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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]

The Brewer‐Dobson circulation (BDC) characterizes the large‐scale meridional overturning mass circulation influencing the composition of the whole middle atmosphere. The BDC consists of two separate parts—a shallow branch in the lower stratosphere and a deep branch higher in the middle atmosphere. Climate models robustly project the advective BDC part to accelerate due to greenhouse gas‐induced climate change and this acceleration strongly influences middle atmospheric chemistry and physics in the projections. A prominent quantity that is being studied as a proxy for advective BDC changes is the net tropical upwelling across pressure levels, particularly in the lower stratosphere. The upper branch of the BDC has received considerably less research attention than its shallow part, although, together with the mean mesospheric pole‐to‐pole circulation, it couples the stratosphere and mesosphere and is responsible for a large portion of the interhemispheric transport in the middle atmosphere. Aiming to fill this gap, we here study climatology and trends in advective mass transport across the stratopause. Results based on the analysis of seven CCMI models include quantification of the climatological cross‐stratopause advective transport, characterization of its interannual variability and long‐term trend as well as detailed analysis of inter‐model differences using a novel decomposition methodology. Our results demonstrate that the changes in circulation speed as well as changes in horizontal and vertical structure in the upper stratosphere and mesosphere jointly shape the projected increasing advective mass flux across the stratopause due to increasing greenhouse‐gas emissions. Plain Language Summary: The Brewer‐Dobson circulation (BDC) is an important circulation pattern, affecting the composition of the entire middle atmosphere. Our study focuses on the less‐studied higher part of the BDC, which connects the stratosphere and mesosphere. Analyzing comprehensive climate model simulations, we found changes in circulation speed, along with horizontal and vertical structure changes of the upper BDC branch as a response to increasing greenhouse gas emissions. Understanding these changes aids improvements of climate predictions and highlights the significant influence of human activities on the atmosphere. Key Points: Advective transport between the stratosphere and mesosphere exhibits a robust annual cycleModels project an average amplification of cross‐stratopause transport of about 1% per decade during all seasonsThe transport decomposition reveals inconsistencies between the models regarding the causes of long‐term changes [ABSTRACT FROM AUTHOR]

The Weather Research and Forecasting (WRF) Model is employed to explore gravity waves excited by a storm complex over West Africa on 15 August 2020, propagating to the mesosphere and lower thermosphere. Although the simulation portrays gravity waves in the stratosphere in a complex structure, it reveals a concentric‐ring pattern of gravity waves in the mesosphere. The waves closely resemble the airglow image acquired by Suomi National Polar‐orbiting Partnership satellite near the mesopause. This intriguing phenomenon of the emergence of concentric rings after passing the stratopause is investigated by analyzing wave characteristics, including horizontal wavenumbers, frequency, and phase speed. The analysis reveals that the wind filtering effect directly influences the behavior of gravity waves, leading to variations in their appearance with altitude. The wind filtering effect, which typically breaks symmetric ring waves, could help reveal the ring pattern when the waves are subject to interference. Consequently, the concentric rings, propagating faster than the background winds, are shown to originate from the cloud tops of the storm complex. A wind duct at a specific location is identified through spectral analysis. This mechanism is further elucidated with the assistance of the dispersion relation of a classical linear theory. Plain Language Summary: Storm‐generated gravity waves can propagate through the stratosphere into the mesosphere and lower thermosphere. They often appear in the form of concentric rings and observe as airglow. However, the presence of concentric rings is not consistently observed. The thermal and dynamic structures of the atmosphere can significantly influence the vertical propagation of gravity waves. We demonstrate that the wind filtering effect does not always lead to the disruption of the symmetric ring pattern. When the pattern is already affected by interference from other waves, the wind filtering effect can actually aid in revealing the ring structure by filtering out the complex interference pattern. We employ a numerical weather prediction model to simulate a real case observed in West Africa on 15 August 2020. The result shows that the simulated gravity waves closely align with observations at the airglow altitude. Subsequently, we differentiate the waves into slow and fast‐moving modes through spectral analysis, revealing that the fast‐moving mode, originating from the storm, propagates consistently up to the mesopause. Moreover, spectral analysis, in conjunction with the dispersion relation of classical linear theory, enables the identification of ducting waves—standing waves along the vertical axis—at a specific location. Key Points: Storm‐generated gravity waves propagating to the mesopause are simulated using a numerical weather prediction modelThe gravity waves exhibiting a concentric pattern arriving at the mesopause are those that have survived the wind filtering effectThe critical level enhances the concentric pattern by suppressing the slow waves that would otherwise dim the concentric pattern [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]

• Impact of Interplanetary shock on thermospheric Nitric Oxide cooling is studied. • Nitric Oxide cooling exhibits a linear enhancement during Interplanetary shock. • Particle precipitation and temperature enhancement occur due to IP shock. The impact of interplanetary (IP) shock on Nitric Oxide (NO) 5.3 µm cooling emission is studied during geomagnetic quiet periods. The Active Magnetosphere and Planetary Electrodynamics Response Experiment measurements of field-aligned-currents intensify during IP shock with a relatively higher magnitude in southern hemisphere as compared to the northern hemispheric counterpart. The Defense Meteorological Satellite Program spacecraft observations displayed an early and strong enhancement in the precipitating particle flux of energy less than 1 keV. The particle flux of higher energy responds at later time. The NO density exhibited a significant, pre-event increase by an order of magnitude due to low-energy particle precipitation. The thermospheric temperature increased by about 100 K at 400 km. The superposed epoch analysis study revealed a linear enhancement in SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) measurements of NO cooling emission onboard the TIMED (Thermosphere Ionosphere Mesosphere Energetics Dynamics) satellite due to the prompt increase in particle precipitations and thermospheric temperature triggered by IP shock. [ABSTRACT FROM AUTHOR]

Global average upper atmosphere temperature changes linked with the Hunga volcanic eruption (January 2022) are analyzed based on satellite measurements and compared with chemistry‐climate model simulations. Results show stratospheric cooling of −0.5 to −1.0 K in the middle and upper stratosphere during 2022 through middle 2023, followed by stronger cooling (−1.0 to −2.0 K) in the mesosphere after middle 2023. The cooling patterns follow the upward propagating water vapor (H2O) anomalies from Hunga, and similar behavior is found between observations and model simulations. While the stratospheric cooling is mainly due to radiative cooling from enhanced H2O, the mesospheric temperature changes result from ozone losses in the mesosphere, which are in‐turn driven by HOx radicals from Hunga H2O. Comparisons with the multi‐decade climate record show that Hunga impacts on stratospheric temperatures have similar magnitude, but opposite sign, to temperature effects from the large El Chichón (1982) and Pinatubo (1991) volcanic eruptions. Plain Language Summary: We analyze temperature changes in the upper atmosphere due to the Hunga volcanic eruption in January 2022 using satellite observations and simulations from a state‐of‐the‐art chemistry‐climate model. Hunga was an underwater volcano that emitted a large amount of water vapor (H2O) directly into the stratosphere, and this H2O has moved slowly upwards over time and mostly remains in the upper atmosphere. Observations show cooling of the stratosphere (20–50 km) by 0.5–1.0 K during 2022 to middle 2023, and cooling of the mesosphere (50–80 km) by 1.0–2.0 K after middle 2023. The cooling patterns follow the upward moving Hunga H2O plume. The model simulations show good agreement with observations, and the model is used to diagnose that the cooling is due to Hunga H2O impacts. The long‐lasting, global‐scale cooling from Hunga has similar magnitude, but opposite sign, to stratospheric warming observed from previous large volcanic eruptions (El Chichón in 1982 and Pinatubo in 1991); the differences are due to the large H2O emitted by Hunga. Key Points: Global average cooling of 1–2 K is observed in the upper atmosphere during 2022–2024 following Hunga eruption in January 2022Good agreement of observations with chemistry‐climate model simulations; cooling is mainly due to Hunga H2O impactsHunga‐forced cooling is comparable in magnitude to stratospheric warming following El Chichón (1982) and Pinatubo (1991) eruptions [ABSTRACT FROM AUTHOR]

