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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]

The variation trends and characteristics of polar mesospheric clouds (PMCs) are important for studying the evolution of atmospheric systems and understanding various atmospheric dynamic processes. Through observation and analysis of PMCs, we can gain a comprehensive understanding of the mechanisms driving atmospheric processes, providing a scientific basis and support for addressing climate change. Ultraviolet (UV) imaging technology, adopted by the Cloud Imaging and Particle Size (CIPS) instrument on board the Aeronomy of Ice in the Mesosphere (AIM) satellite, has significantly advanced the research on PMCs. Due to the retirement of the AIM satellite, there is currently no concrete plan for next-generation instruments based on the CIPS model, resulting in a discontinuity in the observation data sequence. In this study, we propose a compact and cost-effective wide-field-of-view ultraviolet imager (WFUI) that can be integrated into various satellite platforms for future PMC observation missions. A forward model was built to evaluate the detection capability and efficiency of the WFUI. CIPS and Solar Occultation for Ice Experiment (SOFIE) data were fused to reconstruct a three-dimensional PMC scene as the input background. Based on the scattering and extinction characteristics of ice particles and atmospheric molecules, the radiative transfer was calculated using the solar radiation path through the atmosphere and PMCs. The optical system and satellite platform parameters of the WFUI were selected according to CIPS, enabling the calculation of the number of photons received by the WFUI. The actual detection signal is then simulated by photoelectric conversion, and the PMC information can be obtained by removing detector noise. Subsequently, a comparison with the input background field was conducted to compute and analyze the detection efficiency. Additionally, a sensitivity analysis of the instrument and platform parameters was conducted. Simulations were performed for both individual orbits and for the entire PMC seasons. The research results demonstrate that the WFUI performs well in PMC detection and has high detection efficiency. Statistical analysis of the detection efficiency using data from 2008 to 2012 revealed an exponential relationship between the ice water content (IWC) of PMCs and detection efficiency. During the initial and final durations of the PMC season, when the IWC was relatively low, the detection efficiency remained limited. However, as the season progressed and the IWC increased, the detection efficiency significantly improved. We note that regions at lower latitudes exhibited a lower IWC and, consequently, lower detection efficiency. In contrast, regions at higher latitudes, with a greater IWC, demonstrated better detection efficiency. Additionally, the sensitivity analysis results suggest that increasing the satellite orbit altitude and expanding the field of view (FOV) of the WFUI both contribute to improving the detection efficiency. [ABSTRACT FROM AUTHOR]

We explore the effects of lower thermospheric water vapor deposited by launch vehicle plumes on polar mesospheric cloud (PMC) frequencies at 80°N. We use July‐averaged PMC frequencies from 2007 to 2022 from the Cloud Imaging and Particle Size (CIPS) instrument on NASA's Aeronomy of Ice in the Mesosphere (AIM) satellite. Launch sites worldwide are typically located near northern mid‐latitudes. Using the orbital launch record for the same time period, we find that the number of launches correlates with PMC frequencies with a coefficient of r = 0.60, which increases to r = 0.75 when only selecting launches from 2.5 to 21.5 local time (LT), indicating a weak LT dependence on global‐scale transport to 80°N. To support our findings, we use meridional winds from the Michelson Interferometer for Global High‐resolution Imaging experiment on NASA's Ionospheric Connection Explorer satellite and winds from the Horizontal Wind Model climatology to interpret the northward motion of air parcels at 105 km. We find the launch LT window that maximizes the correlation coefficient to be consistent with the expected maximum northward motion from the diurnal variation of mid‐latitude meridional winds. Comparisons with Microwave Limb Sounder satellite observations of upper mesospheric temperature and water vapor reveal a strong dependence of cloud frequency on water vapor (r = 0.86) but not on temperature (r = −0.26), indicating that water vapor is the primary source of PMC variability for the bright PMCs at 80°N. We therefore find that launch vehicle plumes originating primarily from northern mid‐latitudes modulate PMC frequency at 80°N in July. Plain Language Summary: Formation of clouds in the mesosphere in the polar summer depends heavily on atmospheric water vapor. Main engine exhaust plumes from space shuttles and other launch vehicles contain many tons of water vapor which can be transported poleward at an altitude of ∼105 km. Over several days this water vapor descends to ∼80–85 km and aids in the formation of summertime polar mesospheric clouds (PMCs). This work shows how the number of launches per year correlates with the mean interannual PMC frequency averaged over July at 80°N latitude. The correlation maximizes for vehicles launched from 2.5 to 21.5 local time. To provide additional important evidence for the poleward transport of plumes launched at these local times, we use wind data to quantify the maximum northward motion of an exhaust plume over the diurnal cycle near 105 km altitude. We also use satellite temperature and water vapor data at PMC altitudes to show that bright PMCs vary more with the water vapor content than with the air temperature. We conclude that the total water vapor deposited by summertime space traffic worldwide modulates the PMC frequency at 80°N. Key Points: The correlation between orbital rocket launches and polar mesospheric clouds at 80°N in July from 2007 to 2022 is quantifiedMeridional wind, temperature, and water vapor data are used to support causal interpretation of the correlation coefficientsEvidence indicates that water vapor in space traffic exhaust plumes modulates frequencies of polar mesospheric clouds at 80°N [ABSTRACT FROM AUTHOR]

While 30 Dor C is a unique superbubble in the Large Magellanic Cloud for its luminous non-thermal X-ray emission, the thermal X-ray emission it emanates has not yet been thoroughly investigated and well constrained. Based on the separate ∼1 Ms deep XMM–Newton and Chandra observations, we report the discovery of the thermally emitting plasma in some portions of the western half of 30 Dor C. The thermal emission can be reproduced by a collisional-ionization-equilibrium plasma model with an average electron temperature of ∼0.4 keV. We find a significant overabundance of the intermediate-mass elements such as O, Ne, Mg, and Si, which may be indicative of a recent supernova explosion in 30 Dor C. Dynamical properties in combination with the information of the OB association LH 90 suggest that the internal post-main-sequence stars dominate the power of the superbubble and blow it out in the past ∼1 Myr. [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]

This paper considers observational results of noctilucent clouds (NLC) performed by Michael Noble, who was an outstanding amateur observer. One of his special efforts was driving outside Edmonton to avoid weather clouds, hence obtaining clear skies to get more probability of observing NLC. It is demonstrated that this technique can greatly increase the number of NLC-active nights. Doubling the number of sites used or quadrupling the area between all sites can increase seasonal NLC-active night totals by half. [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]

Abstract 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.

