. The applications where precise knowledge about the resonance frequency of this ozone line can be important were discussed in this paper. [ABSTRACT FROM AUTHOR]

There is currently limited research on the structure-property relationship of reduction stimuli-responsive polymeric crosslinked micelles using mesoscopic simulations. Herein, dissipative particle dynamics (DPD) simulations were used to simulate the self-assembly process of the blank non-crosslinked micelle, the structure and doxorubicin (DOX) distribution of diselenide crosslinked micelle with different crosslinker contents (CCs) based on the nearest-neighbor bonding principle. The results revealed that the formation of a three-layer spherical micelle and the loaded DOX mainly distributed in the polycaprolactone (PCL) core and hydroxyethyl methacrylate (HEMA) mesosphere. The larger the dosage of DOX, the more DOX encapsulated, but the encapsulation of DOX in the hydrophobic domain would reach saturation when the dosage increased to 6.0 %. In micelles with lower CCs or crosslinking levels (CLs), DOX entered the middle layer and the inner core faster. Then, based on the nearest media-bead bond breaking principle and subsequently DPD simulation, the effects of different CCs on the micelle structure and DOX release properties were investigated. Low CC could cause fast drug release. With the increase of CCs, the micelle showed a slower DOX release trend. The multilayer crosslinked network system also affected the DOX release rate. Hence, this work can provide some mesoscale guidance for the structural design and structure-property relationship of stimuli-responsive reversible crosslinked micelles for drug delivery. Entry for the Table of Contents [Display omitted]. • The structure-property relationship of diselenide crosslinked micelles was utilized mesoscopic simulations. • The larger the dosage of DOX, the more DOX encapsulated. • Both crosslinking content and crosslinking level could affect the desired stable structure and drug release performance. [ABSTRACT FROM AUTHOR]

We examine hemispheric asymmetry of the annual and semiannual variation of the ratio of O and N2 concentrations (O/N2) using observations by the Global Ultraviolet Imager (GUVI) instrument onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite and compare them with Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCM‐X) model simulations. We found that in the equatorial region, the "equinox peaks" of the observed O/N2 are near the end of March and October, and the two annual lows are near the beginning of July and January. Compared to the equatorial region, in the northern hemisphere (NH) low latitudes, the first "equinox peak" clearly shifts toward the December solstice, whereas in the southern hemisphere (SH) low latitudes, the "equinox peaks" shift toward the June solstice (JS), forming the hemispheric asymmetry characteristics of the annual and semiannual variation. Seasonal variation of O/N2 shows no apparent phase variation with altitude, and the annual and semiannual pattern is consistent from year to year. WACCM‐X reproduces the observed annual and semiannual pattern in NH but in SH, it simulates an annual variation instead of the observed annual and semiannual variation. The largest discrepancy occurs near JS in the lower and middle thermosphere: the simulated O density has an annual high near JS in SH; the simulated N2 density has an annual high near JS in NH but a predominant annual low near JS in SH. These are not in the GUVI data. A weaker thermospheric meridional circulation in the winter hemisphere, or a reduced summer‐to‐winter latitudinal gradient of neutral temperature in WACCM‐X simulations would make model‐data comparisons more consistent. Plain Language Summary: We examine the difference of seasonal variation of the ratio of thermosphere O and N2 (O/N2) between the two hemispheres using data observed from a satellite from 2003 to 2007 and compare them with numerical model simulations. We found that in the equatorial region, the two equinox peaks of the observed O/N2 are somewhere in March/April and October, respectively, and the two annual lows are near the beginning of July and January. Compared to the equatorial region, in the northern low latitudes, the first "equinox peak" clearly shifts toward the December solstice, whereas in the southern low latitudes, the two "equinox peaks" shift toward the June solstice (JS). The seasonal variation of O/N2 is consistent from year to year and at different altitudes. The model reproduces the observed seasonal variation in the northern hemisphere but in the southern hemisphere, it only produces an annual variation. A weaker thermospheric meridional circulation in the winter hemisphere, or a decrease of the model simulated temperature in the summer (northern) hemisphere near the JS would make the simulated O and N2 densities, and thus O/N2, more consistent with the observed ones. Key Points: "Equinox peaks" of Global Ultraviolet Imager (GUVI) O/N2 shift toward the December and June solstices in the northern and southern low latitudes, respectivelyWhole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCM‐X) reproduces GUVI observed annual and semiannual variation of O/N2 in northern hemisphere but misses semiannual variation in southern hemisphereMisrepresentation of gravity wave forcing in regular resolution WACCM‐X may be an important contributor to the discrepancy [ABSTRACT FROM AUTHOR]

Most experimental investigations on planetary-scale waves in the mesosphere and lower thermosphere (MLT) region are based on single-station or -satellite spectral analysis methods, which suffer from intrinsic spectral aliasing/ambiguity. To overcome the aliasing, the author has developed and utilized dual- and multi-station spectral methods in a series of recent works. These methods were implemented on meteor radar observations and surface magnetometer observations. In the implements, a variety of waves were discovered or investigated in terms of seasonal variations and responses to sudden stratospheric warming events, such as lunar and solar tides (migrating and non-migrating), Rossby wave normal modes, ultra-fast Kelvin waves, and secondary waves of wave–wave nonlinear interactions between the previous waves. The current paper illustrates these methods using synthetic data, comparatively reviews the methods and results in plain language, and proposes a new representation, termed the adjusted Feynman diagram, to summarize the nonlinear interactions and explain their implications. [ABSTRACT FROM AUTHOR]

In this study, the neutral wind observations from the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) instrument onboard Ionospheric CONnections (ICON) are used to investigate the longitudinal structure of zonal wind between 100 and 300 km during daytime. The four-peaked structure in the longitudinal distribution between June and August is visually clear in the whole altitudinal range. The longitudinal wavenumber 1–4 patterns (WN1–WN4) are extracted, and the altitude–month distributions of WN1–WN4 and their contributions to the longitudinal structure are compared. The amplitudes of WN3 and WN4 show seasonal dependence, and the amplitude of WN4 exhibits obvious vertical propagation from the mesosphere and lower thermosphere (MLT) to the upper thermosphere in summer and autumn. WN1 is an important contributor to the longitudinal structure, WN4 is the primary contributor in the lower altitude ranges in summer and autumn at three latitudes. The contributions of WN3 (WN1) increase holistically with latitude in summer (spring, autumn, and winter). And the main wave sources of WN1–WN4 are further investigated in the 100–106 km and 210–300 km altitude regions. The main wave sources of WN1 and WN2 have complex variations with altitude, latitude, and season, while WN3 (WN4) is clearly influenced by DE2 (DE3 and SE2). [ABSTRACT FROM AUTHOR]

This Opinion article begins with a review of important advances in the science of the Mesosphere and Lower Thermosphere (MLT) region of the atmosphere that have occurred over the past two decades since the founding of Atmospheric Chemistry and Physics. The emphasis is on chemistry (although, of course, this cannot be decoupled from discussion of atmospheric physics and dynamics), and the primary focus is on work during the past 10 years. Topics that are covered include: observations (satellite, rocket and ground-based techniques); the variability and connectedness of the MLT on various length- and time-scales; airglow emissions; the cosmic dust input and meteoric metal layers; and noctilucent (or polar mesospheric) ice clouds. The paper then concludes with a discussion of important unanswered questions and likely future directions for the field over the next decade. [ABSTRACT FROM AUTHOR]

As of now, direct measurements of middle and upper atmospheric wind are still scarce, and the observation method is limited, especially for the upper stratosphere and lower mesosphere. A new method to derive line-of-sight wind from to more than 120 km is proposed using a high-resolution Terahertz (THz) radiometer, which can fill the measurement between lidar and interferometer. Simulations from 0.1 THz to 5 THz for evaluating the feasibility of the spaceborne THz limb sounder are performed in this study. The results show that high-precision wind (better than 5m s-1) can be obtained from 40 to 70 km by covering a cluster of strong O3 lines. By choosing strong O2 or H2O lines, the high-quality measurement be extended to 105 km. The O atom (OI) lines can provide wind signals in the higher atmosphere. In addition, performance of different instrument parameters include resolution, bandwidth and measurement noise are analyzed and four different band combinations are suggested at last. [ABSTRACT FROM AUTHOR]