Meteoroids of sub‐milligram sizes burn up high in the Earth's atmosphere and cause streaks of plasma trails detectable by meteor radars. The altitude at which these trails, or meteors, form depends on a number of factors including atmospheric density and the astronomical source populations from which these meteoroids originate. A previous study has shown that the altitude of these meteors is affected by long‐term linear trends and the 11‐year solar cycle related to changes in our atmosphere. In this work, we examine how shorter diurnal and seasonal variations in the altitude distribution of meteors are dependent on the geographical location at which the measurements are performed. We use meteoroid altitude data from 18 independent meteor radar stations at a broad range of latitudes and investigate whether there are local time (LT) and seasonal variations in the altitude of the peak meteor height, defined as the majority detection altitude of all meteors within a certain period, which differ from those expected purely from the variation in the visibility of their astronomical source. We find a consistent LT and seasonal response for the Northern Hemisphere locations regardless of latitude. However, the Southern Hemisphere locations exhibit much greater LT and seasonal variation. In particular, we find a complex response in the four stations located within the Southern Andes region, which indicates that the strong dynamical atmospheric activity, such as the gravity waves prevalent here, disrupts, and masks the seasonality and dependence on the astronomical sources. Plain Language Summary: Small meteoroids burn up high in the Earth's atmosphere producing trails of plasma detectable by ground‐based meteor radar instruments. The altitude at which these trails occur depends on a number of factors including atmospheric density and the astronomical source populations from which these meteoroids originate. Previous work demonstrated that the altitude at which the majority of these meteoroids burn up (termed "peak meteor altitude") is affected by long‐term atmospheric changes, such as those related to greenhouse gas emissions and the 11‐year solar cycle. Here, we focus on shorter timescales and analyze meteoroid altitude data from 18 geographically diverse meteor radars to examine the local time (LT) and seasonal variation in the peak meteor altitudes on a latitude basis. We find a consistent LT and seasonal response among the six Northern Hemisphere meteor radar station locations irrespective of latitude. However, we find a more complex response among the 12 Southern Hemisphere stations with much greater LT and seasonal variation. In particular, we found a complex response in the four stations located within the Southern Andes region, a geographic region known for intense atmospheric gravity wave activity, which acts to mask and disrupt the seasonality and dependence on the astronomical sources. Key Points: Local time (LT) and seasonal variations in the peak meteor height exist, which differ from those expected from astronomical variation aloneThere is a consistent LT and seasonal response in the Northern Hemisphere locations regardless of latitudeA complex response in the Andes region where a strong gravity wave component acts to mask seasonality and dependence on astronomical sources [ABSTRACT FROM AUTHOR]

An array of SuperDARN meteor radars at northern high latitudes was used to investigate the sources and characteristics of eastward-propagating planetary waves (EPWs) at 95 km, with a focus on wintertime. The nine radars provided the daily mean meridional winds and their anomalies over 180 degrees of longitude, and these anomalies were separated into eastward and westward waves using a fast Fourier transform (FFT) method to extract the planetary wave components of zonal wavenumbers 1 and 2. Years when a sudden stratospheric warming event with an elevated stratopause (ES-SSW) occurred during the winter were contrasted with years without such events and composited through superposed epoch analysis. The results show that EPWs are a ubiquitous—and unexpected—feature of meridional wind variability near 95 km. Present even in non-ES-SSW years, they display a regular annual cycle peaking in January or February, depending on the zonal wavenumber. In years when an ES-SSW occurred, the EPWs were highly variable but enhanced before and after the onset. [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]

The solar cycle includes multi‐scale variations in the near‐Earth space regions including plasmasphere, inner radiation belt (IRB), ionosphere, mesosphere and lower thermosphere (MLT). We present a thorough analysis of the extent of solar cycle effect on those four regions by using mesospheric and thermospheric geopotential height and temperature from SABER on TIMED, ionospheric hmF2 from Chinese Meridian Project, high‐energy protons in IRB and electron density in plasmasphere from Van Allen Probes within 2013–2018 intervals. By analyzing evolutions of these quantities, we find that entire IRB, ionosphere and MLT region shrink at solar minimum and stretch at solar maximum by ∼103, 50–102, and 1 km scales, respectively, while plasmapause shows an opposite trend. Fourier spectra of these quantities have been investigated by Lomb–Scargle periodogram. The mid‐term periodic oscillations (13.5‐day, 45‐day, and 52‐day) have been observed in MLT region, matching well with plasmapause locations and geomagnetic indices, which have not been observed in solar EUV radiation and IRB. This may indicate that those oscillations facilitate energy exchange and mass transportation between MLT region and plasmasphere due to magnetic storms and substorms. The oscillation periods of higher energy (102.6 MeV) in IRB have not been observed in MLT region except for annual variations. The impact of higher energy protons on MLT regions may not be significant, although they could penetrate deeper into MLT region. Our results reveal relationships between some quantities and solar cycle multi‐scale modulation, which may provide assistance and monitors for mass transportation in the near‐Earth space regions. Key Points: Inner radiation belt, ionosphere, MLT region shrink at solar minimum and stretch at solar maximum, while plasmapause shows an opposite trendLong‐term and mid‐term oscillations are considered as monitors of energy exchange and mass transport in the near‐Earth space regionsThe entire inner radiation belt, ionosphere and mesosphere/thermosphere shrink on about 103, 50–102, and 1 km scales, respectively [ABSTRACT FROM AUTHOR]

Understanding the temporal and spatial variations in the ideal inert tracer helium can provide insight into the dynamic evolution of the thermosphere. The magnitude of the thermospheric winter helium bulge was inversely correlated with the level of solar activity. However, this feature has been found to be not reproduced by the Thermosphere‐Ionosphere Electrodynamic General Circulation Model (TIEGCM), and the associated physical mechanisms remain unknown. Using the Thermosphere‐Ionosphere‐Mesosphere Electrodynamic General Circulation Model (TIME‐GCM), we found that mesospheric gravity wave drag (GWD) is a factor contributing to this inverse correlation. Specifically, the summer‐to‐winter circulation in thermosphere becomes the main cause of the helium bulge, as mesospheric GWD can play a role in strengthening this circulation. The GWD contributions to temperature change below the lower thermosphere do not depend prominently on solar activity. However, because the temperature impacts on the pressure gradient force are height‐integrated according to the background temperature of the neutral gas, the higher background temperature in the thermosphere at the solar maximum corresponds to a relatively weaker response in pressure gradient force in the thermosphere. Therefore, the response of the thermospheric circulation that might be expected to accompany increasing solar activity is suppressed due to the influence of mesospheric GWD, which results in a decrease in the magnitude of the winter helium bulge with increasing solar activity. Thus, our results demonstrated that lower atmosphere forcing can play a significant role in the response of thermospheric helium to solar activity. Plain Language Summary: A winter helium bulge is a phenomenon in which the density of helium in the winter thermosphere is greater than that in the summer thermosphere by 1–2 orders of magnitude. This bulge is closely related to the large‐scale summer‐to‐winter circulation in the thermosphere. Interestingly, the magnitude of the winter helium bulge is inversely related to the level of solar activity. However, the associated physical mechanisms remain unknown. In this study, we focused on the influence of mesospheric gravity wave drag on the solar cycle variation associated with thermospheric winter helium bulges. We found that mesospheric gravity wave drag can play a role in suppressing the enhancement of thermospheric circulation with increasing solar activity. Thus, this factor can lead to an inverse correlation of the thermospheric winter helium bulge with solar activity. Our simulation results indicate that lower atmosphere forcing may play a significant role in the response of thermospheric helium to changes in solar activity. Key Points: Mesospheric gravity wave drag (GWD) contributes to the inverse correlation between the thermospheric helium bulge and solar activityThe relative influence of mesospheric GWD on thermospheric circulation depends on solar activityThe variation in the winter helium bulge depends on both the circulation in the thermosphere and the lateral transport in the exosphere [ABSTRACT FROM AUTHOR]