The variation trends and characteristics of polar mesospheric clouds (PMCs) are important for studying the evolution of atmospheric systems and understanding various atmospheric dynamic processes. Through observation and analysis of PMCs, we can gain a comprehensive understanding of the mechanisms driving atmospheric processes, providing a scientific basis and support for addressing climate change. Ultraviolet (UV) imaging technology, adopted by the Cloud Imaging and Particle Size (CIPS) instrument onboard the Aeronomy of Ice in the Mesosphere (AIM) satellite, has significantly advanced the research on PMCs. Due to the retirement of the AIM satellite, there is currently no concrete plan for next-generation instruments based on the CIPS model, resulting in a discontinuity in the observation data sequence. In this study, we propose a compact and cost-effective wide field-of-view ultraviolet imager (WFUI) that can be integrated into various satellite platforms for future PMCs observation missions. A forward model was built to evaluate the detection capability and efficiency of the WFUI. The fusion of CIPS and solar occultation for ice experiment (SOFIE) data was employed to reconstruct a three-dimensional PMCs scene as the input background. Based on the scattering and extinction characteristics of ice particles and atmospheric molecules, the radiative transfer was calculated using the solar radiation path through the atmosphere and PMCs. The optical system and satellite platform parameters of the WFUI were selected according to the CIPS, enabling the calculation of the number of photons received by the WFUI. The actual detection signal is then simulated by photoelectric conversion, and the PMCs information can be obtained by removing detector noise. Subsequently, a comparison with the input background field was conducted to compute and analyze the detection efficiency. Additionally, a sensitivity analysis of the instrument and platform parameters was conducted. Simulations were performed for both individual orbits and for the entire PMCs seasons. The research results demonstrate that using data from 2008 to 2012 revealed an exponential relationship between the ice water content (IWC) of PMCs and detection efficiency. During the initial and final durations of the PMCs season, when the IWC was relatively low, the detection efficiency the WFUI performs well in PMCs detection and has high detection efficiency. Statistical analysis of the detection efficiency remained limited. However, as the season progressed and the IWC increased, the detection efficiency significantly improved. We note that regions at lower latitudes exhibited a lower IWC and, consequently, lower detection efficiency. In contrast, regions at higher latitudes, with a greater IWC, demonstrated better detection efficiency. Additionally, the sensitivity analysis results suggest that increasing the satellite orbit altitude and expanding the field of view (FOV) of the WFUI both contribute to improving the detection efficiency. [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 expected increase in climate change related methane emissions will result in an increase in middle atmospheric water vapor abundance. This will in turn amplify the brightness of noctilucent clouds (NLC). To examine how NLC will impact the absorption of solar radiation, we utilized both an atmospheric background model and a microphysical model spanning the period from 1950 to 2100. At a latitude of 69 ± 3°N, UV absorption at λ = 126 nm is projected to rise from ∼3% to ∼7%. In specific regions, the absorption may spike to approximately 30% by the year 2100. In the visible spectrum, we observe an absorption increase from 0.0030% in 1950 to 0.020% by 2100. Local absorption reach up to 0.35% by the year 2100. These trends are similar at 79 ± 3°N, but are smaller at 58 ± 3°N. Future average absorptions are comparable to solar cycle fluctuations, but local increases are significantly more pronounced. The ice mass contained in NLC is projected to surge from 677 to 1871 tons between 1950 and 2100. Plain Language Summary: Noctilucent clouds (NLC) consist of water ice particles and appear in the summer season in the upper mesosphere at high/middle latitudes where temperatures are very low. Methane is photochemically converted to water vapor in the middle atmosphere. Therefore, the future increase of methane concentration will lead to an increase in water vapor, and to an enhancement of NLC occurrence and brightness. We apply an atmospheric background model and a microphysical ice particle model to study the associated absorption of solar radiation. At 69°N mean absorptions in the UV will increase from ∼3% to ∼7% from 1950 to 2100, respectively. Locally, the absorption can increase to ∼30% in 2100. In the visible (λ = 532 nm) the corresponding numbers are 0.0030% (1950) to 0.020% (2100), that is, an increase by a factor of ∼7, and local maxima up to 0.35% in 2100. Mean absorptions are comparable to variations throughout a solar cycle, but may locally be much larger. Effects on the photochemistry are therefore expected. The total amount of ice mass bound in NLC also increases with time, namely from 677 tons in 1950 to 1871 tons in 2100. NLC will be easier to observe by naked eye, that is, they will be more frequent and brighter. Key Points: Noctilucent clouds (NLC) are ice clouds in the summer mesopause region at middle and polar latitudesThe expected methane related increase in water vapor at NLC heights will lead to more and larger ice particlesLarger ice particles will lead to an enhanced absorption of solar radiation [ABSTRACT FROM AUTHOR]

This article analyzes fields of noctilucent clouds (NLCs) over the territory of the Russian Federation recorded by a ground-based network of cameras and by aircraft photography over two nights in June 2021. It is demonstrated that aircraft photography can significantly improve the coverage of the territory of probable appearance of NLCs. The NLC fields are compared with model regions of water vapor condensation derived from satellite measurements of temperature and water vapor mixing ratio. Practical steps are proposed for the development of aircraft observations of NLCs. [ABSTRACT FROM AUTHOR]