Sudden stratospheric warmings (SSWs) are large‐scale phenomena characterized by dramatic dynamic disruptions in the stratospheric winter polar regions. Previous studies, especially those employing whole atmosphere models, indicate that SSWs have strong impacts on the circulation of the mesosphere lower thermosphere (MLT) and drive a reversal in the mean meridional circulation (MMC) near 90–125 km altitude. However, the robustness of these effects and the roles of SSW‐induced changes in global‐scale wave activity to drive the reversal have been difficult to observe simultaneously. This work employs horizontal lower thermospheric (∼93–106 km altitude) winds near 10°S‐40°N latitude from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging instrument onboard the Ionospheric Connection Explorer (ICON) to present observational evidence of a prominent MLT MMC reversal associated with the January 2021 major SSW event and to demonstrate connections to semidiurnal tidal activity and possible associations with a ∼3‐day ultra‐fast Kevin wave. Plain Language Summary: The winds in the mesosphere lower thermosphere (MLT) are strongly impacted by dramatic changes in the stratospheric winter polar regions associated with Sudden stratospheric warmings (SSWs). Models have shown that the climatological direction of the MLT north‐south and vertical circulation, characterized by equatorward flow near ∼100–120 km and poleward flow near ∼80–100 km, reverses following the onset of SSWs. Yet, the impacts and causes of these dynamical effects are not well established observationally due to the lack of comprehensive global measurements of the MLT region. This study evaluates the evolution of MLT winds and associated tidal and ultra‐fast Kevin wave variations during the January 2021 SSW using horizontal wind observations from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging instrument onboard the Ionospheric Connection Explorer to present observational evidence of a large MLT north‐south wind reversal due to the SSW and associated global‐scale wave influences. Key Points: A prominent (∼30 m/s) reversal in the mesosphere lower thermosphere (MLT) mean meridional circulation during the January 2021 major sudden stratospheric warming (SSW) is observed in Michelson Interferometer for Global High‐resolution Thermospheric Imaging windsStrong (∼45 m/s) MLT westward flow enhancements and latitudinal (10S–40N) dependencies in the wind response occur as a result of the SSWAmplification in MLT SW2 and ultra‐fast Kevin wave1 zonal wind amplitudes are consistent with the observed westward flow enhancements [ABSTRACT FROM AUTHOR]

Previous studies have demonstrated that the diurnal variability of mesospheric water vapor involves a non‐linear interplay between dynamics, photochemistry, and catalytic processes. The inherent difficulty in measurement of winds and water vapor in this region makes the study of mesospheric water vapor complex. Here we study the sources of diurnal variability of mesospheric water vapor with the help of the specified dynamics configuration of the Whole Atmosphere Community Climate Model (WACCM) with thermosphere‐ionosphere eXtension (SD‐WACCM‐X) for typical equinox conditions during the solar minimum year 2009. Special emphasis is given to the advective transport by the migrating diurnal tide and its impact on the mean water vapor in the mesosphere. To isolate the contribution from advective and non‐advective processes, a Lagrangian parcel trajectory model is employed. Using this model, we incorporated the effect of both vertical and horizontal tidal advection on the diurnal variability of mesospheric water vapor. Our results indicate that the vertical tidal advection dominates the transport of water vapor in the upper mesosphere and that there is a significant contribution from horizontal tidal advection below 70 km. Among non‐advective processes, chemical loss of water vapor has the largest contribution. The Lagrangian parcel trajectory approach is further extended using the recently released version 2.07 of water vapor observations from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument and dynamical tides from the empirical Climatological Tidal Model for the Thermosphere (CTMT). Differences in SD‐WACCM‐X and CTMT + SABER results are discussed in light of the veracity of SABER water vapor measurements. Plain Language Summary: In the mesosphere, water vapor is found to have significant diurnal variations. Both transport and non‐transport‐related processes are responsible for the diurnal variations, and it is challenging to separate their individual contributions. We accomplish this isolation by employing a parcel trajectory approach on outputs from the specified dynamics configuration of the Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (SD‐WACCM‐X). We use the parcel trajectory technique to estimate the transport of water vapor induced by the migrating diurnal tide in the mesosphere both vertically and horizontally. We find that vertical transport dominates the variability of water vapor above 70 km. Additionally, we demonstrate and quantify the non‐negligible contribution from horizontal transport below 70 km. We apply the same method to satellite‐based tides and water vapor observations and analyze how the results differ from model simulations. Key Points: We investigate the contribution of transport and non‐transport‐related processes to the diurnal variability of mesospheric water vaporA Lagrangian parcel trajectory model is used to study the tidal advection contribution to water vapor variabilityAmong transport‐related processes, most of the diurnal variability is contributed by vertical tidal advection [ABSTRACT FROM AUTHOR]

The purpose of this article is to reopen from a practical perspective the question of the extent in altitude of Earth's biosphere. We make a number of different suggestions for how searches for biological material could be conducted in the mesosphere and lower thermosphere, colloquially referred to as the "ignore-osphere" because it has been generally ignored in the meteorological community compared to other regions. Relatively recent technological advances such as CubeSats in very low Earth orbit or more standard approaches such as the rocket-borne MAGIC meteoric smoke particle sampler are shown as potentially viable for sampling biological material in the ignore-osphere. The issue of contamination is discussed, and a potential solution to the problem is proposed by means of a new detector design that filters for particles based on their size and relative velocity to the detector. [ABSTRACT FROM AUTHOR]

Electrons are produced in the Earth's quiet nighttime mesosphere by ionization by cosmic rays and ionization of NO by Lyman- α  radiation. They are removed by attachment or recombination processes that are usually assumed in modelling to occur at the ambient temperature. However, the electrons have initial energies that are much higher than at thermal equilibrium, and so must have a range of energies as they progress towards equilibrium via interactions with atoms and molecules. As attachment and recombination rates are dependent on the electron energy, it is possible that modelling that considers the actual energy of the electrons will give different results to those based on assuming that the electrons are at the ambient temperature. In this work, starting with electrons at a higher initial energy, the detailed electron interactions (including elastic scattering and vibrational excitation of molecules) are tracked in a time-step simulation. This simulation is implemented by treating electrons in subranges of the electron energy spectrum as chemical species. This allows an investigation of two phenomena in the nighttime mesosphere: the origin of the D-region ledge and the production of radiative emissions from vibrationally excited molecules. It is found that there is negligible difference in the electron densities calculated using the ambient temperature or detailed interaction models, so this study does not provide an explanation for the D-region ledge. However, in the latter model, emissions at various wavelengths are predicted due to reactions involving vibrationally excited molecules. It is also found, using the time-step calculation, that it would take several hours for the predicted electron density to approach equilibrium. [ABSTRACT FROM AUTHOR]

Front cover: A rare occurrence of multiple Sprites above a cluster of thunderstorms active in the Chongming District of Shanghai (China) and captured using a four second exposure. Sprites are large scale electrical discharges that occur in the Mesosphere (50‐85km), high above Cumulonimbus. (© Wang Xin: Winner, Standard Chartered Weather Photographer of the Year 2024) [ABSTRACT FROM AUTHOR]

The ionosphere as a part of Earth’s atmosphere supports a wide range of oscillations, of which acoustic–gravity waves (AGWs) form an important part. AGWs distribute energy and momentum from the source region over large distances. A significant portion of AGWs originates in the lower atmosphere and propagates through the atmosphere up to the ionospheric heights where, due to the coupling between neutral and ionized particles, it could be detected as wavelike disturbances of the plasma. Primarily, the ionospheric behavior is driven by solar and geomagnetic activity, while the influence from neutral and below-laying regions of the atmosphere most of the time forms a substantially smaller part of the observed variability. However, it could significantly alter ionospheric behavior. Our study is limited to a time span of rather low solar and geomagnetic activity in order to highlight neutral atmosphere influence. In this study, we focus on two tropospheric situations above Europe that may lead to AGW generation, which propagate up to the F-layer where they potentially induce variability that we observe within ionospheric plasma parameters.