The influence of nonmigrating diurnal tides on Na layer variability in the mesosphere and lower thermosphere regions is investigated for the first time using data from the Optical Spectrograph and InfraRed Imaging System (OSIRIS) on the Odin satellite and Specified Dynamics Whole Atmosphere Community Climate Model (SD-WACCM) with metal chemistry. The Na density from OSIRIS exhibits a clear longitudinal variation indicative of the presence of tidal components. Similar variability is seen in the SD-WACCM result. Analysis shows a significant relationship between the nonmigrating diurnal tides in Na density and the corresponding temperature tidal signal. Below 90 km , the three nonmigrating diurnal tidal components in Na density show a significant positive correlation with the temperature tides. Conversely, the phase mainly indicates a negative correlation above 95 km. Around the metal layer peak, the response of the Na density to a 1 K change in tidal temperature is estimated to be 120 cm-3. [ABSTRACT FROM AUTHOR]

Molecular scattering (Rayleigh scattering) has been extensively used from the ground with lidars and from space to observe the limb, thereby deriving vertical temperature profiles between 30 and 80 km. In this study, we investigate how temperature can be measured using the new Ozone Mapping and Profiler Suite (OMPS) sensor, aboard the Suomi NPP and NOAA-21 satellites. The OMPS consists of three instruments whose main purpose is to study the composition of the stratosphere. One of these, the Limb Profiler (LP), measures the radiance of the limb of the middle atmosphere (stratosphere and mesosphere, 12 to 90 km altitude) at wavelengths from 290 to 1020 nm. This new data set has been used with a New Simplified Radiative Transfer Model (NSRTM) to derive temperature profiles with a vertical resolution of 1 km. To validate the method, the OMPS-derived temperature profiles were compared with data from four ground-based lidars and the ERA5 and MSIS models. The results show that OMPS and the lidars are in agreement within a range of about 5 K from 30 to 80 km. Comparisons with the models also show similar results, except for ERA5 beyond 50 km. We investigated various sources of bias, such as different attenuation sources, which can produce errors of up to 120 K in the UV range, instrumental errors around 0.8 K and noise problems of up to 150 K in the visible range for OMPS. This study also highlighted the interest in developing a new miniaturised instrument that could provide real-time observation of atmospheric vertical temperature profiles using a constellation of CubeSats with our NSRTM. [ABSTRACT FROM AUTHOR]

During sudden stratospheric warming (SSW) events, significant modifications occur, not only in the neutral atmosphere, but also in the ionosphere. Specifically, sporadic E layers in the mesosphere and lower thermosphere regions significantly disrupt satellite communication. Although research has frequently focused on ionospheric alterations during SSW events, detailed studies on sporadic E layers remain limited. Examining these variations during SSW events could enhance our understanding of the interaction mechanisms between the ionosphere and the neutral atmosphere, and provide insights into the patterns of sporadic E layer alterations. This study analyzed the behavior of sporadic E layers during the 2008/2009 winter SSW period using data from three Japanese stations and satellite observations. The principal findings included the following: (1) The enhancement in the critical frequency of the sporadic E layers was most notable following the transition from easterly to westerly winds at 60° N at a 10 hPa altitude, accompanied by quasi 6-day and quasi 16-day oscillations in frequency. (2) The daily average zonal and meridional wind speeds in the MLT region also exhibited quasi 6-day and quasi 16-day oscillations, aligning with the observed periodicities in the critical frequency of the sporadic E layers. (3) Planetary waves were shown to modulate the amplitude of diurnal and semidiurnal tides, influencing the sporadic E layers. Furthermore, a wavelet analysis of foEs data with a time resolution of 0.25 h demonstrated that planetary waves also modulate the frequency of diurnal tides, thereby affecting the sporadic E layers. This research contributes to a deeper understanding of the formation mechanisms and prediction of sporadic E layer behavior. [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]

Interplanetary (IP) shock is one of the most common phenomena that controls the shape and size of the magnetosphere. It affects the whole magnetosphere‐ionosphere‐thermosphere (MIT) system. We utilized the NO 5.3 μ ${\upmu }$m radiative emission, as observed by SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) onboard NASA's TIMED (Thermosphere Ionosphere Mesosphere Energetics Dynamics) satellite, to investigate its response to fast forward shock during 26 January 2017. The high latitude NO emission exhibits a strong enhancement (∼ ${\sim} $three times with respect to pre‐event value) during IP shock within 5 hr of onset. We analyzed both the energy dissipation sources and subsequent chemical mechanisms. The Field‐Aligned‐Current observations from Active Magnetosphere and Planetary Response Experiment (AMPERE), EISCAT measurements of Pederson conductivity and the defense Meteorological Satellite Program (DMSP F18) calculated hemispheric power demonstrate a strong intensification. The low energy particle precipitation from DMSP F18 spacecraft shows an early enhancement for energy less than 1 keV. The particle flux of higher energy responds later which remained elevated for longer duration. The thermospheric density and temperature also experience significant variation during IP shock. The NO molecule and temperature displayed an early enhancement. NO density increased by an order of magnitude with respect to the pre‐event value. About 20% $\%$ increase is noticed in the temperature variation. The atomic oxygen and atomic nitrogen illustrate an early depletion during IP event. The enhanced response of NO cooling to IP shock can be attributed to the combined effects of energy input and subsequent chemical mechanisms. Key Points: Response of thermospheric NO cooling emission to IP shock is studied during 26 January 2017The NO cooling emission shows a strong enhancement due to IP shockIt can be attributed to the increase in low energy particle precipitation, NO density and thermospheric temperature [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]

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]

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]

This paper details the research work carried out over some 35 years at the University of Canterbury's research station near Rolleston in mid-Canterbury New Zealand on radar meteors and their radiants; on the ionic processes that control the duration of both radar and visual long enduring trains; radar work on the aurora; and an extensive programme of radar work mapping the details of the dynamics of the Earth's mesosphere. [ABSTRACT FROM AUTHOR]