MATS (Mesospheric Airglow/Aerosol, Tomography & Spectroscopy) is a Swedish satellite designed to investigate atmospheric dynamics in the Mesosphere and Lower Thermosphere (MLT). From observing structures in noctilucent clouds over polar regions, and oxygen A-band emissions globally, MATS will supply the research community with properties of the MLT atmospheric wave field. Individual A-band images taken by the MATS main instrument, a 6-channel limb imager, are through tomography and spectroscopy turned into three-dimensional temperature fields in which the wave structures are embedded. To identify wave properties, in particular 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 by 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. By using synthetic tomography data based on the HIAMCM (High Altitude Mechanistic Circulation Model) we demonstrate that smoothing polynomials can be applied to MLT temperatures to obtain fields corresponding to a global scale separation at zonal wavenumber 18. The simplicity of the method makes it a promising candidate for studying wave dynamics in the MATS temperature fields. [ABSTRACT FROM AUTHOR]

Noctilucent clouds (NLC) are sensitive indicators in the upper mesosphere, reflecting changes in the background atmosphere. Studying NLC responses to the solar cycle is important for understanding solar-induced changes and assessing long-term climate trends in the upper mesosphere. Additionally, it enhances our understanding of how increases in greenhouse gas concentration in the atmosphere impact the Earth's upper mesosphere and climate. This study presents long-term trends in the response of NLC and the background atmosphere to the 11-year solar cycle variations. We utilised model simulations from the Leibniz Institute Middle Atmosphere (LIMA) and the Mesospheric Ice Microphysics and Transport (MIMAS) over 170 years (1849 to 2019), covering 15 solar cycles. Background temperature and water vapour (H2O) exhibit an apparent response to the solar cycle, with an enhancement post-1960, followed by an acceleration of greenhouse gas concentrations. NLC properties, such as maximum brightness (βmax), calculated as the maximum backscatter coefficient, altitude of βmax (referred to as NLC altitude) and ice water content (IWC), show responses to solar cycle variations that increase over time. This increase is primarily due to an increase in background water vapour concentration caused by an increase in methane (CH4). The NLC altitude positively responds to the solar cycle mainly due to solar cycle-induced temperature changes. The response of NLC properties to the solar cycle varies with latitude, with most NLC properties showing larger and similar responses at higher latitudes (69° N and 78° N) than mid-latitudes (58° N). [ABSTRACT FROM AUTHOR]

Some aspects of increasing the efficiency of noctilucent clouds ground-based observations are considered. The study of such objects is highly relevant in connection with the general problems of climate change. It is shown that the limitations on the possibility of registering the phenomenon in the optical range are associated both with the relatively low brightness of clouds of this type against the background of the twilight segment, and with the strong absorption of light by dust aerosol in the surface layer of the atmosphere. The paper substantiates the idea that the transition to observations of noctilucent clouds in the near infrared range will increase the contrast of their images in the twilight segment. This will make it possible to detect noctilucent clouds during civil twilight, including at low altitudes above the horizon. To test this assumption, shooting was carried out during the 2022 season using a CANON 2000 D camera and RG780 and RG830 infrared filters. The images revealed features morphologically similar to noctilucent clouds. An analysis of the images showed that they can hardly be associated with tropospheric clouds or anthropogenic formations. The results obtained were compared with ground-based observation data from other points, as well as with satellite information on the state of noctilucent cloud fields. This comparison showed that noctilucent clouds, which were not detected in visible light images, were highly likely to be detected in near-infrared images. The prospects for the application and development of the proposed method for ground-based registration of noctilucent clouds are also considered. [ABSTRACT FROM AUTHOR]

We investigate the collection of dust from the mesosphere with the MESS instrument that is designed to fly on a sounding rocket. The instrument consists of a collection device with an opening and closure mechanism and an attached conic funnel. Attaching the funnel increases the sampling area in comparison to the collection area. The instrument will collect primary particles that directly hit the collection area and secondary particles that form from mesospheric dust hitting the funnel. We simulate the entry and impact of dust onto the detector considering their trajectories in the airflow and the fragmentation at the funnel. We estimate the collection efficiency of the instrument and the impact energy of particles at the collecting area. The design considered has a sampling area of 5 cm diameter and a collection area of 1.8 cm diameter. We use the results of the calculations to estimate the amount of dust that MESS (MEteoric Smoke Sampler) can collect during a rocket flight. We consider meteoric smoke particles (MSP) based on a model of the MSP distribution. In addition we assume that water ice particles that form close to the mesopause contain a fraction of smaller MSP. The water ice sublimates during the collection or later during rocket flight so that only refactory material remains. Assuming the collected particles contain 3% volume fraction of MSP, we find that the instrument would collect of the order of 1014 to 1015 amu of refractory MSP particles. The estimate basis on the assumption that the ice components are melting and the flow conditions in the instruments are for typical atmospheric pressures at 85 km. Aside from the instrument conditions that we investigate in this paper, our estimate of the mass that we expect to collect with MESS applies the results from a particle transport model for the meteoric smoke particles and from the description of noctilucent cloud particles based on published model and observational results. [ABSTRACT FROM AUTHOR]

Abstract The expected increase in climate change related methane emissions will result in an increase in middle atmospheric water vapor abundance. This will in turn amplify the brightness of noctilucent clouds (NLC). To examine how NLC will impact the absorption of solar radiation, we utilized both an atmospheric background model and a microphysical model spanning the period from 1950 to 2100. At a latitude of 69 ± 3°N, UV absorption at λ = 126 nm is projected to rise from ∼3% to ∼7%. In specific regions, the absorption may spike to approximately 30% by the year 2100. In the visible spectrum, we observe an absorption increase from 0.0030% in 1950 to 0.020% by 2100. Local absorption reach up to 0.35% by the year 2100. These trends are similar at 79 ± 3°N, but are smaller at 58 ± 3°N. Future average absorptions are comparable to solar cycle fluctuations, but local increases are significantly more pronounced. The ice mass contained in NLC is projected to surge from 677 to 1871 tons between 1950 and 2100.