Mesospheric gravity‐wave (GW) phase velocity spectra and total powers at two Antarctic stations, Davis and Syowa, were derived using OH airglow data from March to October in 2016. The total powers have similar seasonal variation, that is, maxima in winter at both stations. The average powers at both stations in winter were not significantly different. However, the power at Davis in September was three times smaller than that at Syowa. This lower power at Davis was attributed to GWs with omnidirectional phase velocity. These lower GW activities at Davis could be attributed to a longitudinal variation in wave filtering; a stronger wind at Davis filtered out more GWs than at Syowa. Also, to explore possible sources in the middle atmosphere, we investigated one event, in which GWs with ∼100 ms−1 southeastward phase velocity appeared at Davis on 29 August. The raytracing method was applied, and its result indicated that those GWs with high southeastward phase velocity propagated from ∼45 km altitude or higher over the Southern Ocean. A large residual of the non‐linear balanced equation was found at 50 km on its ray path. GWs, very likely emitted from a tropospheric jet, were also found near the ray path at the termination altitude over the Southern Ocean and possibly appeared saturated between 45 and 50 km. Therefore, the OH imager at Davis probably captured GWs generated by a spontaneous adjustment in the upper stratosphere and/or secondary GWs produced by the breaking of the GWs that have originated from the tropospheric jet. Plain Language Summary: A gravity wave (GW) is a type of atmospheric wave that transports momentum from the Earth's surface to the edge of the atmosphere where it can drive atmospheric circulations. The temporal and spatial variation of GWs is not well understood, and their sources remain unclear. We observed GWs at the upper edge of the atmosphere over two Antarctic stations (Syowa and Davis) and calculated their phase velocity and total power spectra. The total powers have winter maxima at both stations. Although the average powers at both stations in winter were not significantly different, the power at Davis in September was three times smaller than that at Syowa. The lower GW activity at Davis could be attributed to a stronger wind at Davis, which filtered out more GWs than at Syowa. Also, to explore possible sources in the middle atmosphere, we investigated one event at Davis on 29 August 2016. We found that some GWs propagated from ∼45 km altitude or higher over the Southern Ocean. Those GWs could be emitted from the polar night jet (flow imbalance) and/or secondary GWs produced by the breaking of the GWs that have originated from the tropospheric jet. Key Points: Mesospheric gravity‐wave activity at two Antarctic stations was larger in winter than in spring and fallWave activity at Davis was not significantly different with Syowa in winter but smaller in fall, which can be explained by filtering effectWe find that GWs at Davis on 29 August 2016, were very likely generated by a spontaneous adjustment and/or a tropospheric jet wave breaking [ABSTRACT FROM AUTHOR]

Measurements of kinetic energy in vortical and divergent fluctuations in the mesosphere and lower thermosphere can be used to study stratified turbulence (ST) and gravity waves. This can be done using horizontal correlation functions of the fluctuating component of velocity. This study introduces a novel method for estimating these correlation functions using radars that observe Doppler shifts of ionized specular meteor trails. The technique solves the correlation functions directly on a longitudinal‐transverse‐up coordinate system, assuming axial symmetry. This procedure is more efficient and leads to smaller uncertainties than a previous approach. The new technique is applied to a year‐long data set from a multistatic specular meteor radar network in Germany, to study the annual variability of kinetic energy within turbulent fluctuations at 87–93 km of altitude. In monthly averages, the kinetic energy is found to be nearly equipartitioned between vortical and divergent modes. Turbulent fluctuations maximize during the winter months with approximately 25% more energy in these months than at other times. The horizontal correlation functions are in agreement with the inertial subrange of ST, exhibiting a 2/3 power law in the horizontal lag direction, with an outermost scale of ST to be about 380 km. This suggests that horizontal correlation functions could be used to estimate turbulent energy transfer rates. Plain Language Summary: Flows exhibit a phenomenon called turbulence, which transfers energy from large scales into smaller scales. This effect is important to quantify the energy budget of the Earth's upper atmosphere. The range of length scales where this phenomenon occurs is called the inertial subrange of turbulence. The classical theory of isotropic turbulence predicts that this energy transfer occurs on length scales smaller than ∼100 m, at 60–110 km altitude. Recent work has shown that horizontal velocity fluctuations can extend the inertial subrange to length scales of up to hundreds of kilometers horizontally. This type of turbulence is called stratified turbulence (ST). So far no comprehensive study has been made to experimentally examine ST in the mesosphere and lower thermosphere (MLT) region on horizontal mesoscales. This study introduces a method for doing so by measuring how the wind fluctuations are correlated as a function of horizontal separation. This is achieved by using meteor radar measurements. The technique is applied to a year‐long data set over Germany. It is found that the MLT wind fluctuations are compatible with ST theory. The introduced method could potentially be used for routinely measuring how kinetic energy flows from large‐scale to small‐scale atmospheric fluctuations. Key Points: A more efficient estimator for horizontal correlation functions is introducedThe rotational and divergent correlation functions of mesosphere and lower thermosphere wind fluctuations are found to be balanced at horizontal mesoscalesHorizontal correlations of wind fluctuations follow a 2/3‐power law for horizontal separations of up to 300–400 km [ABSTRACT FROM AUTHOR]

Temperature observations made by the SABER infrared radiometer from January 2002 through December 2021 are used to study the structure and variability of the migrating diurnal temperature tide in the middle atmosphere (∼17–105 km). In the lower stratosphere, and in the mesosphere and lower thermosphere (MLT), tidal structure is dominated by the gravest latitudinally symmetric mode, with a smaller contribution from the first antisymmetric mode; in the middle and upper stratosphere, vertically nonpropagating modes are prominent. Consistent with previous work, low-frequency variability is mainly semiannual, with maxima at the equinoxes. Quasi-biennial variability is also present and evident in low-passed time series. There are robust relationships between the semiannual and quasi-biennial variability of the tide and the semiannual and quasi-biennial tropical zonal wind oscillations, respectively, which persist throughout the 20-yr dataset. While the physical mechanisms responsible for these relationships cannot be ascertained from the observations, the present results should be useful for hypothesis testing with numerical models. It is also found that the diurnal tide breaks due to convective instability in the MLT. This is reflected in its mean vertical structure, which grows as expected for a nondissipating wave below ∼85 km, but ceases to grow at higher altitudes. Direct confirmation that dissipation is due to breaking is obtained from the potential temperature field, which shows frequent instances of reversed vertical gradient, particularly at the equinoxes. Breaking of the diurnal tide has a major impact on the zonal-mean temperature and zonal wind structure of the MLT at the equinoxes. [ABSTRACT FROM AUTHOR]

The temporal variations in the sudden stratospheric warming (SSW) events in the winter stratosphere always coincide with the quasi-2-day wave (Q2DW) in the summer mesosphere, and the impact of SSW on Q2DW is interesting but still a mystery. Major SSWs occurred in both 2006 and 2009, while the Q2DW activity was quite different. The Second Modern Era-Retrospective Analysis for Research and Applications (MERRA-2) reanalysis dataset was used to comparatively analyze these two major SSW events and elucidate the reasons for the different Q2DW behaviors. We noticed that the summer easterly jet shows a large interannual variability. We conclude that the summer mesospheric Q2DWs are modulated by the winter SSW, whereas the modulation process is also affected by the interannual variability of the summer easterly flow itself. The effects of the SSW on the Q2DWs may differ from year to year due to the variability of the summer easterly flow itself, resulting in different anomalous Q2DW behavior. This conclusion may also be true for the interannual variability of other phenomena during the SSW period. [ABSTRACT FROM AUTHOR]