The dynamical characteristics of the zonal wavenumber 1 (s = 1) Rossby-gravity (RG) wave are examined using recently available reanalysis data for the whole neutral atmosphere over 16 years. An isolated peak is detected in the two-dimensional zonal wavenumber-frequency spectra that likely corresponds to the theoretically-expected s = 1 RG mode at heights of z = 30, 50, 65, and 80 km. The wave period of the spectral peak is approximately 1.3 days, which is close to one day. The s = 1 RG wave is successfully extracted using a band-pass filter after removing the diurnal tide with quite large amplitudes. The s = 1 RG wave exhibits a characteristic seasonal variation: the geopotential height (GPH) amplitudes are largest in the winter hemisphere in the stratosphere and lower mesosphere while enhancement is observed in both the winter and summer hemispheres in the upper mesosphere. Phase structures are examined in detail for a strong case. The horizontal phase structure at each height is consistent with the normal mode theory. The vertical phase structure is approximately barotropic from the lower stratosphere to the upper mesosphere at 30°N and 30°S where the amplitudes are large. [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 temperature trend in the mesosphere and lower-thermosphere (MLT) region can be regarded as an indicator of climate change. Using temperature profiles measured by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument during 2002–2023 and binning them based on the yaw cycle, we obtain a continuous dataset with a wide local time coverage at 50° S–80° N or 80° S–50° N. The seasonal change in temperature, caused by the forward drift in the SABER yaw cycle, is removed using the climatological temperature of the Naval Research Laboratory's Mass Spectrometer Incoherent Scatter Radar model (MSIS2.0). The corrected temperature without any waves is regarded as the mean temperature. At 50° S–50° N, the cooling trends in the mean temperature are significant in the MLT region and are in agreement with previous studies. The novel finding is that the cooling trends of ≥ l2 K per decade exhibit seasonal symmetry and reach peaks of ≥ 6 K per decade at high latitudes around the summer solstice. Moreover, there are warming trends of 1–2.5 K per decade at an altitude range of 10 -2 –10 -3 hPa , specifically at latitudes higher than 55° N in October and December and at latitudes higher than 55° S in April and August. Over the past 22 years, the mesopause temperature (altitude) in the northern summer polar region has been ∼ 5–11 K (∼ 1 km) colder (lower) than that in the corresponding southern region. The trends in the mesopause temperature are dependent on latitudes and months, but they are negative at most latitudes and reach larger magnitudes at high latitudes. These results indicate that the temperature in the high-latitude MLT region is more sensitive to dynamic changes. [ABSTRACT FROM AUTHOR]

This study analyzes H2O and HDO vertical profiles in the Venus mesosphere using Venus Express/Solar Occultation in the InfraRed data. The findings show increasing H2O and HDO volume mixing ratios with altitude, with the D/H ratio rising significantly from 0.025 at ~70 km to 0.24 at ~108 km. This indicates an increase from 162 to 1,519 times the Earth's ratio within 40 km. The study explores two hypotheses for these results: isotopic fractionation from photolysis of H2O over HDO or from phase change processes. The latter, involving condensation and evaporation of sulfuric acid aerosols, as suggested by previous authors [X. Zhang et al., Nat. Geosci. 3, 834-837 (2010)], aligns more closely with the rapid changes observed. Vertical transport computations for H2O, HDO, and aerosols show water vapor downwelling and aerosols upwelling. We propose a mechanism where aerosols form in the lower mesosphere due to temperatures below the water condensation threshold, leading to deuterium-enriched aerosols. These aerosols ascend, evaporate at higher temperatures, and release more HDO than H2O, which are then transported downward. Moreover, this cycle may explain the SO2 increase in the upper mesosphere observed above 80 km. The study highlights two crucial implications. First, altitude variation is critical to determining the Venus deuterium and hydrogen reservoirs. Second, the altitude-dependent increase of the D/H ratio affects H and D escape rates. The photolysis of H2O and HDO at higher altitudes releases more D, influencing long-term D/H evolution. These findings suggest that evolutionary models should incorporate altitude-dependent processes for accurate D/H fractionation predictions. [ABSTRACT FROM AUTHOR]

The inversion of temperature by detecting the ratio of the intensity of airglow vibrational and rotational spectral lines is a traditional method for obtaining mesopause temperature. However, previous studies have shown that there is significant uncertainty in the temperature inversion using this technology. A spectrograph instrument called the Mesosphere Airglow Fine Spectrometer (MAFS) was previously developed by our research team. Based on the MAFS, this work systematically evaluated the impact of the spectral line extraction methods and residual background noise elimination methods on temperature inversion results of the OH (6-2) Q-branch as the target. The fitting of residual background noise using different numbers of sampling points can cause the inverted temperature to vary by 5 K to 10 K without changing the overall trend. The temperature inversion results obtained using the three-region single-fit method were generally 3 K to 5 K higher than those obtained using the two-region double-fit method. Moreover, the temperature obtained using the Gaussian fitting area varied by approximately 15 K, with changes in the residual background noise fitting method; however, when using a spectrum peak instead of the Gaussian fitting area, this variation decreased to approximately 10 K. When the temperature is higher, both the residual background noise fitting and the spectral line intensity extraction methods have a more significant impact on the uncertainty of temperature inversion. [ABSTRACT FROM AUTHOR]

Solar proton events (SPEs) affect the Earth's atmosphere, causing additional ionization in the high-latitude mesosphere and stratosphere. Ionization rates from such solar proton events maximize in the stratosphere, but the formation of ozone-depleting nitrogen and hydrogen oxides begins at mesospheric altitudes. The destruction of mesospheric ozone is associated with protons with energies of about 10 MeV and higher and will strongly depend on the intensity of the flux of these particles. Most studies investigating the impact of SPEs on the characteristics of the middle atmosphere have been based on either simulations or reanalysis datasets, and some studies have used satellite observations to validate model results. We study the impact of SPEs on cold-season ozone loss in both the northern and southern hemispheres using Aura MLS mesospheric ozone measurements over the 2004 to 2024 period. Here, we show how strongly SPEs can deplete polar mesospheric ozone in different hemispheres and attempt to evaluate this dependence on the intensity of solar proton events. We found that moderate SPEs consisting of protons with an energy of more than 10 MeV and a flux intensity of more than 100 pfu destroy mesospheric ozone in the northern hemisphere up to 47% and in the southern hemisphere up to 33%. For both hemispheres, the peak of winter ozone loss was observed at about 76 km. In the northern hemisphere, maximum winter ozone loss was observed on the second day after a solar proton event, but in the southern hemisphere, winter ozone depletion was already detected on the first day. In the southern hemisphere, mesospheric ozone concentrations return to pre-event levels on the ninth day after a solar proton event, but in the northern hemisphere, even on the tenth day after a solar proton event, the mesospheric ozone layer may not be fully recovered. The strong SPEs with a proton flux intensity of more than 1000 pfu lead to a maximum winter ozone loss of up to 85% in the northern hemisphere, and in the southern hemisphere winter, ozone loss reaches 73%. [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]

We present the high and equatorial mesospheric dynamical response to the minor stratospheric warming that occurred in 2014/15 and compared it with the major stratospheric warming events of 2005/06 and 2008/09. Meteor radar observations over Esrange (67.88oN, 21.07o E), Mohe (52.97oN, 122.53oE) and Kototabang (0.20oS, 100.32oE) have been extensively utilized in addition to ERA 5 Reanalysis datasets. Possessing the unique feature of a vortex displacement and split, the minor warming of 2014/15 was observed on 27 December 2014 followed by four subsequent temperature peaks. During the 2014/15 minor SSW, the tropical stratospheric temperature decreased, causing upwelling similar to the major SSW events 2005/06 and 2008/09. The equatorial mesospheric zonal wind in 2014/15 displayed maximum westward wind with a delay of ~ 19 days after the vortex disruption comparable to the major SSW events. Whereas, over Esrange and Mohe, the westward wind maxima occurred about the vortex disruption during all the warming events. During the minor SSW 2014/15, the ~ 16-day planetary wave is observed to be relatively stronger in the equatorial mesosphere than the high latitude mesosphere. The Eliassen Palm flux diagnostics revealed the intrusion of planetary wave energy from high latitudes to the tropical band, suggesting meridional and equatorward propagation of the planetary waves. [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]