We present a new version of O3 data retrieved from the three Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) observation modes that we refer to for simplicity as the modes of the middle atmosphere (middle atmosphere, MA; upper atmosphere, UA; and noctilucent cloud, NLC). The O3 profiles cover altitudes from 20 up to 100 km for the daytime and up to 105 km at nighttime, for all latitudes, and the period 2005 until 2012. The data have been obtained with the IMK–IAA (Institute of Meteorology and Climate Research and Instituto de Astrofísica de Andalucía) MIPAS level-2 data processor and are based on ESA version-8 re-calibrated radiance spectra with improved temporal stability. The processing included several improvements with respect to the previous version, such as the consistency of the microwindows and spectroscopic data with those used in the nominal-mode V8R data, the O3 a priori profiles, and updates of the non-local thermodynamic equilibrium (non-LTE) parameters and the nighttime atomic oxygen. In particular, the collisional relaxation of O3 (v1 , v3) by the atomic oxygen was reduced by a factor of 2 in order to obtain a better agreement of nighttime mesospheric O3 with "non-LTE-free" measurements. Random errors are dominated by the measurement noise with 1 σ values for single profiles for the daytime of < 5 % below ∼ 60 km , 5 %–10 % between 60 and 70 km , 10 %–20 % at 70–90 km , and about 30 % at 95 km. For nighttime, they are very similar below 70 km but smaller above (10 %–20 % at 75–95 km , 20 %–30 % at 95–100 km and larger than 30 % above 100 km). The systematic error is ∼ 6 % below ∼ 60 km (dominated by uncertainties in spectroscopic data) and 8 %–12 % above ∼ 60 km , mainly caused by non-LTE uncertainties. The systematic errors in the 80–100 km range are significantly smaller than in the previous version. The major differences with respect to the previous version are as follows: (1) the new retrievals provide O3 abundances in the 20–50 km altitude range that are larger by about 2 %–5 % (0.2–0.5 ppmv); (2) O3 abundances were reduced by ∼ 2 %–4 % between 50 and 60 km in the tropics and mid-latitudes; (3) O3 abundances in the nighttime O3 minimum just below 80 km were reduced, leading to a more realistic diurnal variation; (4) daytime O3 concentrations in the secondary maximum at the tropical and middle latitudes (∼ 40 %, 0.2–0.3 ppmv) were larger; and (5) nighttime O3 abundances in the secondary maximum were reduced by 10 %–30 %. The O3 profiles retrieved from the nominal mode (NOM) and the middle-atmosphere modes are fully consistent in their common altitude range (20–70 km). Only at 60–70 km does daytime O3 of NOM seem to be larger than that of MA/UA by 2 %–10 %. Compared to other satellite instruments, MIPAS seems to have a positive bias of 5 %–8 % below 70 km. Noticeably, the new version of MIPAS data agrees much better than before with all instruments in the upper mesosphere–lower thermosphere, reducing the differences from ∼± 20 % to ∼± 10 %. Further, the diurnal variation in O3 in the upper mesosphere (near 80 km) has been significantly improved. [ABSTRACT FROM AUTHOR]

Motivated by an improved European Space Agency (ESA) version of calibrated Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) spectra (version 8.03), we have released version 8 of MIPAS temperatures and pointing information retrieved from 2005–2012 MIPAS measurements at 12–15 µ m in the Middle Atmosphere (MA), Upper Atmosphere (UA) and Noctilucent Cloud (NLC) measurement modes. The Institute of Meteorology and Climate Research–Instituto de Astrofisica de Andalucia (IMK–IAA) retrieval processor in use considers non-local thermodynamic equilibrium (non-LTE) emission explicitly for each limb scan. This non-LTE treatment is essential to obtain accurate temperatures above the mid-mesosphere because at the altitudes covered, up to 115 km, the simplified climatology-based non-LTE treatment employed for the Nominal (NOM) measurements is insufficient. Other updates in MA/UA/NLC version 8 non-LTE temperature retrievals from previous data releases include more realistic atomic oxygen and carbon dioxide abundances, an updated set of spectroscopic data, an improved spectral shift retrieval, a continuum retrieval extended to altitudes up to 58 km, consideration of an altitude-dependent radiance offset retrieval, the use of wider microwindows above 85 km to capture the offset, an improved accuracy in forward model calculations, new a priori temperature information, improved temperature horizontal gradient retrievals and the use of MIPAS version 5 interfering species where available. The resulting MIPAS MA/UA/NLC IMK–IAA temperature dataset is reliable for scientific analysis in the full measurement vertical range for the MA (18–102 km) and the NLC (39–102 km) observations and from 42 to 115 km for the UA observations. The random temperature errors, dominated by the instrumental noise, are typically less than 1 K below 60 km, 1–3 K at 60–70 km, 3–5 K at 70–90 km, 6–8 K at 90–100 km, 8–12 K at 100–105 km and 12–20 K at 105–115 km. Random pointing correction errors, also mainly arising from instrumental noise, are on average 50 m for tangent altitudes up to 60 km and decrease linearly to values smaller than 20 m for altitudes above 95 km. The vertical resolution is 3 km at altitudes below 50 km, 3–5 km at 50–70 km, 4–6 km at 70–90 km, 6–10 km at 90–100 km and 8–11 km at 100–115 km. The systematic errors in retrieved temperatures below 75 km are driven by uncertainties in the CO 2 spectroscopic data and, above 80 km, by uncertainties in the non-LTE model parameters (including collisional rates and atomic oxygen abundance) and the CO 2 abundance. These lead to systematic temperature errors of less than 0.7 K below 55 km, 1 K at 60–80 km, 1–2 K at 80–90 km, 3 K at 95 km, 6–8 K at 100 km, 10–20 K at 105 km and 20–30 K at 115 km. Systematic errors in the tangent altitude correction, mainly arising from CO 2 spectroscopic uncertainties, are 250 m at 20 km, 200 m at 40–60 km, 100 m at 80 km and smaller than 50 m above 90 km. The consistency between the MA/UA/NLC and the NOM IMK–IAA datasets is excellent below 70 km (typical 0.5–1 K differences). The comparison of this temperature dataset with co-located Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature measurements shows excellent agreement, with differences typically within 1.5 K below 90 km, 1–3 K at 90–95 km, 1–5 K at 95–100 km, 1–8 K at 100–105 km and 1–10 K above. The agreement with SABER improves with respect to previous MIPAS IMK–IAA data versions. [ABSTRACT FROM AUTHOR]