We report observations of the wave number transition between two types of westward-propagating quasi-2 day wave (QTDW) with zonal wave number s = 3 and s = 4 in the mesosphere. The Aura/MLS temperature datasets from January and February 2011 are utilized in the analysis. A strong W4 was observed in the Southern Hemisphere in early February, following the fading away of a strong W3 in late January. The W3 and W4 were maximized on days 23 and 36 with maximum amplitudes of ~15 K and ~8 K, respectively. Both the amplitudes and periods were nearly equal on day 30, which coincided with the peak of the Sudden Stratospheric Warming (SSW) on day 31 in the polar region of the Northern Hemisphere. Our analysis shows that the equatorial barotropic/baroclinic and inertial instabilities, which are related with the winter planetary wave activities that lead to the SSW, may have triggered this wavenumber transition event and contributed significantly to the growth of the QTDW. Meanwhile, the relatively strong summer easterly jet at middle and high latitude regions in early February also provided favorable conditions for the amplification of W4 during 2011, and thus facilitated the wavenumber transition. [ABSTRACT FROM AUTHOR]

In this research, we analyze the gravity wave (GW) energy density, ice water content (IWC), particle radius, and cloud albedo data of 16 polar mesospheric cloud (PMC) seasons in the Northern Hemisphere (NH) from 2007 to 2014 and Southern Hemisphere (SH) from 2007/2008 to 2014/2015, based on observations from the Cloud Imaging and Particle Size and Solar Occultation For Ice Experiment instruments. The influence of GW activity on the formation of PMCs is studied by hemispheric contrast. In the NH, the GW flux generally starts to increase significantly around the summer solstice during the 8 PMC seasons. In 6/8 of these seasons, the IWC is positively correlated with the variation of GW. When the GW activity is enhanced to reach the maximum, the IWC will start to increase and reach the peak within 0–23 days. In comparison, in the SH, the GW peaks around 55 days after the solstice. The timing of PMC appearance also varies, with the IWC starting to grow 20 days after the solstice and the GW increasing in 55 days after the solstice. In particular, the IWC starts even earlier than the solstice in the seasons of 2012/2013 and 2013/2014. [ABSTRACT FROM AUTHOR]

The meteor radar wind observations over Esrange (67.9°N, 21.10°E) and Rothera (67.5°S, 68.1°W), located respectively in the northern and southern polar latitudes show seasonal variation in the upper mesosphere and lower thermosphere (UMLT) winds with strong westward flow in the zonal wind and equatorward flow in the meridional wind during April‐September over Esrange and October‐February over Rothera. The semi‐diurnal tide shows larger amplitudes over Esrange than over Rothera. However, the diurnal tide (DT) shows comparable amplitudes in both stations. The DT amplitude decreases with height over both stations. Significant correlations between the DT amplitude and mean zonal wind (−0.74 for Esrange and −0.54 and −0.77 for Rothera) indicate the possible role of DT in driving the westward winds. The space‐time spectral analysis of the temperature obtained from the radiance observations of the SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) instrument on board the TIMED (Thermosphere Ionosphere Mesosphere Energetics Dynamics) satellite reveals the summer maximum of migrating diurnal tide of zonal wavenumber 1 (DW1) over both 60°N and 60°S, indicating that the DT is mostly composed of DW1. These results suggest that the westward winds in the polar UMLT region are largely driven by the westward momentum contributed by the DW1 tide due to its interaction with the background wind. Plain Language Summary: The study provides an observational evidence for the influence of tides on the background wind flow at the polar upper mesospheric heights. The meteor radar observations over two polar latitude stations are used to study the seasonal and interannual variation of winds and investigate whether there is any relation between winds and tides. It is observed that stronger westward wind flows at the upper mesosphere lower thermospheric heights when the amplitude of diurnal tide is larger. Moreover, the amplitude of the diurnal tides is found to decrease with height, unlike the semi‐diurnal tidal amplitude, indicating the interaction of diurnal tide with background flow. Due to the tidal mean flow interaction, the diurnal tide which is mostly the westward propagating migrating diurnal tide (DW1), can drive the large westward wind through its westward momentum deposition into the background flow. Key Points: Strong westward winds are observed when the amplitude of westward propagating diurnal tide is largerThe semi‐diurnal tidal amplitude increases with height, while the diurnal tidal amplitude decreases with heightDiurnal tide being the major source of westward momentum possibly drives the zonal wind flow due to its interaction with background flow [ABSTRACT FROM AUTHOR]

The Solar Occultation (SO) channel of the Nadir and Occultation for Mars Discovery (NOMAD) instrument scans the Martian atmosphere since 21 April 2018. In this work, we present a subset of the NOMAD SO data measured at the mesosphere. We focused on a spectral range that started to be recorded in Martian year (MY) 35. A total of 968 vertical profiles of carbon dioxide density and temperature covering MY 35 and the beginning of MY 36 were investigated until 135° of solar longitude. We compared 47 profiles with co‐located profiles of the Mars Climate Sounder onboard the Mars Reconnaissance Orbiter. Most profiles show a good agreement as SO temperatures are only 1.8 K higher, but some biases lead to an average absolute difference of 7.4°K. The SO data set is also compared with simulations from the Global Environmental Multiscale‐Mars general circulation model. Both data sets are in good agreement except for the presence of a cold layer in the winter hemisphere and a warm layer at dawn in the Northern hemisphere for solar longitudes between 240° and 360°. Five profiles contain temperatures lower than the limit for CO2 condensation. Strong warm layers were found in 13.5% of the profiles. They are present mainly at dawn and in the winter hemisphere, while the Northern dusks appear featureless. The data set mainly covers high latitudes around 60° and we derived some non‐migrating tides. In the Southern winter hemisphere, we derived apparent zonal wavenumber‐1 (WN‐1) and WN‐3 tidal components with a maximum amplitude of 10% and 5% at 63 km, respectively. Plain Language Summary: The vertical profiles analyzed in this work extend over more than a Martian year (687 days) and are located in the mesosphere (50–100 km altitude) at the transition between the day and night sides. Temperature can sometimes be so low that CO2 ice clouds form. Here, we report some features in our data set that are likely due to those CO2 ice clouds. We also report some warm layers which appear in the winter hemisphere, and we analyzed the components of the apparent atmospheric tides at the terminator. We compared our results to highlight possible biases with data from another instrument (Mars Climate Sounder) and a model (Global Environmental Multiscale‐Mars). The measured temperatures are higher than those of the model in the winter hemisphere. Compared to the other instrument, we find similar profile curves but with stronger variations in our profiles, probably due to a higher vertical resolution. Key Points: We show vertical distributions of temperature with high vertical resolution in the Mars mesosphere at the terminatorThe retrieved temperatures show a good agreement with the results measured by Mars Climate Sounder/Mars Reconnaissance Orbiter and a general circulation modelThe temperature has distinct features such as tidal components, warm layers, and cold layers below the CO2 condensation temperature [ABSTRACT FROM AUTHOR]

We have studied the convective (or static) and dynamic instabilities between 80 and 100 km above Tromsø (69.6° N, 19.2° E) using temperature and wind data of 6 min and 1 km resolutions primarily almost over a solar cycle obtained with the sodium lidar at Tromsø. First, we have calculated Brunt–Väisälä frequency (N) for 339 nights obtained from October 2010 to December 2019, and the Richardson number (Ri) for 210 nights obtained between October 2012 to December 2019. Second, using those values (N and Ri), we have calculated probabilities of the convective instability (N2 < 0) and the dynamic instability (0 ≤ Ri < 0.25) that can be used for proxies for evaluating the atmospheric stability. The probability of the convective instability varies from about 1% to 24% with a mean value of 9%, and that of the dynamic instability varies from 4 to 20% with a mean value of 10%. Third, we have compared these probabilities with the F10.7 index and local K-index. The probability of the convective instability shows a dependence (its correlation coefficient of 0.45) of the geomagnetic activity (local K-index) between 94 and 100 km, suggesting an auroral influence on the atmospheric stability. The probability of the dynamic instability shows a solar cycle dependence (its correlation coefficient being 0.54). The probability of the dynamic instability shows the dependence of the 12 h wave amplitude (meridional and zonal wind components) (C.C. = 0.52). The averaged potential energy of gravity waves shows decrease with height between 81 and 89 km, suggesting that dissipation of gravity waves plays an important role (at least partly) in causing the convective instability below 89 km. The probability of the convective instability at Tromsø appears to be higher than that at middle/low latitudes, while the probability of the dynamic instability is similar to that at middle/low latitudes. [ABSTRACT FROM AUTHOR]