The quasi-2-day wave (Q2DW) is a large temperature disturbance in the austral summer-time mesosphere. Its decay-phase movement and phase-speed are poorly understood. Q2DW events observed by the microwave limb sounder (MLS) onboard the NASA Aura satellite reveal that during the temperature Q2DW's decay-phase, the Q2DW temperature disturbance is still substantial, but its phase-speed reduces exponentially, on average, from around −70 to −20 m/s in around 30-days. Observations also reveal significant interannual variability in these phase-speed values. Q2DW events simulated by the extended Canadian middle atmosphere model (eCMAM) reveal that during the Q2DW decay-phase, the temperature Q2DW phase-speed is strongly correlated with the summer mesosphere easterly jet (SMEJ) wind values. eCMAM also shows that the phase-speed interannual variabilities partially reflect the interannual variabilities of the SMEJ. Planetary-wave diagnostics indicate that during the Q2DW's decay phase, there is still an active transfer of momentum from the SMEJ into Q2DW. The model simulations therefore indicate that the SMEJ plays a substantial role in the decaying temperature Q2DW phase-speed variabilities observed by MLS. This adds to our knowledge of how the SMEJ affects the Q2DW. [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]

The geopotential height observations from the Aura Microwave Limb Sounder show that the quasi‐6‐day wave (Q6DW) events with westward zonal wavenumber 1 (W1) in the Northern Hemisphere (NH) mesosphere and lower thermosphere (MLT) during September 2013 and 2014 had a prolonged lifetime of ∼45–50 days and reached their maximum amplitudes after the September equinox, while the climatological Q6DW‐W1 is completely dissipated in the background atmosphere before the equinoxes. The Eliassen‐Palm flux diagnostic results indicate that Q6DW‐W1 during September 2013 and 2014 obtained an additional source at ∼60°‐80°S and ∼40–50 km as the September equinox approached, which is related to the baroclinic/barotropic instability in this region. Further investigation on the background atmosphere reveals that several stratospheric minor warming (SMW) events occurred in the Antarctic region during September 2013 and 2014 due to the enhancement of wavenumber 1 activities, accompanied by the increase in the stratospheric temperature, changes in the shape of the polar vortex and the reversal in the zonal mean circulation. The strong planetary wave breaking during the September 2013 and 2014 Antarctic SMW events significantly weakened the strength of the polar night jet (PNJ) and made its peak height descend below ∼35 km, which generated the baroclinic/barotropic instability at the new upper boundary of the PNJ (∼60°‐80°S and ∼40–50 km) by an anomalous double‐jet configuration in the background winds. This unusual instability provided additional wave source and energy for the trans‐equatorial propagation of Q6DW‐W1, which finally led to prolonged wave activities in the NH MLT region. Plain Language Summary: Previous studies have suggested that sudden stratosphere warming (SSW) during wintertime can additionally trigger large‐amplitude traveling planetary waves (PWs) in the mesosphere and lower thermosphere (MLT) region. However, due to the lower occurrence rate and intensity, there are few opportunities to study the impact of Southern Hemisphere (SH) stratospheric warming on global PW behaviors, especially for the climatological event of the eigenmode in the Earth's atmosphere. The Aura satellite observations show that the third climatological event of quasi‐6‐day wave (Q6DW) with westward zonal wavenumber 1 (W1) over one year had an exceptionally long duration in the Northern Hemisphere (NH) MLT region in 2013 and 2014, and abnormally peaked after the September equinox, which is not observed in other years since 2004. The reanalysis data confirms that the stratospheric minor warming (SMW) events occurred over the Antarctic region during September 2013 and 2014, which generated instability under an unusual double‐jet structure in the background winds to provide additional energy for the extension of Q6DW‐W1. The current results establish the relationship between the abnormal PW activities in the NH MLT region and the Antarctic SMW events and provide the first observational evidence that the double‐jet structure can influence the westward propagating PW. Key Points: The duration of the quasi‐6‐day wave (Q6DW) with s = 1 (W1) in the Northern Hemisphere (NH) was greatly extended in September 2013 and 2014The Antarctic stratospheric minor warmings provided additional wave sources to disrupt the typical dissipation pattern of Q6DW‐W1 in the NHThe impact of the double‐jet structure in the Southern Hemisphere on the behavior of westward propagating planetary waves is first observed [ABSTRACT FROM AUTHOR]

Neuromorphic sensors have inherently‐fast speeds and low data rates, which potentially make them ideal for the observation of transient sources, such as lightning and sprites. Particularly, for remote observations. In this article, we report the first observations of sprites from the ground with a neuromorphic sensor. These observations are accompanied by measurements with established instruments such as low‐light level and high‐frame rate cameras. We determine that neuromorphic sensors can capture sprites and determine their duration to an accuracy of roughly 6 ms. Average sprite durations were found to be 55 ms within our data set. We have also ascertained that sprites may be too dim for the neuromorphic sensors to resolve the internal spatiotemporal dynamics, at least without the aid of intensifiers. Plain Language Summary: Neuromorphic sensors are a relatively new tool with great potential for imaging transient sources. Neuromorphic sensors track light changes in individual pixels and only store information when the light intensity surpasses a pre‐set threshold, a design choice made with the intent of mimicking how the human eye works. Sprites and lightning are fast, transient light sources and, thus, are an ideal target to test this new technology. In this work, we report the first observations of sprites from the ground with a neuromorphic sensor, and compare these observations with recordings made with an established and well‐understood instrument: an intensified high‐speed camera. Our results show that there is a lot of promise in using this new technology for observation of sprites and lightning, particularly for autonomous, long‐duration observations. We conclude by speculating that the use of neuromorphic sensors in atmospheric sciences will grow substantially in the coming years. They will find many uses in autonomous optical instruments, such as employed for monitoring the aurora, atmospheric gravity waves, and bolide/fireball detection. Key Points: Fast speeds and low data rates potentially make neuromorphic cameras ideal for observation of sprites and lightningNeuromorphic sensors can measure sprite durations in good agreement with high frame rate camerasSprites are very dim making it challenging for the neuromorphic sensor to resolve their internal spatiotemporal dynamics [ABSTRACT FROM AUTHOR]

• Observation of Multiple layer structures (Temperature Sheets) around the tropopause region and lower stratosphere. • Strong hydrostatic stability responsible for the intensity gradient in the radio refractive index. • Temperature sheets in the troposphere have significant implications for weather and atmospheric stability. • Higher-order spectral analysis has been applied to the atmospheric signals to detect the nonlinearities associated with the temperature sheets near the tropopause region. • Large shear region shows the temperature variation, which results in temperature sheet formation near the tropopause region. Temperature sheets have been observed in the lower stratosphere up to 20 km, with strong (positive) temperature gradients within very thin layers. These were observed from vertical profiles obtained from the Stratosphere and Troposphere (ST) radar located in Kolkata. The sheets tend to occur in groups and contribute to the regions of high static stability, inhibiting vertical air motion. Several experiments were carried out to understand the backscattering mechanism of the Very High Frequency (VHF) radar. Many experiments have reported the multiple layered structures at and around the tropopause regions, especially in the lower stratosphere, it is well established that these layered structures are due to presence of strong negative and positive temperature gradients, which is called as temperature sheets. In order to understand the nonlinearities associated with temperature sheets, Higher-order spectral analysis has been carried out on backscattered signals received from the Mesosphere Stratosphere and Troposphere (MST) radar. This approach allowed for a more detailed examination of the nonlinearities associated with temperature sheets. Therefore, this study proposes a new way of distinctly identifying temperature sheets and the process associated with them using Nonlinear Index based measurements. [ABSTRACT FROM AUTHOR]

