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Sudden stratospheric warmings (SSWs) could act as an important mediator in the vertical coupling of atmospheric regions and dramatic variations in the mesosphere and lower thermosphere (MLT) in response to SSWs have been documented. However, due to rare occurrences, SSWs in the Southern Hemisphere (SH) and their impacts on the MLT dynamics are not well understood. This study presents an analysis of MLT winds at ∼80–98 km altitudes measured by meteor radars located at Tierra del Fuego (53.7°S, 67.7°W), King Edward Point (54.3°S, 36.5°W) and King Sejong Station (62.2°S, 58.8°W) near 60°S latitude during the Antarctic winter. Eastward zonal winds from these stations are observed to decrease significantly near the peak date of the 2019 Antarctic SSW, and both zonal and meridional winds in 2019 exhibit considerable differences to the mean winds averaged over other non‐SSW years. A quasi 6‐day oscillation is observed at all three radar locations, being consistent with the presence of the westward propagating zonal wave‐1 planetary wave. The vertical wavelength of this wave is estimated to be ∼55 km, and the enhancement of the wave amplitude during this SSW is noticeable. Evidence of the interaction between the 6‐day wave and the semidiurnal diurnal tide is provided, which suggests a possible mechanism for SSWs to impact the upper atmosphere. This study reports the large‐scale variations in winds in the MLT region at SH midlatitudes to high latitudes in a key dynamic but largely unexplored latitudinal band in response to the 2019 Antarctic SSW. Plain Language Summary: Sudden stratospheric warmings (SSWs) manifest dynamic disruptions in the polar winter stratosphere, characterized as rapid changes in temperature and wind within a few days. Although SSWs are by definition a stratospheric phenomenon, they have significant impacts throughout the middle and upper atmosphere. Many studies of the SSW impacts on the mesosphere and lower thermosphere (MLT) have been performed, mostly for the Northern Hemisphere (NH). In the Southern Hemisphere (SH), SSW events are rare and thus the Antarctic SSWs are not well known. An unusual SSW occurred in the SH during September 2019, and the work presented here focuses on studying the MLT winds observed by three meteor radars located at Tierra del Fuego (53.7°S, 67.7°W) in Argentina, King Edward Point (54.3°S, 36.5°W) on South Georgia Island, and King Sejong Station (62.2°S, 58.8°W) in King George Island. These observations are over a key dynamic but largely unexplored region around the Drake Passage. This study presents the large‐scale variations in the MLT winds at SH midlatitudes to high latitudes which are believed to be in response to the 2019 Antarctic SSW. Possible mechanisms for SSWs to impact the upper atmosphere are discussed. Key Points: Large wind disturbances are observed at ∼80–98 km altitude ∼60°S by three meteor radars during the 2019 Antarctic stratospheric warmingA quasi 6‐day oscillation is observed at different longitudes indicating the presence of the westward propagating zonal wave‐1 waveThis study provides observational evidence of the nonlinear interaction between the 6‐day planetary wave and the semidiurnal tide [ABSTRACT FROM AUTHOR]

Sporadic potassium (Ks) and sodium (Nas) layers were investigated using more than 1,500 h of simultaneous Na/K LIDAR data at São José dos Campos, Brazil (23.1°S, 45.9°W). These events were observed on more than 88% of the analyzed nights. Although Na and K are both alkali metals, their sporadic layers show different behaviors. The Nas layers were in general 0.7 km wider and occurred ∼2 km higher than Ks layers. Moreover, the Nas layers progression is faster than that of Ks layers not only in the process of reaching their maximum amplitude but also in terms of dissipation. Our study showed that most Ns events (80% of Ks layers and 90% of Nas layers) have a strength factor of upto 5. Moreover, on average, Ks presented a strength factor ∼1.3 times stronger than that of Nas. The relative frequency of occurrence for both Ks and Nas layers exhibits an almost semiannual variation, with maxima in January/October and February/September, respectively. On the other hand, no similar variation was observed in the peak density or in the peak height. The peak density shows an annual variation, with a maximum occurring during winter for Ks events and a semiannual variation with a maxima occurring at the equinoxes for Nas. Different seasonal behaviors were also seen in the peak height of the identified layers, with Ks presenting a semiannual variation with maxima around January/October, while an annual variation occurred in Nas with lower layers occurring around winter. Key Points: This study presents the first simultaneous analysis of averaged K and Na sporadic layer parameters in the Southern HemisphereSeasonal and nocturnal variations in sporadic events were observed in K and Na layersThe differing behaviors of neutral sporadic layers were described as seen in K and Na mesospheric layers [ABSTRACT FROM AUTHOR]