Using the temperature data observed from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER), we investigate the response of the mesosphere and lower thermosphere (MLT) to two medium geomagnetic storms with disparate durations, on 20 April 2018 and 10 April 2022. The high-latitude MLT temperature increase in the Southern hemisphere can reach 40 K during April 2018 geomagnetic storm with a longer duration (Kp values greater than 4 for 15 h), while the temperature variations are less than 10 K for the April 2022 event (Kp values greater than 4 for 6 h). To investigate the different temperature responses to disparate geomagnetic storm durations and understand what physical process results in this difference, we simulated the two events using the thermosphere ionosphere mesosphere electrodynamics general circulation model (TIMEGCM). The simulations show that more particles and energy input in longer-duration geomagnetic storms produce larger ion drag force and pressure gradient force at ~130 km, and then the enhanced two forces cause faster horizontal wind, leading to larger horizontal divergence. Subsequently, the stronger downward vertical wind is transported to the MLT region (below 110 km) and ultimately makes greater temperature increases through adiabatic heating/cooling and vertical advection. Therefore, the effects of the storm's duration on the MLT temperature are also important. [ABSTRACT FROM AUTHOR]

One of the remarkable properties of Polar Mesosphere Summer Echoes (PMSE) is their occurrence in the form of distinct multiple layers (double- and tri-layers). Using the PMSE observations of E-ISCAT VHF radar, for the first time, we present a combination of heating with multiple layers. Our main focus is to compare the heating effects between the mono-, double-, and tri-layers. Our results show that, as compare to mono-layer, PMSE is greatly affected by HF heating in multiple layers. As a result, the modulated characteristics (e.g., PMSE intensity reduction, recovery, and overshoot) produced in multiple layers are greater than those in mono-layer. Inside multiple layers, the PMSE heating decreases with increasing altitude. Due to heating, the enhancement produced in electron temperature is estimated which is also greater in multiple layers than those in mono-layer. The considerable increase in electron temperature may be one of the possible reasons for the strong heating effects in multiple layers. In addition, we show the dependency of the PMSE heating on the HF pump parameters (effective radiated power (ERP), polarization, frequency, and heating cycle time). It is found that in all layers, the PMSE heating increases with increasing ERP. X-mode and O-mode polarizations produce strong heating in mono-layer and in multiple layers, respectively. We found no significant effect of Heater frequency on PMSE heating. The heating cycle time is the sum of Heater on and off times. In both, mono- and multiple layers, the increasing Heater on time does not greatly affect the PMSE heating, on the other hand, the increasing Heater off time significantly increases the PMSE overshoot effect. [ABSTRACT FROM AUTHOR]

In this study, the Total Electron Content (TEC) variability over Asian-Australian low-latitude sector is investigated during the 2013 Sudden Stratospheric Warming (SSW) event that overlapped with a moderate geomagnetic storm. These investigations are about the latitudinal distribution of ionospheric TEC measured from 10 Global Positioning System (GPS) receivers along 115° E in the Asian-Australian sector. We used a pair of magnetometers to reveal the equatorial electrojet (EEJ) strength equivalent to the magnetometer-inferred drift. Also, the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite airglow instrument was used to unveil the changes in the neutral thermospheric ratio during the event. The current work was compared with a similar investigation in the American sector. During SSW onset (7 January 2013), the northern EIA crest that moved poleward in the Asian-Australian sector contradicted the equatorward movement of the crest observed in the American sector. At both the Asian-Australian and American longitudes in mid-January, the poleward northern crests are at higher latitudes in the American longitude than in the Asian-Australian longitude. This longitudinal difference in mid-January was evident in significant enhancement of the magnetometer inferred upward-directed drift in the American sector compared to the Asian-Australian sector. Compared to the American sector, the moderate geomagnetic storm that overlapped the ongoing major SSW on 17 January 2013 did not significantly affect the Asian-Australian sector. The storm-time effect on the TEC on 18 January 2013 in the Asian-Australian sector reduced (increased) the SSW (photo-ionization) effect in the northern (southern) hemisphere. [ABSTRACT FROM AUTHOR]

The middle atmosphere plays an important role in the research of various dynamical and energy processes. Microwave Limb Sounder (MLS), reanalyses and model simulations with NCAR's Whole Atmosphere Community Climate Model (WACCM) data in the range between 100 and 0.1 hPa from 2005 to 2020 have been analyzed with a focus on the temperature semi-annual oscillations (SAO). Significant SAO of temperature is prominent in the tropical region (20 ° S–20 ° N) around 1–3 hPa, which is consistent with previous studies. We also found significant SAO in the northern hemisphere (NH) high latitudes between 8 and 0.3 hPa and southern hemisphere (SH) high latitudes between 0.5 and 0.1 hPa, which has been of less concern in previous studies. The thermal budget based on MERRA2 and simulations is used to explain the mechanism of SAO in the middle atmosphere. In the tropics, the two temperature peaks are mainly determined by radiative processes. In the NH high latitudes of the stratosphere, the temperature peak in January is mainly related to dynamical processes, while the temperature peak in July is determined by a combination of dynamical and radiative processes. In the NH high latitudes of the lower mesosphere, the first peak in June is primarily associated with dynamical and radiative processes, while the second peak in December is primarily associated with the dynamical processes. In the SH high latitudes of the lower mesosphere, the first temperature peak in July is mainly due to dynamical processes while the second temperature peak in December is mainly due to radiative processes. Various features are present in the SH and NH high latitude SAO in the lower mesosphere. Furthermore, we performed model simulations with and without SAO in sea surface temperatures (SST-SAO) to study the connection between SST and temperature SAO. WACCM6 results indicate that the SAO in the middle atmosphere is partially affected by the existence of an SST-SAO. By removing SAO in SST, the PSD magnitude of the SAO decreases in the tropical region and increases in the polar region. The amplitudes of total heating rates are also modified. The WACCM experiment confirms the relationship between SST-SAO and temperature SAO in the middle atmosphere. [ABSTRACT FROM AUTHOR]

"Wave turbopause" is defined as the mesospheric altitude level where the temperature fluctuation field indicates a substantial increase in wave amplitude in the vertical direction. It is similar to turbopause in seasonal and latitudinal variations, providing an almost global analysis of turbopause on the basis of satellite measurements rather than localized detection. Previous studies of "wave turbopause" were based on standard deviation of temperature, which is an integrated measure of wave activity. In this study, we distinguish different atmospheric waves and investigate their influences on "wave turbopause". By comparing the altitude of "wave turbopause" with peak height of amplitude (PHA) for gravity waves and 6.5 days waves (6.5 DWs), whose period is approximately 6.5 days in the mesosphere and lower thermosphere, we find that the seasonal variation in altitude of "wave turbopause" is higher at the winter pole and lower at the summer pole, correlated with PHA for 6.5 DWs but anti-correlated with PHA for gravity waves. We infer that gravity waves reach saturation and break at lower altitudes in the winter when Brunt–Vaisala frequency is also lower between 80 and 100 km altitudes. Finally, the results may imply that seasonal variations of the "wave turbopause" are driven mainly by 6.5 DWs. [ABSTRACT FROM AUTHOR]