This article begins with a review of important advances in the chemistry and related physics of the mesosphere and lower thermosphere (MLT) region of the atmosphere that have occurred over the past 2 decades, since the founding of Atmospheric Chemistry and Physics. The emphasis here is on chemistry, but we also discuss recent findings on atmospheric dynamics and forcings to the extent that these are important for understanding MLT composition and chemistry. Topics that are covered include observations, with satellite, rocket and ground-based techniques; the variability and connectedness of the MLT on various length scales and timescales; airglow emissions; the cosmic dust input and meteoric metal layers; and noctilucent/polar mesospheric ice clouds. The paper then concludes with a discussion of important unanswered questions and likely future directions for the field over the next decade. [ABSTRACT FROM AUTHOR]

Continuous observations at specified locations and chronicling of astronomical phenomena provide a good opportunity to study ancient space weather. There are 248 white, 125 red, and 44 blue color aurora-like descriptions, also known as aurora candidates, recorded in Chinese official historical records during the 1365-year period of 511–1876. Qualitative descriptions of the color, location, and appearance time of these candidates are quantitatively denoted. The red, white, and blue aurora candidates occurred most frequently 34% in autumn, 32% in summer, and 49% in summer, respectively. The white and red aurora as well as the overall candidates tend to appear during high solar activity periods. By contrast, the blue candidates frequently occur during low solar activity periods. Statistical results with 90% confidence intervals further show that the relationship between solar activities and overall/red (white/blue) aurora candidates is significant (insignificant). The red aurora candidates that frequently occurred in autumn during the periods of high solar activity agree well with those of low/middle latitude auroras, while the white aurora candidates might be confounded by noctilucent clouds or other atmospheric optical events, such as airglows, moon halo, etc. The study of ancient space weather/climate based on historical records shows that aurora occurrences are related to solar activities, and in particular, red auroras frequently appear in low/middle latitudes during high solar activity periods. [ABSTRACT FROM AUTHOR]

This article discusses the layering phenomena in the middle/upper atmosphere and ionosphere, including mesospheric metal layers, sporadic E layers, equatorial plasma bubbles, and polar mesospheric clouds. The research articles in this topic provide new insights into atmospheric dynamics, chemistry, and the vertical coupling of atmospheric layers. The authors highlight the challenges in forecasting changes in ionospheric irregularities and dynamic processes in the Earth's upper atmosphere. The findings contribute to our understanding of the behavior and interaction processes of the upper atmospheric layers. [Extracted from the article]

Polar mesospheric clouds (PMC), or noctilucent clouds, can be observed over high latitudes with the naked eye from the ground or from space near the summer solstice. PMC are considered a direct and sensitive indicator of climate change and have been reported to appear more frequently in recent decades. How PMC will change in the future under the influence of natural variability and anthropogenic forcing is uncertain. In this study, we utilize model output from the Whole Atmosphere Community Climate Model under several shared socioeconomic pathway (SSP) scenarios and input the water vapor, temperature, and pressure information into a 0‐d PMC model to project the trend and variation of PMC over the 21st century, and their relationship to future changes of temperature, water vapor, and the solar cycle. The 0‐d model calculations indicate that PMC ice water content (IWC) will increase and PMC will extend to lower latitudes under high SSP scenarios. Under these scenarios, more mesospheric water vapor leads to an increased IWC of PMC over the polar region, and colder mesopause temperature leads to more PMC over the mid‐latitudes. There is a significant anti‐correlation between the solar cycle and PMC IWC over the 21st century, but the anti‐correlation is not always significant on the decadal scale. Finally, methane oxidation in the stratosphere and water vapor entering from the troposphere are both responsible for future changes in mesospheric water vapor and thus PMC. Plain Language Summary: Polar mesospheric clouds (PMC), also known as noctilucent clouds, are a direct and sensitive indicator of climate change and have been reported to appear more frequently in recent decades. Our numerical model projection of future changes in PMC indicates a possible increase in their ice water content (IWC) with higher future greenhouse gas emissions and a decrease with lower greenhouse gas emissions. The IWC of the PMC can be influenced by the solar cycle, mesospheric water vapor, and temperature. With more greenhouse gas emissions and global surface warming, more water vapor will enter the middle atmosphere or form via the oxidation of methane, thus increasing PMC IWC over the polar region. The associated middle atmospheric cooling increases the IWC of PMC over the mid‐latitudes, and PMC can extend to lower latitudes where PMC have not been commonly seen before. Key Points: Future projections of Polar mesospheric clouds (PMC) for high greenhouse gas emission scenarios indicate an increasing ice water content (IWC) over the 21st centuryThe variation of PMC is mainly determined by mesospheric water vapor over polar region and by mesospheric temperature over mid‐latitudesOver the 21st century, PMC IWC anti‐correlates with the solar cycle. The correlation is not always significant on a decadal time scale [ABSTRACT FROM AUTHOR]