Based on the long‐term observations of the Wuhan and Beijing Mesosphere‐Stratosphere‐Troposphere (MST) radars, the statistic characteristics of the mesospheric vertical winds at midlatitudes are investigated for the first time. The vertical wind data come from the Wuhan MST radar (29.51°N, 114.13°E) during 2012–2017 and from the Beijing MST radar (39.75°N, 116.96°E) during 2012–2014. The subdaily, seasonal, and annual variations of the mesospheric vertical winds at Wuhan and Beijing are presented. No obvious subdaily and seasonal variations are found and the annual variations of the mean vertical winds are downward in most of time. The mean vertical winds at Wuhan become smaller in summer and turn to upward at some heights, while those of Beijing only become smaller in summer. The difference of summer vertical winds over Wuhan and Beijing is closely related to the gravity wave activity. The Wuhan MST radar is located to the east side of the Qinghai‐Tibet plateau and also in the East Asian monsoon region, thus the mesospheric winds over Wuhan may experience more influence of gravity waves compared with those over Beijing. The possible bias sources in the vertical wind measurement of the two MST radars are carefully analyzed and the presented wind velocities are proved to be credible. The measured velocity magnitude is similar to other Very High Frequency radar observations, but much larger than that of the WACCM simulation. The Stokes drift, multicellular circulation structures, the wave influence on the radar echoes are raised as the potential sources for this discrepancy. Key Points: Wuhan and Beijing MST radars are used to study the statistical characteristics of the midlatitude mesospheric vertical windsEstimated mesospheric vertical winds at Wuhan and Beijing are generally downward all the year, and much larger than the model dataMuch more gravity wave activities are responsible for the summer difference of the mesospheric vertical winds [ABSTRACT FROM AUTHOR]

We have demonstrated a remote magnetometer based on sodium atoms in the Earth's mesosphere, at a 106‐km distance from our instrument. A 1.33‐watt laser illuminated the atoms, and the magnetic field was inferred from backscattered light collected by a telescope with a 1.55‐m‐diameter aperture. We theoretically predict a shot noise limited measurement sensitivity of 19nT/Hz. The measured sensitivity was 162nT/Hz due to a smaller returned intensity and smaller resonance strength than expected. The value of magnetic field inferred from our measurement is consistent with several models of the Earth's field shape to within a fraction of a percent. Projected improvements in optics, plus the use of advanced lasers or a large telescope, could result in 1‐nT/Hz sensitivity. Plain Language Summary: The upper end of a spinning top will slowly revolve around a vertical axis, a phenomenon known as precession. The precession frequency of the top is proportional to the strength of the gravitational field in which the top sits. This work uses the precession of atomic spins to analogously measure the strength of a magnetic field. The interaction of light with the atoms is sensitive to this precession and allows existing devices to observe the precessing atoms within an instrument. We have, for the first time, measured the magnetic field 100 km from our instrument using naturally occurring sodium atoms in the Earth's mesosphere. Atomic‐based measurements, such as optical magnetometers, are typically performed in well‐controlled environments. Here we demonstrate the ability to initialize quantum spin states and readout the evolution of the state at a significant standoff distance using a modified laser guidestar. The technique provides unique information about the mesospheric region of the atmosphere that is inaccessible by either aircraft or satellites. The small amount of light collected by our telescope creates a much less sensitive measurement than typical optical magnetometers, but methods are discussed that can improve the performance of this initial setup to reach a sensitivity needed for advancing understanding of geomagnetic field fluctuations such as those due to solar eruptions. This novel measurement technique is also an example of feasibility for other remote quantum measurements in the ambient atmosphere. Key Points: Remotely measured the geomagnetic field at a 90‐km altitude, this is an altitude that was previously difficult to monitorMeasured geomagnetic field was 45,441 nT, comparable to geomagnetic models, with a sensitivity of 162 nT/√HzSeveral improvements to the present measurements are discussed; the sensitivity could be improved to better than 1 nT/√Hz [ABSTRACT FROM AUTHOR]

In this paper an attempt is made to study equatorial Kelvin waves using a network of three radars: Kototabang (0.204° S, 100.320° E) meteor radar, Pameungpeuk (7.646° S, 107.688° E) medium-frequency radar, and Pontianak (0.003° S, 109.367° E) medium-frequency radar. We have used the continuous data gathered from the three radars during April-May 2010. Empirical mode decomposition (EMD), Lomb-Scargle periodogram (LSP) analysis, and wavelet techniques are used to study the temporal and altitude structures of planetary waves. Here, we used a novel technique called EMD to extract the planetary waves from wind data. The planetary waves of ~6.5 and ~3.6 days periodicity are observed in all three radar stations with peak amplitudes of about 12 and 11ms-1, respectively. The 3.6- day wave has an average vertical wavelength from the three radars of about 42 km. The 3.6- and 6.5-day planetary waves are particularly strong in the zonal wind component. We find that the two waves are present at the 84-94 km height region. The observed features of the 3.6- and 6.5-day waves at the three tropical-latitude stations show some correspondence with the results reported for the equatorial-latitude stations. [ABSTRACT FROM AUTHOR]