We present long‐term density trends of the Earth's upper atmosphere at altitudes between 71 and 116 km, based on atmospheric occultations of the Crab Nebula observed with X‐ray astronomy satellites, ASCA, RXTE, Suzaku, NuSTAR, and Hitomi. The combination of the five satellites provides a time period of 28 years from 1994 to 2022. To suppress seasonal and latitudinal variations, we concentrate on the data taken in autumn (49 < doy < 111) and spring (235 < doy < 297) in the northern hemisphere with latitudes of 0°–40°. With this constraint, local times are automatically limited either around noon or midnight. We obtain four sets (two seasons × two local times) of density trends at each altitude layer. We take into account variations due to a linear trend and the 11‐year solar cycle using linear regression techniques. Because we do not see significant differences among the four trends, we combine them to provide a single vertical profile of trend slopes. We find a negative density trend of roughly −5%/decade at every altitude. This is in reasonable agreement with inferences from settling rate of the upper atmosphere. In the 100–110‐km altitude, we found an exceptionally high density decline of about −12%/decade. This peak may be the first observational evidence for strong cooling due to water vapor and ozone near 110 km, which was first identified in a numerical simulation by Akmaev et al. (2006, https://doi.org/10.1016/j.jastp.2006.03.008). Further observations and numerical simulations with suitable input parameters are needed to establish this feature. Plain Language Summary: Numerical simulations have shown that, while an increase of greenhouse gases such as CO2 in the atmosphere causes heating of the troposphere (near surface), it causes cooling of the middle and upper atmosphere, which is the so‐called "greenhouse cooling." The greenhouse cooling should result in atmospheric contraction and consequently a temporal density decrease at a fixed height. However, observational evidence for the density decrease has been scarce in the mesosphere and lower thermosphere (MLT: 80–110 km), owing to difficulty in measuring the density in this region. Here, we present the first direct measurements of long‐term variations for combined N and O atom number density in the MLT, based on atmospheric occultations of the Crab Nebula observed with X‐ray astronomy satellites. The combination of five X‐ray astronomy satellites, ASCA, RXTE, Suzaku, NuSTAR, and Hitomi, allows us to explore density trends for a long period from 1994 to 2022. We take into account variations due to a temporal linear trend and the 11‐year solar cycle, using linear regression techniques. As a result, we find a negative density trend of roughly −5%/decade at every altitude, with a local minimum of −12%/decade near 105 km. This is in reasonable agreement with the state‐of‐the‐art numerical simulations. Key Points: Time series of combined O and N densities are measured in the MLT, based on atmospheric occultations of the Crab Nebula using X‐ray astronomy satellitesThe density is decreasing everywhere, with a local minimum of −12%/decade near 105 kmThe local minimum in density trends may be due to strong cooling by water vapor and ozone, as was first predicted by Akmaev et al. (2006, https://doi.org/10.1016/j.jastp.2006.03.008) [ABSTRACT FROM AUTHOR]

The strong quasi 2‐day wave (Q2DW) event that occurred during January 2015 was studied by employing medium frequency and meteor radar observations of winds from three different locations at low latitudes with considerable longitudinal separation. The large‐amplitude wave activity that occurred in mid‐January is the focus of this study. Using the advantage offered by the longitudinal separation among the stations, the zonal structure of the Q2DW has been examined. The presence of a dominant westward propagating zonal wavenumber 3 (W3) mode was revealed in the analysis. The availability of geomagnetic field data from a number of stations at low latitudes enabled us to observe the wave event at ionospheric dynamo region heights. The phase‐longitude cross‐section of the quasi 2‐day oscillation in the Y component of the geomagnetic field reiterates the presence of the dominant W3 mode observed at lower altitudes. Similar zonal characteristics and periods of the wave signatures found at mesospheric altitudes and in the manifestation of quiet‐time ionospheric currents in geomagnetic field are in conformity with the notion of the wave coupling of the atmosphere‐ionosphere system. Plain Language Summary: This study examines the strong quasi 2‐day wave event that occurred during January 2015 at low latitudes using medium frequency and meteor radar observations. The analysis reveals the presence of a dominant westward propagating zonal wavenumber 3 mode. Geomagnetic field data from multiple stations at low latitudes show similar zonal characteristics and periods of the wave signatures, which suggests wave coupling of the atmosphere‐ionosphere system. Key Points: Strong quasi 2‐day wave in the mesosphere during January 2015Simultaneous presence of quasi 2‐day oscillation in magnetic field with similar zonal structureEvidence for vertical coupling by quasi 2‐day wave [ABSTRACT FROM AUTHOR]

This work uses the Specified Dynamics‐Whole Atmosphere Community Climate Model with Ionosphere/Thermosphere eXtension (SD‐WACCM‐X) to determine and explain the seasonality of the migrating semidiurnal tide (SW2) components of tropical upper mesosphere and lower thermosphere (UMLT) temperature, zonal wind, and meridional wind. This work also quantifies aliasing due to SW2 in satellite‐based tidal estimates. Results show that during equinox seasons, the vertical profiles of tropical UMLT temperature SW2 and zonal‐wind SW2's amplitudes have a double‐peak structure while they, along with meridional‐wind SW2, have a single‐peak structure in their amplitudes in June solstice. Hough mode reconstruction reveals that a linear combination of five SW2 Hough modes cannot fully reproduce these features. Tendency analysis reveals that for temperature, the adiabatic term, nonlinear advection term, and linear advection term are important. For the winds, the classical terms, nonlinear advection term, linear advection term, and gravity wave drag are important. Results of our alias analysis then indicate that SW2 can induce an ∼60% alias in zonal‐mean and DW1 components calculated from sampling like that of the Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics satellite and the Aura satellite. This work concludes that in situ generation by wave–wave interaction and/or by gravity waves plays significant roles in the seasonality of tropical UMLT temperature SW2, zonal‐wind SW2, and meridional‐wind SW2. The alias analysis further adds that one cannot simply assume that SW2 in the tropical UMLT is negligible. Key Points: Thermodynamic and momentum budget of tropical migrating semidiurnal tide in the upper mesosphere and lower thermosphere calculatedHough modes cannot fully reconstruct tropical migrating semidiurnal tide component of temperature and windsWave–wave interaction and gravity waves significantly affect tropical migrating semidiurnal tide component of temperature and winds [ABSTRACT FROM AUTHOR]

An annular solar eclipse shifted across the earth from the middle of Africa to the Pacific on 21 June 2020. The Wuhan Mesosphere‐Stratosphere‐Troposphere (MST) radar was located on the eclipse path to provide us a rare opportunity to investigate the atmospheric responses to the rapid variation of solar radiations. The east, north, and vertical beams of the Wuhan MST radar were applied to record the echoes from mesosphere with 10 min sampling period. A Digisonde located near the MST radar was also used to investigate the variations of ionospheric Es layer during the solar eclipse. The mesosphere region between 70 and 80 km heights presented obvious responses to the eclipse. The echoes disappeared at around the eclipse maximum, indicating the dominant role of the solar radiation. The descending and ascending echo structures were recorded in the first and recovery phases of the eclipse, respectively. The Digisonde also found the evidence of the eclipse induced gravity waves in the Es layer that just above the mesosphere. The lunar shadow produced southward winds on the northside of the eclipse path is considered to induce the descending echo structure. The upward propagating gravity waves generated during the eclipse may be responsible for the ascending echo structure. Key Points: It is very rare for Mesosphere‐Stratosphere‐Troposphere radar observations of mesospheric responses to solar eclipseEclipse induced depletion was recorded between 70 and 80 km altitudes, and anomalous echo structures emerged unexpectedly during the eclipseMeridional winds and gravity waves are suspected to induce the anomalous echoes [ABSTRACT FROM AUTHOR]