Results of experimental studies of red-colored plasma diffuse jets are presented. Such jets are initiated by a capacitive discharge in air or nitrogen at pressures of 0.2–3 Torr fed by voltage pulses with an amplitude of 5–7 kV following with a frequency of 21 kHz. They can be considered as a lab analog of a columnar sprite. The jet is formed by successive ionization waves (streamers). A significant effect of the reduced electric field strength E/N on the color (emission spectrum) of a plasma diffuse jet has been established. It is shown that the transition from red to blue as the jet approaches the additional electrodes and the end flange of the discharge tube is due to an increase in E/N in these regions. This, in turn, explains the change in color of sprites as they approach the top of the storm clouds. An assumption about the influence of noctilucent clouds on the formation of the beaded structure of sprites is made. The plasma parameters (electron Te, vibrational Tv, rotational Tr, and translational Ttr temperatures, as well as E/N) in the region of the capacitive discharge and along the plasma diffuse jet were measured by optical emission spectroscopy. The measurements have shown that with the increase in distance from the electrode assembly, E/N decreases from ∼3500 to ∼200 Td, while Te changes from ∼50 to 3 eV. The gas temperature varies slightly from 400 to 360 K. The measurement results are compared with those of natural red sprites. [ABSTRACT FROM AUTHOR]

Motivated by an improved ESA version of MIPAS calibrated spectra (version 8.03), we have released version 8 of MIPAS temperatures and pointing information retrieved from 2005-2012 MIPAS measurements at 12-15 µm in the Middle Atmosphere (MA), Upper Atmosphere (UA) and Noctilucent Cloud (NLC) measurement modes. The IMK/IAA retrieval processor in use considers non-local thermodynamic equilibrium (non-LTE) emission explicitly for each limb scan. This non-LTE treatment is essential to obtain accurate temperatures above the mid-mesosphere, because at the altitudes covered, up to 115 km, the simplified climatology-based non-LTE treatment employed for the nominal (NOM) measurements is insufficient. Other updates in MA/UA/NLC V8 non-LTE temperature retrievals from previous data releases include: more realistic atomic oxygen and carbon dioxide abundances; an updated set of spectroscopic data; an improved spectral shift retrieval; a continuum retrieval extended to altitudes up to 58 km; consideration of an altitude-dependent radiance offset retrieval; the use of wider microwindows above 85 km to capture the offset; an improved accuracy in forward model calculations; new temperature a priori information; improved temperature horizontal gradient retrievals; and, the use of MIPAS version 5 interfering species, where available. The resulting MIPAS MA/UA/NLC IMK/IAA temperature dataset is reliable for scientific analysis in the full measurement vertical range for the MA (18-102 km) and the NLC (39-102 km) observations, and from 42 to 115 km for the UA observations. The random temperature errors, dominated by the instrumental noise, are typically less than 1K below 60 km, 1-3K at 60-70 km, 3-5K at 70-90 km, 6-8K at 90-100 km, 8-12K at 100-105 km and 12-20K at 105-115 km. Pointing correction random errors, also mainly arising from instrumental noise, are on average 50m for tangent altitudes up to 60 km and decrease linearly to values smaller than 20m for altitudes above 95 km. The vertical resolution is 3 km at altitudes below 50 km, 3-5 km at 50-70 km, 4-6 km at 70-90 km, 6-10 km at 90-100 km and 8-11 km at 100-115 km. The systematic errors of retrieved temperatures below 75 km are driven by uncertainties in the CO2 spectroscopic data and, above 80 km, by uncertainties in the non-LTE model parameters (including collisional rates and atomic oxygen abundance) and the CO2 abundance. These lead to systematic temperature errors of less than 0.7K below 55 km, 1K at 60-80 km, 1-2K at 80-90 km, 3K at 95 km, 6-8K at 100 km, 10-20K at 105 km and 20-30K at 115 km. Systematic errors in the tangent altitude correction, mainly arising from CO2 spectroscopic uncertainties, are 250m at 20 km and 200m at 40-60 km, 100m at 80 km and smaller than 50m above 90 km. The consistency between the MA/UA/NLC and the NOM IMK/IAA datasets is excellent below 70 km (typical 0.5-1K differences). The comparison of this V8 temperature dataset with co-located SABER temperature measurements shows an excellent agreement, even better than in previous MIPAS IMK/IAA versions. [ABSTRACT FROM AUTHOR]

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

Using the IMK/IAA data processor, methane and nitrous oxide distributions were retrieved from version 8 limb emission spectra recorded with the Michelson Inferferometer for Passive Atmospheric Sounding (MIPAS). The dataset includes measurements from the Nominal, UTLS-1, Middle Atmosphere, Upper Atmosphere and Noctilucent Cloud observation modes. The processing differs from the previous version 5 data with respect to the atmospheric state variables jointly retrieved with the target gases CH4 and N2O, the treatment of the radiance offset, the selection of microwindows, the regularization, the spectroscopic data used and the treatment of horizontal variability of the atmospheric state. Besides the regular data product, a coarse-grid representation of the profiles with unity averaging kernels is available, as well as a specific research product for Middle Atmosphere measurements resulting from a slightly different retrieval approach. The CH4 errors are dominated by the large spectroscopic uncertainty for line intensities, which probably is too pessimistic, and estimated to be 21–34 % in the altitude range 6–68 km for northern midlatitude summer day conditions. The N2O errors are 7–17 % below 45 km. At higher altitudes they increase strongly due to nearly vanishing N2O amounts. Analysis of the horizontal averaging kernels reveals that for both gases the horizontal resolution is sampling-limited, i.e., information is not smeared over consecutive limb scans. Zonal mean seasonal composites of both CH4 and N2O exhibit the typical distribution of source gases with strong upwelling in the tropics and subsidence above the winter poles. Comparison with the previous data version shows several improvements: First, the vertical resolution of the retrieved CH4 (N2O) profiles has generally been significantly enhanced and varies between 2.5 (2.5) and 4 (5) km at altitudes between 10 and 60 km, with the best resolution around 30 km for both species. Secondly, the number of not converged retrievals has been clearly reduced, and thirdly, formerly strongly oscillating profiles are now considerably smoother. [ABSTRACT FROM AUTHOR]

