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

The ground-based measurements obtained from a lidar network and the six-year OSIRIS limb-scanning radiance measurements made by the Odin satellite are used to study the climatology of the middle- and low-latitude sodium (Na) layer. Up to January 2021, four Na resonance fluorescence lidars at Beijing (40.2° N, 116.2° E), Hefei (31.8° N, 117.3° E), Wuhan (30.5° N, 114.4° E), and Haikou (19.5° N, 109.1° E) collected vertical profiles of Na density for a total of 2,136 nights (19,587 h). These large datasets provide routine long-term measurements of the Na layer with exceptionally high temporal and vertical resolution. The lidar measurements are particularly useful for filling in OSIRIS data gaps since the OSIRIS measurements were not made during the dark winter months because they utilise the solar-pumped resonance fluorescence from Na atoms. The observations of Na layers from the ground-based lidars and the satellite are comprehensively compared with a global model of meteoric Na in the atmosphere (WACCM-Na). The lidars present a unique test of OSIRIS and WACCM, because they cover the latitude range along 120° E longitude in an unusual geographic location with significant gravity wave generation. In general, good agreement is found between lidar observations, satellite measurements, and WACCM simulations. Whereas the Na number density from OSIRIS is slightly larger than that from the Na lidars at the four stations within one standard deviation of the OSIRIS monthly average, particularly in autumn and early winter arising from significant uncertainties in Na density retrieved from much less satellite radiance measurements. WACCM underestimates the seasonal variability of the Na layer observed at the lower latitude lidar stations (Wuhan and Haikou). This discrepancy suggests the seasonal variability of vertical constituent transport modeled in WACCM is underestimated because much of the gravity wave spectrum is not captured in the model. [ABSTRACT FROM AUTHOR]

In this paper we present an overview of measurements conducted during the WADIS-2 rocket campaign. We investigate the effect of small-scale processes like gravity waves and turbulence on the distribution of atomic oxygen and other species in the MLT region. Our analysis suggests that density fluctuations of atomic oxygen are coupled to fluctuations of other constituents, i.e., plasma and neutrals. Our measurements show that all measured quantities, including winds, densities, and temperatures, reveal signatures of both waves and turbulence. We show observations of gravity wave saturation and breakdown together with simultaneous measurements of generated turbulence. Atomic oxygen inside turbulence layers shows two different spectral behaviors, which might imply change of its diffusion properties. [ABSTRACT FROM AUTHOR]

Two important approaches for satellite studies of Polar Mesospheric Clouds (PMC) are nadir measurements adapting phase function analysis and limb measurements adapting spectroscopic analysis. Combining both approaches enables new studies of cloud structures and microphysical processes but is complicated by differences in scattering conditions, observation geometry, and sensitivity. In this study, we compare common volume PMC observations from the nadir viewing Cloud Imaging and Particle Size instrument (CIPS) on the AIM satellite and a special set of tomographic limb observations from the Optical Spectrograph and InfraRed Imager System (OSIRIS) on the Odin satellite. While CIPS provides preeminent horizontal resolution, the OSIRIS tomographic analysis provides combined horizontal and vertical PMC information. This first direct comparison is an important step towards co-analyzing CIPS and OSIRIS data, aiming at unprecedented insights into horizontal and vertical cloud processes. We perform the first thorough error characterization of OSIRIS tomographic cloud brightness and cloud ice. We establish a consistent method for comparing cloud properties from limb tomography and nadir observations, accounting for differences in scattering conditions, resolution and sensitivity. Based on an extensive common volume, and a temporal coincidence criterion of only 5 minutes, our method enables a detailed comparison of PMC regions of varying brightness and ice content. We find that the primary OSIRIS tomography product, cloud scattering coefficient, shows very good agreement with the primary CIPS product, cloud albedo with a correlation coefficient of 0.96. However, OSIRIS systematically reports brighter clouds than CIPS and the bias between the instruments (OSIRIS - CIPS) is 3.4e-6sr-1 (±2.9e-6sr-1) on average. The OSIRIS tomography ice mass density agrees well with the CIPS ice water content, with a correlation coefficient of 0.91. However, the ice water content reported by OSIRIS is lower than CIPS, and we quantify the bias to -22gkm-2 (±14gkm-2) on average. [ABSTRACT FROM AUTHOR]