Using the temperature provided by the Mars Global Circulation Model (MGCM) reanalysis data from Mars Year 30–32, we investigate the global characteristics of eastward traveling planetary waves (TPWs) with different periods in the Martian atmosphere. Based on the wavenumbers and frequencies, the TPWs in the Martian atmosphere are mainly divided into three modes: quasi‐3‐sol waves with eastward wavenumber 2, quasi‐6.5‐sol waves with eastward wavenumber 1, and quasi‐20‐sol waves with eastward wavenumber 1. The temporal, latitudinal, and altitudinal distributions of TPWs are revealed. The amplitudes of these TPWs are all stronger in the Northern Hemisphere than those in the Southern Hemisphere and are annually enhanced during autumn and winter. Quasi‐3‐sol waves are strongest in the lower troposphere from 40°N to 70°N. Quasi‐6.5‐sol waves dominate in the upper troposphere at the latitude from 55°N to 80°N. Quasi‐20‐sol waves exhibit a double‐peak structure in the upper troposphere and the mesosphere from 55°N to 82.5°N in autumn. Additionally, the features of solsticial pause are observed in the quasi‐3‐sol waves at the surface, in the quasi‐6.5‐sol waves from the surface to the middle troposphere, and in the quasi‐20‐sol waves in the lower troposphere. Our analysis suggests that the appearance of the solsticial pause is mainly related to the lack of strong wave sinks or wave sources during the winter solstice. Key Points: Quasi‐3‐sol waves with wavenumber 2, and quasi‐6.5‐sol and quasi‐20‐sol waves with wavenumber 1 are found dominant in the Mars atmosphereThe solsticial pause of planetary waves on Mars occurs mainly at the surface and in the lower troposphereThe lack of strong wave sinks or wave sources is likely responsible for the solsticial pause during the winter solstice [ABSTRACT FROM AUTHOR]

We present temperature maps derived from number density retrievals of carbon dioxide (CO2) for the upper mesosphere and lower thermosphere of Mars using limb observations from the Imaging Ultraviolet Spectrograph (IUVS) aboard NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We retrieve CO2 densities using O(1S) metastable atoms that radiatively relax by emitting photons at 297.2 nm, producing a double‐peaked emission profile detectable by IUVS. Retrieved CO2 densities are used to derive altitude profiles of temperature as a function of latitude, longitude, local time, season, dust activity, and solar activity. CO2 density and temperature profiles retrieved using the O I 297.2 nm emission feature presented herein extend previous IUVS retrievals from 130–170 km down to 80 km. We validate retrieved CO2 densities and derived temperatures using coincident measurements and corresponding data products produced by MAVEN IUVS, as available. Analysis of this comprehensive data set, which spans Mars years 32–36, shows (a) a consistently well‐defined mesopause at approximately 120 km, (b) warming at high pressures (typically below ∼100 km) for a variety of geophysical conditions, (c) asymmetry in temperatures at dawn and dusk with respect to latitude during different seasons (warmer temperatures at dawn during northern hemisphere autumn/winter and cooler temperatures at dusk during spring/summer), and (d) longitudinal waves with a dominant wave‐3 component in both the upper mesosphere and lower thermosphere, with lower (80–90 km) and upper (135–145 km) atmospheric waves about 65° out of phase. Plain Language Summary: We show the first O I 297.2 nm emission‐derived temperature profiles extending from 80 to 150 km in the atmosphere of Mars obtained from observations by the Imaging Ultraviolet Spectrograph onboard NASA's Mars Atmosphere and Volatile EvolutioN spacecraft. This new analysis enables us to bridge the gap and improve our understanding of the coupling that occurs between the middle and upper atmospheres of Mars. Our analysis has revealed thermal variability in the Martian atmosphere with respect to latitude, longitude, local time, season, dust activity, and solar activity. As suggested by previous studies, waves generated in the lower atmosphere propagate to the upper atmosphere, coupling the atmospheric layers, and produce fluctuations in density (up to 40% locally) that can affect the background atmospheric temperature, which is a key parameter driving thermal escape of atomic hydrogen. The density and temperature data presented here provide an important but heretofore missing source of information that is needed to directly link weather and waves in the lower atmosphere to perturbations in composition and temperature in the upper atmosphere. Key Points: Our analysis reveals a consistently well‐defined mesopause at approximately 120 kmWe observe an asymmetry in temperatures at dawn and dusk with respect to latitude during different seasonsLongitudinal waves with dominant wave‐3 component are characterized in both the upper mesosphere and lower thermosphere [ABSTRACT FROM AUTHOR]

Previous studies suggested that sudden stratospheric warmings (SSW) change the global atmosphere from troposphere to thermosphere/ionosphere. We report the low‐latitude O+ and H+ composition at 840‐km altitude during the 2009 SSW, with the DMSP satellite morning measurements. Our results indicate that the stratospheric variation around 30‐km altitude modulates the ion exchange between the ionosphere and protonosphere via the vertical and field‐aligned plasma drifts due to the enhanced lunar semidiurnal tides. The upward disturbance drift uplifts the ionospheric O+ into the protonosphere, and most O+ is changed to H+ via chemical coupling, while the O+/H+ transition height does not change under combined effects of the southward and upward disturbance drifts on 24–29 January. The disturbance drift turns downward, lowers the O+/H+ transition height, depletes the O+ density in the protonosphere, and the H+ at higher altitudes moves downward to supply the H+ at 840 km from 30 January to 5 February. Plain Language Summary: The stratosphere is a layer of the atmosphere from ∼10–50 km altitudes. Roughly every 2 years, the Northern Hemisphere winter polar stratosphere suddenly warms over a course of few days, and the winds decelerate dramatically, even reversal to easterly winds, which is known as sudden stratospheric warmings (SSW). SSW causes large changes in global atmosphere including the troposphere, mesosphere, and thermosphere via the dynamic coupling. Studies in the last decade revealed that SSW can also lead to the remarkable changes in the ionospheric electron density around the 300–400 km F2 region. A question is whether or not the SSW effects can reach higher altitude. The main ions are O+ around the F2 peak, while change to light ions as the altitude increases. We use the DMSP satellite observations in the morning to present the O+ and H+ distributions at 840 km altitude during the 2009 SSW. The ion composition distributions provide us a channel to study the coupling between the protonosphere and ionosphere. The results illustrate that the SSW occurred at ∼30 km altitude can impact the protonospheric plasma environment in low latitudes that has been not presented previously. Key Points: The sudden stratospheric warmings modulates the ion exchange between the ionosphere and protonosphereThe O+/H+ transition height does not change under the combined effects of the upward and southward disturbance plasma driftsThe enhanced semidiurnal lunar tides may contribute to the ion exchange between two layers [ABSTRACT FROM AUTHOR]

The mesosphere/lower thermosphere (MLT, 80–100 km) region is an important boundary between Earth's atmosphere below and space above and may act as a sensitive indicator for anthropogenic climate change. Existing observational and modeling studies have shown the middle atmosphere and the MLT is cooling and contracting because of increasing greenhouse gas emissions. However, trend analyses are highly sensitive to the time periods covered, their length, and the measurement type and methodology used. We present for the first time the linear and 11‐year solar cycle responses in the meteor ablation altitude distributions observed by 12 meteor radars at different locations. Decreasing altitudes were seen at all latitudes (linear trends varying from −10.97 to −817.95 m dec−1), and a positive correlation with solar activity was seen for most locations. The divergence of responses at high latitudes indicates an important and complex interplay between atmospheric changes and dynamics at varying time scales. Plain Language Summary: High up in our atmosphere lies the mesosphere/lower thermosphere region (80–100 km); an important transition zone between the atmosphere below and space above. Existing studies indicate that this region is changing (cooling and contracting) in response to increasing greenhouse gas emissions, quite unlike the net warming we see near the surface. However these trend studies are often highly sensitive to choice and length of time period covered, and the methodology and type of measurements used. Here we present for the first time a self‐consistent methodology applied to 12 different meteor radar station datasets located at a diverse range of latitudes. We looked at changes in the mean peak altitude of individual meteoroid detections, and found decreasing peak altitudes at all locations examined (linear trends varying from −10.97 to −817.95 m decade−1) consistent with a global cooling and contracting of the upper atmosphere. We also examined the response to the 11‐year solar cycle and found a positive correlation with solar activity (i.e., increased meteoroid peak altitudes during solar maximum, and vice versa) for low and mid‐latitude locations. However we found an anti‐correlation at high latitudes suggestive of an important and complex interplay between atmospheric changes and dynamics at varying time scales. Key Points: Use of geographically diverse meteor radar peak detection altitudes to assess long‐term and 11‐year solar cycle (SC) trends in mesopause regionThe altitude of observed peak meteor height has decreased over time at all locations, regardless of latitude and data setPositive correlation at low‐ and mid‐latitude locations with the 11‐year SC, but more complex response at high‐latitudes [ABSTRACT FROM AUTHOR]