Abstract Global average upper atmosphere temperature changes linked with the Hunga volcanic eruption (January 2022) are analyzed based on satellite measurements and compared with chemistry‐climate model simulations. Results show stratospheric cooling of −0.5 to −1.0 K in the middle and upper stratosphere during 2022 through middle 2023, followed by stronger cooling (−1.0 to −2.0 K) in the mesosphere after middle 2023. The cooling patterns follow the upward propagating water vapor (H2O) anomalies from Hunga, and similar behavior is found between observations and model simulations. While the stratospheric cooling is mainly due to radiative cooling from enhanced H2O, the mesospheric temperature changes result from ozone losses in the mesosphere, which are in‐turn driven by HOx radicals from Hunga H2O. Comparisons with the multi‐decade climate record show that Hunga impacts on stratospheric temperatures have similar magnitude, but opposite sign, to temperature effects from the large El Chichón (1982) and Pinatubo (1991) volcanic eruptions.

Upward‐propagating solar tides are responsible for a large part of atmospheric variability in the mesosphere and lower thermosphere (MLT) region, and they are also an important source of ionospheric variability. Tides can be divided into the parts that are symmetric and antisymmetric about the equator. Their distinction is important, as the electrodynamic responses of the ionosphere to symmetric and antisymmetric tides are different. This study examines symmetric and antisymmetric tides using 21 years of temperature measurements by the Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry. The main focus is on the solar migrating semidiurnal tide (SW2), which is one of the dominant tides in the MLT region. It is shown that symmetric and antisymmetric parts of SW2 are comparable in amplitude. However, their spatiotemporal characteristics are different. That is, the symmetric part is strongest during March–June at 30–35° latitude, while the antisymmetric part is most prominent during May–September with the largest amplitude at 15–20° latitude. The symmetric and antisymmetric parts can be well described by the first two symmetric and antisymmetric Hough modes, respectively. Amplification is observed in the antisymmetric part during the major sudden stratospheric warmings (SSWs) in January 2006, 2009, 2013 and 2019. Atmospheric model simulations for the 2009 and 2019 SSWs confirm the amplification in the antisymmetric part of SW2. The enhanced antisymmetric tidal forcing explains the previously‐reported asymmetric response of the ionospheric solar‐quiet current system to SSWs. Plain Language Summary: Upward‐propagating tides play an important role in vertical atmospheric coupling, as they transfer energy and momentum from the lower atmosphere to the upper atmosphere. Tides can be divided into two parts; one is symmetric and the other is antisymmetric about the equator. Their effects on the upper atmosphere are different. We examine symmetric and antisymmetric parts of tides separately, using 21 years of satellite‐based temperature observations. We focus on the solar semidiurnal tide, which is known to have a significant impact on the upper atmosphere. We describe how the symmetric and antisymmetric parts of the solar semidiurnal tide depend on latitude and height, and how they change from month to month and from year to year. These results help us understand some aspects of spatiotemporal variability in the upper atmosphere. Key Points: Symmetric and antisymmetric parts of the solar migrating semidiurnal tide are examined using satellite observations of air temperatureThe two parts are comparable in amplitude in the mesosphere and lower thermosphere but their spatiotemporal features are differentAmplification occurs in the antisymmetric part during major sudden stratospheric warmings [ABSTRACT FROM AUTHOR]

Upper mesospheric wind data acquired by the medium frequency radar at Kolhapur (16.7°N, 74.2°E) and Modern-Era Retrospective analysis for Research and Application version 2 (MERRA-2) temperature and wind reanalysis datasets are used to investigate the dynamical response of the low-latitude middle atmosphere to the sudden stratospheric warming (SSW) events that occurred during the 2017-18 and 2018-19 winters. When the amplitude of the high-latitude stratospheric planetary wave (PW) of zonal wavenumber one reduces considerably with the onset of the SSW event, the low-latitude mesospheric PW over Kolhapur also shows a considerable reduction in the PW activity. It is noteworthy that the upper mesospheric winds are eastward for approximately 3 weeks after the onset of SSW. The reduced PW activity is associated with the enhanced gravity wave activity in the meridional wind during the SSW 2018-19 event. The plane of propagation of gravity waves obtained from the perturbation ellipse method suggests that their predominant plane of propagation is in the north-south direction. The persistence of the eastward winds is suggested to be due to the interaction of the northward propagating gravity waves with the mean flow, leading to the eastward acceleration due to the Coriolis force. [ABSTRACT FROM AUTHOR]

We investigate the collection of dust particles in the mesosphere with the MESS (MEteoric Smoke Sampler) instrument that is designed to fly on a sounding rocket. We assume that the ice particles that form in the polar mesosphere between 80 and 85 km altitude in summer contain meteoric smoke particles; and these should be collected with MESS. The instrument consists of a collection device with an opening and closure mechanism, as well as an attached conic funnel which increases the sampling area in comparison to the collection area. Dust particles are collected either directly after passing through the instrument or indirectly after colliding with and fragmenting on the funnel wall. We calculate the dust and fragment trajectories in the detector to determine the collection efficiency for different particle sizes, rocket velocities, and heights, and we find the final velocities and the temperatures of the particles. The considered design has a sampling area of 62.78 mm diameter and a collection area of 20 mm diameter. For the conditions at the rocket launch site in Andøya, Norway, we estimate the collection of meteoric smoke particles contained in the ice particles to be ∼ 10 12 –10 14 amumm-2. The estimated temperatures suggest that the composition of these smoke particles is not affected by the collection. Our calculations also show that keeping the instrument open above 85 km altitude increases the amount of small smoke particles that are directly collected. The directly collected smoke particles are heated as they decelerate, which can affect their composition. [ABSTRACT FROM AUTHOR]

MATS (Mesospheric Airglow/Aerosol Tomography and Spectroscopy) is a Swedish satellite designed to investigate atmospheric dynamics in the mesosphere and lower thermosphere (MLT). By observing structures in noctilucent clouds over polar regions and oxygen atmospheric-band (A-band) emissions globally, MATS will provide the research community with properties of the MLT atmospheric wave field. Individual A-band images taken by MATS's main instrument, a six-channel limb imager, are transformed through tomography and spectroscopy into three-dimensional temperature fields, within which the wave structures are embedded. To identify wave properties, particularly the gravity wave momentum flux, from the temperature field, smaller-scale perturbations (associated with the targeted waves) must be separated from large-scale background variations using a method of scale separation. This paper investigates the possibilities of employing a simple method based on smoothing polynomials to separate the smaller and larger scales. Using using synthetic tomography data based on the HIAMCM (HIgh Altitude Mechanistic general Circulation Model), we demonstrate that smoothing polynomials can be applied to MLT temperatures to obtain fields corresponding to global-scale separation at zonal wavenumber 18. The simplicity of the method makes it a promising candidate for studying wave dynamics in MATS temperature fields. [ABSTRACT FROM AUTHOR]

