The Arctic stratosphere during the first half of the 2024/25 winter was characterized by an exceptionally strong polar vortex, comparable to the coldest winter of 2019/20 with record ozone depletion. In mid-February 2025, enhanced propagation of planetary wave activity over northeastern Eurasia led to a deceleration of the wind in the upper stratosphere, followed by downward wave reflection into the troposphere over Canada, northern USA, and northwestern Eurasia. This stratosphere-troposphere interaction resulted in significant surface cooling in these regions and drove the Arctic Oscillation (AO) index to a winter minimum of −5. Such strong AO anomalies, exceeding −2σ, have been observed in February over the past 25 years only in 2010, 2021, and 2025. A major sudden stratospheric warming (SSW) event in early March was preceded by a prolonged preconditioning stage of the polar vortex, characterized by intensified stratospheric vacillations between zonal winds and planetary waves. During the SSW, enhanced wave activity propagation into the stratosphere was identified over northeastern Eurasia and Europe. The amplified upward wave flux over Europe was linked to the eastward redistribution of wave activity fluxes in the upper troposphere over the North Atlantic, originating from the wave reflection region over North America. Using lidar sounding and spectral measurements of excited hydroxyl molecules OH* temperature, the stratosphere and upper mesosphere temperature variations in February–March 2025 were analyzed. Simulation with the CCM (chemistry–climate model) SOCOLv3 estimated the total chemical ozone loss in the Arctic stratosphere in winter 2024/25 as ∼50

A transatlantic scientific balloon flight (TRANSAT) was conducted between 22 and 26 June 2024. The TRANSAT balloon, operated by the French Space Agency (CNES), floated in the stratosphere at approximately 40 km altitude between Esrange (Sweden) and Baffin Island (Canada) for about 3.8 d. The scientific payload comprised nine instruments, including an optical imager for noctilucent cloud (NLC) studies from the Swedish Institute of Space Physics. The NLC imager consisted of three identical visible-range optical cameras, one of which operated successfully throughout the entire flight, capturing thousands of NLC images. The TRANSAT balloon campaign was supported by ground-based lidar measurements and spaceborne observations from the Swedish MATS satellite. Here, we describe the technical characteristics of the balloon experiment and present early results. Nearly continuous observations of NLC were obtained during the entire flight. A localized warm region in the mesopause was identified as the cause of temporary NLC disappearance, while complex NLC structures exhibiting different motions were found to probably result from horizontal wind rotation with altitude within the mesopause region.

The results of simultaneous measurements of noctilucent clouds (NLC) position in a number of ground-based locations are presented. Observational data of 14 bright NLC events over 5 years is used for building the altitude maps of cloud fields using triangulation technique updated for multi-location case. Statistical distribution of NLC altitude and its change during the summer season is considered. Mean NLC altitudes are compared with colorimetric technique based on the same data and simple radiation transfer model. This can be used to check the model and estimate the accuracy of single-camera technique of NLC altitude measurements. Results and methods are suggested for net ground-based survey of noctilucent clouds.

Ground-based observations of the natural hydroxyl (OH) nightglow at altitudes of 85-90 km are used for deriving the rotational temperature of excited OH, which is close to the neutral atmospheric temperature. For filtering of mesoscale perturbations, we use differences between pairs of measured values of OH rotational temperature separated with fixed time intervals in the range of 0.5-2 h. The filtering is applied for studying mesoscale variations of temperature near the mesopause according to the data of spectral OH nightglow measurements at observatories of Zvenigorod (56 degrees N, 37 degrees E.) in the years 2004-2016, Tory (52 degrees N, 103 degrees E) in 2012-2017 and Maymaga (63 degrees N, 130 degrees E) in 2000-2015. Monthly-mean values and variances of temperature disturbances with periods 0.7-8.2 h are determined. Semiempirical and statistical approaches are used to estimate and subtract variances of the instrumental dark current noise and uncorrelated in time fluctuations. Seasonal and interannual variations in standard deviations of correlated in time mesoscale perturbations of the OH rotational temperature at the considered observational sites are studied. They can give information about multiyear changes in the activity of atmospheric acoustic-gravity waves propagating through the OH layer near the mesopause. (c) 2023 COSPAR. Published by Elsevier B.V. All rights reserved.

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.

Long-term trends and dependences on solar activity were derived for wave disturbances at mesopause altitudes (80−100 km) based on temperature variations obtained by spectral observations of the hydroxyl airglow at the Zvenigorod Science Station, Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, in 2000−2024, as well as by methods of statistical analysis. Their activities were determined by digital frequency filtering in three regions of wave periods, such as 0.7−2.0, 1.4−4.1, and 2.7−8.2 h with maxima at 1, 2, and 4 h. The root-mean-square values of temperature half-differences served as an indicator of wave activity. Both year-round and average seasonal (winter and summer) values were analyzed. According to the study results, the wave activity is characterized by positive trends with their dependence on the frequency band of disturbances (in winter, the trend is greater in the high-frequency band, and in summer, the trend is greater in the low-frequency band). The dependence on solar activity is positive. Its values are greater for the high-frequency band of disturbances, as well as in winter.