Explosive volcanic eruptions emitting large amounts of sulfur can alter the temperature of the lower stratosphere and change the circulation of the middle atmosphere. The dynamical response of the stratosphere to strong volcanic eruptions has been the subject of numerous studies. The impact of volcanic eruptions on the mesosphere is less well understood because of a lack of large eruptions in the satellite era and only sparse observations before that period. Nevertheless, some measurements indicated an increase in mesospheric mid-latitude temperatures after the 1991 Pinatubo eruption. The aim of this study is to uncover potential dynamical mechanisms that may lead to such a mesospheric temperature response. We use the upper-atmospheric icosahedral non-hydrostatic (UA-ICON) model to simulate the atmospheric response to an idealized strong volcanic injection of 20 Tg S into the stratosphere (about twice as much as the eminent 1991 Pinatubo eruption). The simulation shows a significant warming of the polar summer mesospause of up to 15–21~K in the first November after the eruption. We argue that this is mainly due to intrahemispheric dynamical coupling in the summer hemisphere and potentially enhanced by interhemispheric coupling with the winter stratosphere. This study will focus on the first austral summer after the eruption, because mesospheric temperature anomalies are especially relevant for the properties of noctilucent clouds whose season peaks around January in the southern hemisphere. [ABSTRACT FROM AUTHOR]

We analyse 43 years of mesosphere/lower thermosphere (MLT) horizontal winds obtained from a joint analysis of low frequency (LF) spaced receiver lower ionospheric drift measurements from late 1978 through 2008 and VHF meteor radar wind observations since summer 2004 at Collm (51.3 ∘ N, 13.0 ∘ E). Due to limitations of the earlier LF measurements, we restrict ourselves to the analysis of monthly mean winds near 90 km, which represents the meteor peak height as well as mean LF reflection heights in the MLT. We observe mainly positive trends of the zonal prevailing wind throughout the year, while the meridional winds tend to decrease in magnitude in both summer and winter. Furthermore, there is a change in long-term trends around the late 1990s, which is most clearly visible in summer MLT winds. We compare these measurements with long-term partial reflection radar observations of winds at 81–85 km over Juliusruh (54.6 ∘ N, 13.4 ∘ E) since 1990, and find general qualitative agreement of trends except for summer. The latter can be explained by the different altitudes considered, and by the latitude dependence of the summer mesospheric jet. [ABSTRACT FROM AUTHOR]

We present an analysis of the response of quasi-10-day waves (Q10DWs) to the sudden stratospheric warming (SSW) event which occurred on March 23, 2020. The Q10DWs are observed in the mesosphere and lower thermosphere (MLT) region by three meteor radars, which are located at middle latitudes along the 120°E meridian from Mohe (MH, 53.5°N, 122.3°E), Beijing (BJ, 40.3°N, 116.2°E), to Wuhan (WH, 30.5°N, 114.6°E). The Q10DWs reveal similar temporal and altitudinal variations during the SSW in the MLT region at the three stations. The activities of Q10DWs are also captured in the temperature measurements from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite in the MLT region. Further analysis of the Q10DW phases indicates that the Q10DWs might be in situ generated due to mesospheric instabilities at higher latitudes around MH and then propagate southward to lower latitudes at BJ and WH. The atmospheric instabilities are not directly responsible for the excitations of Q10DWs at lower latitudes, while the observed equatorward propagation of the Q10DWs is important. Our result provides the observational evidence for latitudinal couplings in the MLT region after the SSW onset, which is achieved by southward propagating planetary waves in the MLT region. [ABSTRACT FROM AUTHOR]

This study presents the analysis of planetary waves (PWs) using daily mean wind velocities for four years (August 2013 to July 2017) of continuous measurements using MF radar over the low latitude Indian region Kolhapur (16.8° N; 74.2° E). The MF radar at Kolhapur was upgraded in 2013. These are the first results of PWs after the upgradation of MF radar. The seasonal and intra-seasonal variabilities of East-West (EW) traveling PWs in the MLT region have been studied. In the present work, the data was analyzed to study the waves with various periodicities (e.g. 3–4, 5–8, 15–17, and 30–60 days). The 3.5 day [Ultra-Fast Kelvin (UFK)] wave shows semiannual variability with burst like wave activity observed during the summer months and December solstice. In addition, it is observed to be stronger in the spring equinoctial period. A strong semiannual oscillation (SAO) has been observed in a 6.5-day wave with peaks near the equinoxes. Similar to SAO over the low latitude MLT region, the wave activity is stronger in April/May than in September/October. The 6.5-day waves are observed to be stronger when the background mean wind is westward. From the analysis, it has been seen that the period before and after the equinoctial period is favorable for the 6.5-day wave propagation. The 16-day wave has no significant seasonal dependence; instead, the waves spread to almost all seasons. The Madden-Julian Oscillations (MJOs) have been observed to be propagating with an average wind speed of ∼ 5 m/s when the background mean wind is eastward. The occurrence of MJO is observed during the summer and winter months. These results are the first of their kind in two aspects: first, they show the PWs with enhanced altitude coverage covering up to 110 km, and second, they show the PWs not contaminated due to equatorial electro jet influence. [ABSTRACT FROM AUTHOR]

Nitric Oxide is a very important trace species which plays a significant role acting as a natural thermostat in Earth's thermosphere during strong geomagnetic activity. In this paper, we present various aspects related to the variation in the NO Infrared radiative flux (IRF) exiting the thermosphere by utilizing the TIMED/SABER (Thermosphere Ionosphere Mesosphere Energetics and Dynamics/ Sounding of the Atmosphere using Broadband Emission Radiometry) observational data during the Halloween storm which occurred in late October 2003. The Halloween storm comprised of three intense-geomagnetic storms. The variability of NO infrared flux during these storm events and its connection to the strength of the geomagnetic storms were found to be different in contrast to similar super storms. The connection between the quantum of energy outflux from the upper atmosphere into space in terms of NO IRF and the duration of storms is established. The NO radiative cooling, and the closely correlated depletion in O/N 2 ratio are controlled by the Joule heating intensity (proxied by AE-index). The collisional excitation rate of NO, calculated using the modelled datasets of WACCM-X (Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension), correlates well with the observed pattern of radiative emission by NO. Observational datasets from TIMED/GUVI (Global Ultra-Violet Imager) and MIT Haystack observatory madrigal GNSS (Global navigation satellite system) total electron content (TEC) database shows that the TEC and O/N 2 enhancement in low-mid northern hemispheric latitudes are mainly controlled by the z-component of Interplanetary magnetic field (IMF-B z). The penetration of eastward electric field during the storm events is found to be responsible for the overall enhancement of TEC. The contribution of enhanced day-side TEC in observed variation of O/N 2 ratio by GUVI is also reported. It is also seen that during substorms related events the night-time polar region experiences more cooling due to NO than the daytime polar region. The connections between the mid- and low-latitude enhancement in NO IRF with the propagation of LSTIDs (Large-scale traveling ionospheric disturbances) in combination with the O/N 2 variability, and the altitudinal variation in NO flux with the progression of the storm is also investigated. This study presents the evidence on the role of diffusion processes in the large scale enhancement of NO in the mesospheric altitudes. [ABSTRACT FROM AUTHOR]