We present the first continuous observations of three‐dimensional spatially resolved wind fields and atmospheric motions in the mesosphere and lower thermosphere in the mid‐latitudes of the Northern Hemisphere. Our observations were performed during a 19‐month campaign from January 2022 to July 2023 and exploited the composite data from the first multistatic meteor radar system in China and an adjacent monostatic meteor radar. To retrieve the atmospheric kinetic properties, we introduce an improved volume velocity processing method including coordinate transformations and non‐linear constraints to minimize errors. The vertical winds are estimated separately from the iteration of the horizontal divergence to avoid potential biases or contamination from the horizontal winds. The winds and air motions show annual/semiannual variation characteristics within certain altitudes, usually more variable around the equinoxes. The vertical winds are basically within the magnitude of 1 m/s and are upward as expected at the mesopause during the summer, which corresponds to the adiabatic cooling. Plain Language Summary: The mesosphere and lower thermosphere (MLT) is a coupling region between the upper and lower atmosphere and contains various dynamic and chemical processes. Meteor is a convenient and reliable tool for MLT mean horizontal wind observations. Multistatic meteor radars can investigate the more sophisticated dynamic properties of the MLT winds from different viewing angles. In this work, for the first time, we reported 3‐dimensional spatially resolved MLT winds and atmospheric motions in the mid‐latitudes of the Northern Hemisphere. To minimize errors in the retrieved winds, we proposed improvements to the volume velocity processing method by performing coordinate transformations and adding non‐linear constraints. The mean winds and air motions show annual/semiannual variations and are more variable near the equinoxes. Due to the small value of the vertical wind, we estimated it from the integration of the horizontal divergence and obtained a reasonable range of less than 1 m/s. The upwelling in the summer mesopause corresponds to the adiabatic cooling. Key Points: The first continuous observations of 3‐D spatially resolved winds and air motions in the Northern Hemisphere mid‐latitudes are reportedA newly developed volume velocity processing method including coordinate transformations and non‐linear constraints is introducedThe vertical wind is basically less than 1 m/s and is upward at the mesopause during summer, corresponding to the adiabatic cooling [ABSTRACT FROM AUTHOR]

Noctilucent cloud forms in the mesosphere at an altitude of about 83 km and appears annually in northern latitudes between the end of May and the beginning of August. A number of observers, situated at widely dispersed locations in the UK and Western Europe, report their sightings to the British Astronomical Association. This paper includes an analysis of these observations for the summer months of 2023, together with background information about the conditions in the mesosphere which contribute to the formation of polar mesospheric ice: the cause of ground-based sightings of noctilucent cloud. [ABSTRACT FROM AUTHOR]

Observations show that mesospheric clouds (MCs) have been increasing in recent decades, presumably due to increased mesospheric water vapor which is mainly caused by greater methane (CH4) oxidation in the middle atmosphere. Past warm climates such as those of the early Cretaceous and Paleogene periods are thought to have had higher CH4 concentrations than present day, and future CH4 concentrations will also likely continue to rise. Here, idealized atmosphere chemistry‐climate model experiments forced with strong polar‐amplified sea‐surface temperatures and elevated carbon dioxide (CO2) and CH4 concentrations predict a substantial spreading of MCs to middle and low latitudes, well beyond regions where they are currently found. Sensitivity tests show that increased water vapor from CH4 oxidation and cooling from increased CO2 and CH4 concentrations create favorable conditions for cloud formation, producing MC fractions of 0.02 in the low latitudes and 0.1 in the mid‐latitudes in the Northern Hemisphere when CH4 concentration is 16× higher than pre‐industrial. Further increases in CH4 result in a monotonic increase in low‐ and mid‐latitude MCs. A uniform surface ocean warming, changes in polar amplification, or the solar constant do not significantly affect our results. While the appearance of these clouds is interesting, their ice and liquid water content is not sufficient to cause a significant radiative effect. On the other hand, dehydration of the mesosphere due to these low‐ and mid‐latitude MCs could potentially lead to a reduction in atomic hydrogen, thereby affecting mesospheric ozone concentration, although further study is required to confirm this. [ABSTRACT FROM AUTHOR]

A theoretical model is presented that describes the settling regime of plasma-dust clouds in the mesosphere of Mars. The values of the characteristic sizes of cloud dust particles predicted by the model are calculated. It is shown that an important factor influencing the formation of plasma-dust structures in the Martian atmosphere is the Rayleigh–Taylor instability, which limits (from above) the permissible sizes of dust particles in the cloud. [ABSTRACT FROM AUTHOR]

Quasi 16‐day waves (Q16DWs) are a prominent and recurrent phenomenon in the middle atmosphere, typically observed over winter mid and high latitudes. This study investigates the intense Q16DW event during the 2018–2019 Northern Hemisphere (NH) winter, and explores its propagation in the middle atmosphere and its notable influence on the E‐region ionosphere. Long‐term geopotential height estimates of Aura Microwave Limb Sounder (MLS) reveal that the wave activity under consideration exhibited the largest amplitudes in the mesosphere for past 16 years. An analysis of wind data obtained from medium frequency (MF) and meteor radars, as well as from Modern‐Era Retrospective analysis for Research and Applications, Version 2 (MERRA‐2) reanalysis, reveals the presence of a westward‐propagating Q16DW with zonal wavenumber 1 exhibiting notable asymmetry about the equator, with the majority of the wave activity being confined to the NH. The prominently large amplitudes and vertical wavelengths of the wave suggest potential for the wave propagation to extend deep into the E‐region ionosphere. Swarm satellite observations reveal concurrent ∼16‐day oscillations in the eastward component of the geomagnetic field at low latitudes. These oscillations can be attributed to the periodic variations in interhemispheric field‐aligned currents (IHFACs). The ∼16‐day oscillations in the IHFACs are likely a consequence of asymmetric wind‐dynamo action, which is directly or indirectly associated with the Q16DW. These findings suggest that planetary waves originating in the middle atmosphere can cause interhemispheric coupling in the ionosphere. Plain Language Summary: This study examines the strong quasi‐16‐day wave activity that occurred during the winter of 2018–2019 over mid‐to‐high latitudes. A data set consisting of medium frequency (MF) and meteor radar winds, reanalysis data, Aura Microwave Limb Sounder (MLS) and Swarm geomagnetic field measurements is utilized to characterize the wave. The simultaneous occurrence of wave signatures in the middle atmosphere and the ionosphere suggests some atmosphere‐ionosphere coupling. The wave signatures observed in the eastward component of the geomagnetic field are attributed to interhemispheric field‐aligned currents. Key Points: Strong quasi‐16‐day wave signatures in middle atmospheric temperatures and winds during the northern hemispheric winter of 2018–2019Simultaneous occurrence of the wave in the eastward component of the geomagnetic fieldPlanetary wave oscillations of ∼16‐day period observed in the interhemispheric field aligned currents [ABSTRACT FROM AUTHOR]