The study analyzes the long-term average annual OH* temperature trend, the values of which were obtained from nighttime spectral measurements of hydroxyl airglow bands at Zvenigorod research station (56° N, 37° E) from 1957 to 2022. At present, this OH* temperature series, which reflects the thermal state of the mesopause region, is the longest in the world. On its basis, the linear trend and response of temperature to changes in solar activity are estimated both in general for the entire data set and for individual time intervals. In the first case, the trend was –0.23 ± 0.04 K/year. In the second case, the analysis showed a strong cooling in the mesopause region (–0.53 ± 0.34 K/yr) until the 1970s, which subsequently slowed to –0.14 ± 0.03 K/yr. Comparison of the results with other measurements and model calculations shows that the latter have lower trend values. It is suggested that the causes of the temperature trend, in addition to the increase in greenhouse gases, the main one being CO2, can be due to long-term changes in the dynamics of the upper atmosphere.

The paper describes technical characteristics and presents the first scientific results of a novel infrared imaging system (imager) for studies of nightglow emissions coming from the hydroxyl (OH) and molecular oxygen (O2) layers in the mesopause region (80–100 km) above northern Scandinavia. The OH imager was put into operation in November 2022 at the Swedish Institute of Space Physics in Kiruna (67.86° N, 20.42° E; 400 m altitude). The OH imager records selected emission lines in the OH(3-1) band near 1500 nm to obtain intensity and temperature maps at around 87 km altitude. In addition, the OH imager registers infrared emissions coming from the O2 IR A-band airglow at 1268.7 nm in order to obtain O2 intensity maps at a slightly higher altitude, around 94 km. This technique allows the tracing of wave disturbances in both horizontal and vertical domains in the mesopause region. Validation and comparison of the OH(3-1) rotational temperature with collocated lidar and Aura Microwave Limb Sounder (MLS) satellite temperatures are performed. The first scientific results obtained from the OH imager for the first winter season (2022–2023) are discussed.

In the history of atmospheric science, the lunar tropical-monthly oscillation with the period of 27.32 days in atmospheric data has been discovered several times. However, up to now, analysis of possible influence of other lunar tidal oscillations on the extracted tropical-monthly oscillation was not provided. The historical and modern discoveries of this tidal oscillation in the atmosphere are reconsidered in the present paper. Although its exis-tence in the atmosphere was predicted by the lunar tidal theory, its amplitude must be roughly one order less than that of the lunar anomalistic tidal oscillation having a very close period of 27.55 days. Moreover, in spite of the different periods these two oscillations appear to be mutually dependent. The analysis of the previous findings demonstrates that, in some papers, the tropical monthly oscillation was not properly separated from the anomalistic monthly oscillation and hence the presence of the tropical monthly oscillation in atmospheric data was not convincing. Krahenbuhl et al. (2011) were first who managed to extract convincingly the measurable tropical monthly oscillation in atmospheric data. Now the existence of the lunar tropical monthly oscillation in the atmosphere should be regarded as a solved problem.

The problem of distinguishing the solar rotation from lunar tidal origin with the period of about 27 days in time series of geophysical data is considered. Although power spectra of some solar indices and of the lunar tidal gravitational potential contain coinciding or very close spectral peaks (with periods of 27.32 and 27.44 days), examination of the fine structure of the spectra for 1962-2019 allows us extracting a pair of determinative spectral peaks with the periods of 27.17 and 27.55 days indicating the solar rotation or lunar (tidal) origin of the quasi 27-day oscillation. This technique is illustrated by analyzing time series of the geomagnetic Ap index demonstrating that its 27-day oscillation is of a solar origin.

The 2020 summer season had more frequent than usual occurrences of noctilucent clouds (NLCs) in the Northern Hemisphere at middle latitudes (45-50 degrees N), with the lowest latitude at which NLCs were seen being 34.1 degrees N. In order to investigate a reason for this extraordinary NLC season, we have analyzed long-term Aura/MLS satellite data for all available summer periods from 2005 to 2021. Both Aura/MLS summer temperature and water vapor in the mesopause region, between about 79 and 89 km altitude, have been considered. There has been a decrease in the summer mesopause temperature between 2016 and 2020. At the same time, water vapor mixing ratio has significantly increased (by about 12-17%) in the zonal mean H2O value in the 2020 summer compared to 2017. There exists a positive linear trend in the H2O amount by about 5% between 2005 and 2021 at middle latitudes 45-50 degrees N at 0.0046 hPa. A combination of lower mesopause temperature and water vapor mixing ratio maximum at middle latitudes is the main reason for frequent and widespread occurrences of NLCs seen around the globe at middle latitudes in the summer of 2020. The 24th solar cycle minimum can explain neither the H2O maximum nor NLC maximum in 2020.