Dusty plasma is generally defined as a plasma system containing a large number of nano‐ or micron‐sized dust particles. Its plasma properties and propagation properties of wave travel through it is of great scientific significance to the understanding of many fundamental problems in space physics and astrophysics. Based on previous studies on rocket exhaust and Polar Mesosphere Summer Echoes, fluid mechanics and dust microphysics are combined to simulate the ionospheric plasma structure generated by neutral gases and charged aerosols using fluid theory and kinetic theory. The Total Electron Content of the in‐situ detection beacon is simulated and compared with the experimental phenomenon given in Charged Aerosol Release Experiment by Bernhardt et al. The ray tracing algorithm is used to simulate the path deflection of the radio wave when it passes through the artificial disturbance area generated by the gas‐solid mixture, and the simulation results of ionosonde and analysis are given. Since the neutral gas expands substantially isotropically after being released, the resulting depleted structure is an approximately isotropic sphere, or an acorn‐like structure. The solid particles are constrained by the nozzle, and particles will distribute within a spray angle, and the depletion structure will appear as a "broom‐like" or "cone‐like" structure. If the size of dust particles is small enough, the particle movement will be restricted by the geomagnetic field, resulting in a "squid‐like" ionospheric structure. Because of the fishtail‐like sharp boundary, radio waves passing through it are deflected in a specific path, which is markedly different from the effect of pure neutral gas injected into the ionosphere. Plain Language Summary: Many studies have focused on ionospheric disturbances caused by natural dust of polar mesospheric clouds or by ionospheric pollutants. However, practical physical scenarios usually have both neutral gases and charged aerosols, whose total contribution to the plasma after injection into the ionosphere and wave propagation through the disturbance have rarely been studied. In this paper, the fluid theory and kinetic theory are used to simulate the ionospheric plasma disturbance structure by combining the fluid mechanics and the dust microphysics. The Total Electron Content of the in‐situ detection, the path of radio waves through the disturbance and the vertical ionogram produced by the injection of neutral gas and dust particles are also simulated. Key Points: An analytic solution model of the ionospheric disturbance produced by artificial neutral gas‐dust particles mixture injection is establishedThe motion of charged dust particles and evolution of ionospheric depletion caused by mixture injection are simulatedThe perturbed Total Electron Content and the path deflection of radio wave passing through the disturbance region are simulated and analyzed [ABSTRACT FROM AUTHOR]

Polar Mesospheric Summer Echoes (PMSE) are a phenomenon that are measured in the upper atmosphere during the summer months and can occur in several layers. In this study, we aimed to investigate the relationship between PMSE layers ranging from 80 to 90 km altitude, and the solar cycle. We used 230 hours of observations from the EISCAT VHF radar located near Tromsø, Norway, and applied a previously developed classification model to identify PMSE layers. The observations were taken during the solar maximum of the solar cycle with the years 2013, 2014 and 2015, and during the solar minimum of the solar cycle with the years 2019 and 2020. Our analysis focused on parameters such as the altitude, thickness, and echo power in the PMSE layers, as well as the number of layers present. Our results indicate that the average altitude of PMSE, the echo power in the PMSE and the thickness of the layers is on average higher during solar maximum than during solar minimum. In the considered observations, the electron density at 92 km altitude and the echo power in the PMSE are positively correlated with the thickness of the layers. In addition, we found that higher electron densities at ionospheric altitudes might be necessary to observe multi-layered PMSEs. Furthermore, we observed that the thickness decreases as the number of multi-layers increase. Based on comparisons with previous studies, we hypothesized that the thickness of PMSE layers may be related to the vertical wavelength of gravity waves, with larger wavelengths potentially resulting in thicker layers. Also, an interesting parallel is seen between the thickness of Noctilucent Clouds (NLC) multi layers and PMSE multi layers, where both NLC and PMSE have a similar distribution of layers greater than 1 km in thickness. [ABSTRACT FROM AUTHOR]

The responses of water vapour (H 2 O) and noctilucent clouds (NLCs) to the solar cycle are studied using the Leibniz Institute for Middle Atmosphere (LIMA) model and the Mesospheric Ice Microphysics And tranSport (MIMAS) model. NLCs are sensitive to the solar cycle because their formation depends on background temperature and the H 2 O concentration. The solar cycle affects the H 2 O concentration in the upper mesosphere mainly in two ways: directly through the photolysis and, at the time and place of NLC formation, indirectly through temperature changes. We found that H 2 O concentration correlates positively with the temperature changes due to the solar cycle at altitudes above about 82 km, where NLCs form. The photolysis effect leads to an anti-correlation of H 2 O concentration and solar Lyman- α radiation, which gets even more pronounced at altitudes below ∼ 83 km when NLCs are present. We studied the H 2 O response to Lyman- α variability for the period 1992 to 2018, including the two most recent solar cycles. The amplitude of Lyman- α variation decreased by about 40 % in the period 2005 to 2018 compared to the preceding solar cycle, resulting in a lower H 2 O response in the late period. We investigated the effect of increasing greenhouse gases (GHGs) on the H 2 O response throughout the solar cycle by performing model runs with and without increases in carbon dioxide (CO 2) and methane (CH 4). The increase of methane and carbon dioxide amplifies the response of water vapour to the solar variability. Applying the geometry of satellite observations, we find a missing response when averaging over altitudes of 80 to 85 km, where H 2 O has a positive response and a negative response (depending on altitude), which largely cancel each other out. One main finding is that, during NLCs, the solar cycle response of H 2 O strongly depends on altitude. [ABSTRACT FROM AUTHOR]