Global three-dimensional data are a key to understanding gravity wave interactions in the mesosphere and lower thermosphere. MATS (Mesospheric Airglow/Aerosol Tomography and Spectroscopy) is a new Swedish satellite mission that addresses this need. It applies space-borne limb imaging in combination with tomographic and spectroscopic analysis to obtain gravity wave data on relevant spatial scales. Primary measurement targets are O2 Atmospheric Band dayglow and nightglow in the near infrared, and sunlight scattered from noctilucent clouds in the ultraviolet. While tomography provides horizontally and vertically resolved data, spectroscopy allows analysis in terms of mesospheric temperature, composition, and cloud properties. Based on these dynamical tracers, MATS will produce a climatology on wave spectra during a 2-year mission. Major scientific objectives concern a characterization of gravity waves and their interactions in the mesosphere and lower thermosphere, as well as their relationship to dynamical conditions in the lower and upper atmosphere. MATS is currently being prepared for launch in 2019. This paper provides an overview over scientific goals, measurement concepts, instruments, and analysis ideas. [ABSTRACT FROM AUTHOR]

Based on self-consistent rocket-borne measurements of temperature, densities of atomic oxygen and neutral air, and volume emission of the Atmospheric Band (762 nm) we examined the one-step and two-step excitation mechanism of O2(b¹Σg+) for night-time conditions. Following McDade et al. (1986), we derived the empirical fitting coefficients, which parameterize the Atmospheric Band emission O2(b¹Σg+ - X³Σg-)(0,0) in terms of the atomic oxygen concentrations. This allows to derive atomic oxygen concentration from night-time observations of Atmospheric Band emission O2(b¹Σg+ - X³Σg-)(0,0). The derived empirical parameters can also be utilised for Atmospheric Band modelling. Additionally, we derived fit function and corresponding coefficients for combined (one- and two-step) mechanism. Simultaneous and true common volume measurements of all the parameters used in this derivation, i.e. temperature and density of the background air, atomic oxygen density, and volume emission rate, is the novelty and the advantage of this work. [ABSTRACT FROM AUTHOR]

In this study the properties of Polar Mesospheric Clouds (PMCs) and the background atmosphere in which they exist are studied using measurements from two instruments, OSIRIS and SMR, on-board the Odin satellite. The data comes from a set of tomographic measurements conducted by the satellite during 2010 and 2011. The expected ice mass density and cloud frequency for conditions of thermodynamic equilibrium, calculated using the temperature and water vapour as measured by SMR, are compared to the ice mass density and cloud frequency as measured by OSIRIS. Similar to previous studies, we find that assuming thermodynamic equilibrium reproduces the seasonal, latitudinal and vertical variations in ice mass density and cloud frequency, but with a high bias of a factor of 2 in ice mass density. To explain this bias we use a simple ice particle growth model to estimate the time it would take for the observed clouds to sublimate completely and the time it takes for these clouds to reform. We find a difference in the median sublimation time (2.1 h) and the reformation time (3.2 h) at peak cloud altitudes (82–84 km). This difference implies that temperature variations on these timescales have a tendency to reduce the ice content of the clouds, explaining the high bias of the equilibrium model. Finally, we detect, and are for the first time able to positively identify, cloud features with horizontal scales of 100 to 300 km extending far below the region of supersaturation (> 2 km). Using the growth model, we conclude these features cannot be explained by sedimentation alone, and suggest that these events may be indication of strong vertical transport. [ABSTRACT FROM AUTHOR]

In this study the properties of Polar Mesospheric Clouds (PMCs) and the background atmosphere in which they exist are studied using measurements from two instruments, OSIRIS and SMR, on-board the Odin satellite. The data comes from a set of tomographic measurements conducted by the satellite during 2010 and 2011. The expected ice mass density and cloud frequency for conditions of thermodynamic equilibrium, calculated using the temperature and water vapour as measured by SMR, are compared to the ice mass density and cloud frequency as measured by OSIRIS. Similar to previous studies, we find that assuming thermodynamic equilibrium reproduces the seasonal, latitudinal and vertical variations in ice mass density and cloud frequency, but with a high bias of a factor of 2 in ice mass density. To explain this bias we use a simple ice particle growth model to estimate the time it would take for the observed clouds to sublimate completely and the time it takes for these clouds to reform. We find a difference in the median sublimation time (2.1 h) and the reformation time (3.2 h) at peak cloud altitudes (82-84 km). This difference implies that temperature variations on these timescales have a tendency to reduce the ice content of the clouds, explaining the high bias of the equilibrium model. Finally, we detect, and are for the first time able to positively identify, cloud features with horizontal scales of 100 to 300 km extending far below the region of supersaturation (> 2 km). Using the growth model, we conclude these features cannot be explained by sedimentation alone, and suggest that these events may be indication of strong vertical transport. [ABSTRACT FROM AUTHOR]