The paper describes technical characteristics and presents first scientific results of a novel infrared imaging system (imager) for studies of nightglow emissions coming from hydroxyl (OH) and molecular oxygen (O2) layers in the mesopause region (80–100 km) above northern Scandinavia. The OH imager was put into operation in November 2022 at the Swedish Institute of Space Physics located in Kiruna (67.86° N, 20.42° E, 400 m altitude). The OH imager records selected emission lines in the OH (3-1) band near 1500 nm to obtain intensity and temperature maps at around 87 km altitude. Also, the OH imager registers infrared emissions coming from the O2 IR A-band airglow at 1268.7 nm in order to obtain O2 intensity maps at a slightly higher altitude around 94 km. This technique allows tracing wave disturbances both in horizontal and vertical domains in the mesopause region. Validation and comparison of the OH (3-1) rotational temperature with collocated lidar and Aura/MLS satellite temperatures are performed. First scientific results obtained with the OH imager for the first winter season (2022–2023) are discussed.

The influence of the 27-day solar oscillation on the temperature and intensity of the airglow of the mesopause region based on the hourly midnight observations of the О 2 А(0-1) and ОН(6-2) bands at the Zvenigorod Scientific Station in 2000−2021 has been studied. It is found that the response to solar activity changes is observed in all characteristics of the atmospheric emissions both by the yearly and seasonal data. They are statistically significant for all characteristics of the atmospheric emissions in winter and only for the О 2 А(0-1) intensity in summer. The positive response of the molecular oxygen emission is lower in the summer than in the winter by approximately a factor of 2. It is found that the variations in the atmospheric emission characteristics are close to antiphase with a 27-day solar oscillation. At the same time, the temperature of the emitting hydroxyl in winter undergoes oscillations with a shift by a third of the period relative to the solar variation maximum.

On the night 16-17 August 2021, a balloon-borne experiment called Stratospheric Observations of Noctilucent Clouds (SONC) was successfully performed. A big scientific balloon, having onboard three automated cameras for studies of noctilucent clouds (NLC), was launched to 32.7 km altitude from Esrange (northern Sweden). All three NLC cameras and electronics were completely operational in the stratosphere for more than 10 h at low tem-peratures of about-30 degrees C. Two wide angle cameras registered an extended NLC field of about 1700 km long in the twilight sky sector from the north-west to the north-east of Esrange. NLC were of a moderate brightness and were located at high latitudes between 68 degrees and 71 degrees N. The NLC field was located in a cold area (138-142 K) below the frost point temperature (145-148 K) in the mesopause region that was confirmed by Aura/MLS sat-ellite and Esrange lidar measurements. The balloon-borne NLC measurements were accompanied by ground -based lidar and radar measurements. The latter have registered Polar Mesosphere Summer Echoes (PMSE) in the same volume of the summer mesopause along with NLC observed from the stratosphere that has been per-formed for the first time above northern Scandinavia. We describe the technique and method of the NLC observation from the stratosphere as well as present the first scientific results of the SONC experiment.

Some important data on both lunar tidal and solar effects on mesopause region characteristics, which were obtained by scientists of the Obukhov Institute of Atmospheric Physics in cooperation with colleagues from other organizations, are discussed. The present-day evidence that supports the hypothesis proposed by A.I. Semenov and N.N. Shefov about the existence of oscillations (with periods of the lunar synodic month and its half) in the mesopause region characteristics is considered. The oscillation amplitudes are estimated based on a statistical analysis of measurement data, possible mechanisms for generating these oscillations are indicated. Statistical data on the effect of solar activity on the mesopause region are discussed. It is shown that the effects of solar activity on some atmospheric characteristics on interannual and intraseasonal time scales have opposite signs, which suggests that there are different physical mechanisms of solar-terrestrial relations within these frequency ranges.

Temporally coherent mesoscale perturbations of the rotational temperature of excited hydroxyl (OH*) are often used as an indicator of wave processes in the mesosphere and lower thermosphere. Digital filters are used in this study to determine the mesoscale component with periods of 0.8–11 h, based on the differences in measured values shifted in time by fixed intervals varying from 10 min to 2 h. The average monthly intensity of mesoscale variations is proportional to the variance of the indicated differences recorded in each calendar month of measurements. These variances contain both information about coherent mesoscale processes and also temporally incoherent noise of an instrumental and turbulent nature. A statistical method for the analysis of the structural functions of the analyzed characteristics of nighttime airglow was developed and applied to estimate the variance of incoherent random noise. These estimates are subtracted from the measured monthly mean variances of mesoscale differences in order to obtain information on the intensity of coherent mesoscale processes near the mesopause. Subtraction of the variance of incoherent noise does not change the character of seasonal variations in all spectral intervals, but it decreases the values of mesoscale standard deviations by 10–20%. The proposed correction makes it possible to better determine the features of seasonal and interannual changes in coherent mesoscale disturbances in different ranges of the frequency spectrum.