Geographical and seasonal variations of gravity wave events in the upper mesosphere were investigated using the nightglow imaging data obtained by the Visible and near-Infrared Spectral Imager (VISI) on the Ionosphere, Mesosphere, upper Atmosphere and Plasmasphere (IMAP) onboard the International Space Station (ISS). The nadir-imaging data of the O2(0–0) atmospheric band (762 nm) with the typical emission peak around 95 km altitude was used to investigate small-scale waves (horizontal wavelengths less than ~ 200 km) on a global scale. To detect gravity wave events, the variance of high-pass filtered nightglow images within a local 100 km radius was evaluated, with a threshold set at three times the standard deviation from the average variance of the background level. A data screening algorithm that evaluates the variance of upwelling contamination light emission was also introduced to remove contaminated data. Applying the variance filter and data screening algorithm to a nearly 3-year data set, from November 2012 to August 2015, occurrence maps of wave events for four seasons were derived. The occurrence maps show a higher frequency of wave events in winter high latitudes (> 40° N/S), considerably attributed to gravity wave activity associated with the polar night jet. Hot spots were observed near orographic sources in winter high latitudes, including the eastern part of North America, Europe, and the southern Andes. In the summer hemisphere, hot spots were detected at mid-to-high latitudes such as North America, Europe, and the eastern side of the Eurasian continent, and at equatorial latitudes just above the intertropical convection zone (ITCZ). They are likely gravity waves from deep convection that arise from mid-latitude summertime thunderstorms and the ITCZ, respectively. During the equinox seasons, hot spots were detected near convective sources such as the Amazon Rainforest, Congo Rainforest, and the Indochina peninsula. [ABSTRACT FROM AUTHOR]

Polar mesospheric cloud (PMC) data obtained from the Aeronomy of Ice in the Mesosphere (AIM)/Cloud Imaging and Particle Size (CIPS) experiment and Himawari-8/Advanced Himawari Imager (AHI) observations are analyzed for multi-year climatology and interannual variations. Linkages between PMCs, mesospheric temperature, and water vapor (H2O) are further investigated with data from the Microwave Limb Sounder (MLS). Our analysis shows that PMC onset date and occurrence rate are strongly dependent on the atmospheric environment, i.e., the underlying seasonal behavior of temperature and water vapor. Upper-mesospheric dehydration by PMCs is evident in the MLS water vapor observations. The spatial patterns of the depleted water vapor correspond to the PMC occurrence region over the Arctic and Antarctic during the days after the summer solstice. The year-to-year variabilities in PMC occurrence rates and onset dates are highly correlated with mesospheric temperature and H2O. They show quasi-quadrennial oscillation (QQO) with 4–5-year periods, particularly in the southern hemisphere (SH). The combined influence of mesospheric cooling and the mesospheric H2O increase provides favorable conditions for PMC formation. The global increase in mesospheric H2O during the last decade may explain the increased PMC occurrence in the northern hemisphere (NH). Although mesospheric temperature and H2O exhibit a strong 11-year variation, little solar cycle signatures are found in the PMC occurrence during 2007–2021. [ABSTRACT FROM AUTHOR]

The impact of geomagnetic storms on the mesosphere temperature has been controversial and lacks direct observational evidence. The intricate chemical and physical processes in the mesosphere, combined with the scarcity of observations, pose challenges to achieving a thorough comprehension of storm‐induced turbulences in this region. Currently, some investigations have characterized temperature responses during geomagnetic storms and the focus has largely been on changes above mesopause (∼90 km). In this work, the responses of temperature in the mesosphere (75–95 km) to the storms on September 8 and 15, 2003 and its causes are studied using observations from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. It is found that (a) temperature responses for the moderate storm on September 15 manifest as increases within the latitude range of 80°S to 65°S, with peak values decreasing from approximately 15 K to around 7 K as latitude decreases, while for the minor strom of September 8 temperature changes only occur at ∼80°S with peaks of 7 K and −10 K, (b) the temperature responses can be transmitted down to 76–81 km, depending on the latitude and storm magnitude, and (c) there are significant fluctuations in both ozone (exceed 45%) and atomic oxygen (exceed 90%) after storm onset, highly correlated with temperature temporal and spatial variations. We suggest that the increases in ozone caused by the increases in atomic oxygen concentrations are the major contributor to rising temperature. Plain Language Summary: Geomagnetic storms may have a non‐negligible effect on the atmospheric circulation, neutral temperature, winds, and composition of the mesosphere and lower thermosphere. However, there is a lack of understanding of the perturbations and its chemical and physical processes in the mesosphere during geomagnetic storms. In this paper, we analyze temperature, ozone, and atomic oxygen observations acquired from the SABER instrument aboard the TIMED satellite. Our work demonstrate that variations in mesosphere temperature occur at high latitudes and are correlated with the magnitude of the storm. Additionally, significant disturbances in ozone and atomic oxygen are observed from 80°S to 65°S latitude. We propose that the variations in mesosphere temperature at high latitudes are dominated to changes in ozone caused by the heightened concentrations of atomic oxygen. Key Points: Temperatures of the mesosphere observed by SABER have remarkable responses to the geomagnetic storms of 8 and 15 September 2003The mesosphere temperature variations occur at high latitudes and can be conveyed to 76–81 km, depending on the latitude and storm magnitudeOzone variations are responsible for the mesosphere temperature variations at high latitudes during storms [ABSTRACT FROM AUTHOR]

Future space missions dedicated to measuring CO2 on a global scale can make advantageous use of the O2 band at 1.27 μm to retrieve the air column. The 1.27 μm band is close to the CO2 absorption bands at 1.6 and 2.0 μm, which allows a better transfer of the aerosol properties than with the usual O2 band at 0.76 μm. However, the 1.27 μm band is polluted by the spontaneous dayglow of the excited state O2 (1∆), which must be removed from the observed signal. We investigate here our quantitative understanding of the O2(1∆) dayglow with a chemistry‐transport model. We show that the previously reported −13% deficit in O2(1∆) dayglow calculated with the same model is essentially due a −20% to −30% ozone deficit between 45 and 60 km. We find that this ozone deficit is due to excessively high temperatures (+15 K) of the meteorological analyses used to drive the model in the mesosphere. The use of lower analyzed temperatures (ERA5), in better agreement with the observations, slows down the hydrogen‐catalyzed and Chapman ozone loss cycles. This effect leads to an almost total elimination of the ozone and O2(1∆) deficits in the lower mesosphere. Once integrated vertically to simulate a nadir measurement, the deficit in modeled O2(1∆) brightness is reduced to −4.2 ± 2.8%. This illustrates the need for accurate mesospheric temperatures for a priori estimations of the O2(1∆) brightness in algorithms using the 1.27 μm band. Plain Language Summary: Future space missions dedicated to measuring CO2 in the atmosphere can make advantageous use of the O2 absorption band at 1.27 μm. Indeed, the 1.27 μm band is close to the wavelengths where CO2 absorbs the solar radiation, which allows more precise calculations. However, the 1.27 μm band is polluted by the spontaneous infrared emission of O2 in its excited state called O2(1∆), which occurs in the upper atmosphere and must be removed from the observed signal. We investigate here our understanding of the O2(1∆) emission with a chemistry‐transport model. When compared to observations, there is a −13% deficit in O2(1∆) in our model. We find that this deficit is due to excessively high temperatures (+15 K) used in the calculations. The use of temperatures more in line with the observations slows down the ozone‐destroying chemical cycles, which leads to an almost total elimination of the O2(1∆) deficit. Once integrated vertically to simulate a satellite measurement, the deficit in modeled O2(1∆) brightness is reduced to −4.2 ± 2.8%. This illustrates the need for accurate temperatures in the middle atmosphere for a reliable prediction of the O2(1∆) emission occurring in the 1.27 μm band. Key Points: Three‐dimensional simulations of the O2(1∆) dayglow at 1.27 μm are compared to satellite dataThe model and observations show that O3 and O2(1∆) near the stratopause and in the lower mesosphere are anti‐correlated with temperatureThe O2(1∆) brightness at 1.27 μm calculated with reanalyzed temperatures is on average −4.2 ± 2.8% lower than the observations [ABSTRACT FROM AUTHOR]