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

Noctilucent clouds (NLC) are sensitive indicators in the upper mesosphere, reflecting changes in the background atmosphere. Studying NLC responses to the solar cycle is important for understanding solar-induced changes and assessing long-term climate trends in the upper mesosphere. Additionally, it enhances our understanding of how increases in greenhouse gas concentration in the atmosphere impact the Earth’s upper mesosphere and climate. This study presents long-term trends in the response of NLC and the background atmosphere to the 11-year solar cycle variations. We utilised model simulations from the Leibniz Institute Middle Atmosphere (LIMA) and the Mesospheric Ice Microphysics and Transport (MIMAS) over 170 years (1849 to 2019), covering 15 solar cycles. Background temperature and water vapour (H2O) exhibit an apparent response to the solar cycle, with an enhancement post-1960, followed by an acceleration of greenhouse gas concentrations. NLC properties, such as maximum brightness (βmax), calculated as the maximum backscatter coefficient, altitude of βmax (referred to as NLC altitude) and ice water content (IWC), show responses to solar cycle variations that increase over time. This increase is primarily due to an increase in background water vapour concentration caused by an increase in methane (CH4). The NLC altitude positively responds to the solar cycle mainly due to solar cycle-induced temperature changes. The response of NLC properties to the solar cycle varies with latitude, with most NLC properties showing larger and similar responses at higher latitudes (69° N and 78° N) than mid-latitudes (58° N).

In the atmosphere, water can be present in liquid and solid phases, but the vapor phase is generally predominant. Condensed phases of water occur under a wide range of conditions, ranging from polar mesospheric clouds at the lowest atmospheric temperatures and at low pressure to the much warmer tropospheric clouds. The temperature range at which ice or water clouds are observed spans from T = 100 to 300 K with pressures ranging from about 10−3 mbar to about 1 bar. Over this wide range, water is known to form several condensed phases, which can be separated into crystalline (hexagonal and stacking disordered ice) and noncrystalline phases (liquid and supercooled liquid water, amorphous solid water). We report on the vapor pressure of these water phases with a focus on metastable amorphous solid water and stacking disordered ice in the light of recent experimental findings and discuss possible implications for the atmosphere. We present evidence that supercooled liquid water and low density amorphous solid water do not belong to the same phase and therefore, no continuous vapor pressure curve can be given. [ABSTRACT FROM AUTHOR]

The quasi 5‐day wave (Q5DW) with zonal wavenumber 1 is a dominant planetary wave (PW) oscillation in the polar summer mesospheric temperature and polar mesospheric cloud (PMC) fields. In this paper, the Q5DW signal derived from 16 years (2007–2022) of Microwave Limb Sounder temperature observations is used to investigate the role of this PW mode on the onset of PMC seasons in the northern hemisphere (NH). PMC data from the Cloud Imaging and Particle Size (CIPS) instrument during this time indicates that NH PMC season onsets ranged from 15 to 28 May, with earliest onsets in 2013, 2015, 2019, 2020, 2021, and 2022. Except 2013 and 2022, the other four earlier onsets were also characterized by enhanced Q5DW activity. The wave amplification appears to be driven by baroclinic instability arising from the negative meridional gradient of potential vorticity in the high‐latitude summer mesosphere. CIPS data show that when the Q5DW was present at the beginning of the season, clouds formed preferentially in the cold troughs of the wave. We thus propose that the much colder troughs due to enhanced Q5DW activity in mid‐May of 2015, 2019, 2020, and 2021 influenced the timing of PMC onset in these years. While the 11‐year solar cycle, inter‐ and intra‐hemispheric coupling due to gravity wave and PW activity have been shown to contribute to earlier onset of PMC seasons in the NH, our analysis suggests that enhanced Q5DW activity also plays a major role. Plain Language Summary: The quasi 5‐day wave (Q5DW), a wave that takes ∼5 days to travel around a latitude circle, is a dominant planetary scale oscillation in the polar summer mesospheric temperature and polar mesospheric cloud (PMC) fields. We present the first study of the Q5DW impact on the northern hemisphere (NH) PMC season onset. The Q5DW activity is derived from 16 years (2007–2022) of satellite temperature observations. This activity is then compared to the onset of NH PMC seasons determined from the Cloud Imaging and Particle Size (CIPS) observations. The season onsets ranged from 15 to 28 May, with earliest onsets in 2013, 2015, 2019, 2020, 2021, and 2022. Earlier onsets in 2015, 2019, 2020, and 2021 were characterized by increased Q5DW power presumably driven by instability due to large vertical shear in the zonal winds. PMCs are found to form in the cold troughs of the Q5DW. We thus propose that the much colder troughs due to enhanced Q5DW activity in mid‐May of 2015, 2019, 2020, and 2021 influenced the timing of PMC onset in these years. While several other factors contribute to earlier onset of PMC seasons in the NH, our analysis suggests that enhanced Q5DW activity also plays a major role. Key Points: There is year‐to‐year variability in the quasi 5‐day wave (Q5DW) activity in the northern hemisphere summerObservations from 2007 to 2022 indicate that four of the six earliest polar mesospheric cloud (PMC) season onsets are characterized by higher Q5DW activityPMC first form in the longitude sector coincident with the cold trough of the Q5DW [ABSTRACT FROM AUTHOR]

The Balloon Lidar Experiment (BOLIDE), which was part of the Polar Mesospheric Cloud Turbulence (PMC Turbo) Balloon Mission has captured vertical profiles of PMCs during a 6 d flight along the Arctic circle in July 2018. The high-resolution soundings (20 m vertical and 10 s temporal resolution) reveal highly structured layers with large gradients in the volume backscatter coefficient. We systematically screen the BOLIDE dataset for small-scale variability by assessing these gradients at high resolution. We find longer tails of the probability density distributions of these gradients compared to a normal distribution, indicating intermittent behaviour. The high occurrence rate of large gradients is assessed in relation to the 15 min averaged layer brightness and the spectral power of short-period (5–62 min) gravity waves based on PMC layer altitude variations. We find that variability on small scales occurs during weak, moderate, and strong gravity wave activity. Layers with below-average brightness are less likely to show small-scale variability in conditions of strong gravity wave activity. We present and discuss the signatures of this small-scale variability, and possibly related dynamical processes, and identify potential cases for future case studies and modelling efforts. [ABSTRACT FROM AUTHOR]