Mesoscale variations of the rotational temperature of excited hydroxyl (OH*) are studied at altitudes 85 – 90 km using the data of spectral measurements of nightglow emission at Russian observatories Zvenigorod (56 ° N, 37°E.) in years 2004 – 2016, Tory (52 ° N, 103°E) in 2012 – 2017 and Maimaga (63° N, 130° E) in 2014 - 2019. The filtering of mesoscale variations was made by calculations of the differences between the measured values of OH* rotational temperature separated with time intervals of dt ~ 0.5 - 2 hr. Comparisons of monthly variances of the temperature differences for various dt allow us to estimate coherent and non-coherent in time components of the mesoscale temperature perturbations. The first component can be associated with mesoscale waves near the mesopause. The non-coherent component may be produced by instrument errors and atmospheric turbulence. The results allow us correcting the observed mesoscale temperature variances at all listed sites for contributions of instrumental and turbulent errors. Seasonal and interannual changes in the coherent component of mesoscale variances of the temperature at the observational sites are studied, which may reflect respective changes in the intensity of mesoscale internal gravity waves in the mesosphere and lower thermosphere region. The analysis of nightglows data was supported by the grant #19-35-90130 of the Russian Foundation for Basic Research. Hydroxyl nightglow data at the Tory site were obtained with the equipment of the Center for Common Use «Angara» http://ckp-rf.ru/ckp/3056/ at the ISTP SB RAS within budgetary funding from the Basic Research Program (Project 0278-2021-0003). Data of the “Geomodel” Resource Center of Saint-Petersburg State University were used.

Spectral observations of the mesopause airglow at the Zvenigorod Scientific Station have been used to obtain the midnight emission intensities of molecular oxygen (О2А(0-1) band) and hydroxyl (OH (6-2) band) for 2000–2019. Spectral analysis of the variations has made it possible to determine the annual variability for each emission, which is described by the sum of four harmonics. The time lag in seasonal variations of the hydroxyl emission relative to variations in the emission of molecular oxygen is 5–18 days. Long-term changes in the average annual emission intensities have been studied. The linear trend (–3.3 ± 0.3% per year for О2А(0-1) and –2.6 ± 0.2% per year for ОН(6-2)), the dependences on the 11-year solar cycle (response to changes in the Lyman-alpha solar radiation (18.5 ± 3.3% per 1011 photons cm–2 s–1 for О2А(0-1) and 10.5 ± 2.5% per 1011 photons cm–2 s–1 for OH (6-2)) and the 22-year solar cycle (response to changes in the solar magnetic field strength (23.2 ± 4.5% per 100 μТ for О2А(0-1) and 12.1 ± 3.5% per 100 μТ for ОН (6-2)), as well as quasi-eight-year oscillations have been found.

Abstract. The 2020 summer season has revealed frequent occurrences of noctilucent clouds (NLCs) around the Northern hemisphere at middle latitudes (45–55° N), with the lowest latitude at which NLCs were seen being 34.1° N. In order to investigate a reason for this NLC extraordinary summer season, we have analyzed long-term Aura/MLS satellite data for all available summer periods from 2005 to 2020. Both Aura/MLS summer temperature and water vapor in the upper mesosphere and the mesopause region, between 74 and 89 km altitude, have been considered. We have found that there has been a moderate decrease in the upper mesosphere temperature between 2016 and 2020 and no dramatic changes have been observed in temperature in the summer of 2020 at the middle latitude mesopause. At the same time, water vapor concentration has significantly increased (by about 12–15 %) in the zonal mean H2O value in the 2020 summer compared to 2017, meaning that the summer mesopause at middle latitudes has become more wet. At the same time, no increase in water vapor has been detected at the high latitude high altitude mesopause. A combination of lower mesopause temperature and water vapor concentration maximum at middle latitudes is the main reason for frequent and widespread occurrences of NLCs seen around the globe at middle latitudes in the summer of 2020. The 24th solar cycle minimum cannot explain the H2O maximum in 2020 since the correlation between Lyman-α flux and the amount of water vapor is low. The increase in volcanic activity from 2013 to 2015 (and its recent maximum occurred in 2015) explains the increased amount of water vapor in the upper mesosphere for the past years and its maximum in 2020 due to volcanic water vapor being injected into the atmosphere and transported into the upper mesosphere. The 5-year delay between volcanic activity and water vapor maximum is well explained by a general meridional-vertical atmospheric circulation.