Mesospheric airglow measurements of two or three layers were used to characterize both vertical and horizontal parameters of gravity waves. The data set was acquired coincidentally from a multi-channel filter (Multi-3) photometer and an all-sky imager located at São João do Cariri (7:4° S, 36:5° W) in the equatorial region from 2001 to 2007. Using a least-square fitting and wavelet analysis technique, the phase and amplitude of each observed wave were determined, as well as the amplitude growth. Using the dispersion relation of gravity waves, the vertical and horizontal wavelengths were estimated and compared to the horizontal wavelength obtained from the keogram analysis of the images observed by an all-sky imager. The results show that both horizontal and vertical wavelengths, obtained from the dispersion relation and keogram analysis, agree very well for the waves observed on the nights of 14 October and 18 December 2006. The determined parameters showed that the observed wave on the night of 18 December 2006 had a period of ~ 43:8 ±; 2:19 min, with the horizontal wavelength of 235:66 ±; 11:78 km having a downward phase propagation, whereas that of 14 October 2006 propagated with a period of ~ 36:00 ±; 1:80 min with a horizontal wavelength of ~ 195±; 9:80 km, and with an upward phase propagation. The observation of a wave taken by a photometer and an allsky imager allowed us to conclude that the same wave could be observed by both instruments, permitting the investigation of the two-dimensional wave parameter. [ABSTRACT FROM AUTHOR]

A capacitively coupled discharge source equipped with liquid nitrogen cooling system has been developed to create water-ice grains in a plasma environment at the Korea Atomic Energy Research Institute. We found that the gravitational force exerted on 5- $\mu \text{m}$ water-ice grains is nearly compensated by the thermophoretic force when the upper electrode is cooler than that of the bottom electrode by 10–15 K and we observed two distinct, axisymmetric vortex flows are formed in the upper and lower parts of the plasma. The numerical calculation solving a set of equations including the vorticity equation confirms that the observed axisymmetric vortices result from the nonconservative ion drag force. [ABSTRACT FROM AUTHOR]

Mesospheric hydroxyl, molecular oxygen, and ozone airglow emission data from the SABER instrument on the NASA TIMED satellite are analyzed. Particular attention is paid to the dynamics of the altitude airglow volume emission rate (VER) profiles with the goal of establishing the cause or causes of occasional observed multi-peak and bifurcation anomalies in the tangent point limb emissions and in the derived VER profiles. These anomalies are important because they may lead to discrepancies in the analysis of the VER profiles. Knowledge about the causes and characteristics of the mesospheric airglow are important to understanding of the Earth's dynamic environment including climate change. [ABSTRACT FROM AUTHOR]

New homogeneous time series of daily standard phase-height (SPH) and daily plasma scale-height (PSH) have been derived from a 50-year long-radio-wave measurement of the broadcasting station Allouis (France, 162 kHz). The signal was received at Kühlungsborn (54°N, 12°E, Mecklenburg, Germany) and the present series is a third release. The daily time series of SPH shows in its spectrum dominant modes which are typical for the solar cycle (SC), for El Niño-Southern Oscillation (ENSO) and for quasi-biannual oscillation (QBO), indicating solar and lower atmospheric influences. Surprisingly, the time series of daily PSH shows a band of dominant cycles larger than 16 years. In order to exclude the influence of the winter anomaly in the determination of column-integrated mesospheric temperature trends the phase-height-temperature procedure is confined to summer months. The derived thickness temperature of the mesosphere decreased statistically significant over the period 1959–2008 after pre-whitening with summer mean of solar sun spot numbers. The trend value is in the order of about −1.05 K/decade if the stratopause trend is excluded. The linear regression is more pronounced, −1.35 K/decade for the period of 1963–1985 (2 SCs), but weaker, −0.51 K/decade during 1986-2008 (last 2 SCs). The linear regression is in very good agreement with a mean column-integrated mesospheric trend derived from OHP-Lidar temperatures on a monthly mean basis for the last two SCs. This clearly shows that the thickness temperature of the mesosphere derived from phase-height measurement is a useful proxy for the long-term summer temperature change in the mesosphere from 1959 until 2008. [ABSTRACT FROM AUTHOR]

We investigate long-term Polar Mesosphere Summer Echoes (PMSEs) observations conducted by the northernmost geographically located MST radar at Eureka (80°N, 86°W). While PMSEs are a well recognized summer phenomenon in the polar regions, previous calibrated studies at Resolute Bay and Eureka using 51.5 MHz and 33 MHz radars respectively, showed that PMSE backscatter signal strengths are relatively weak in the polar cap sites, compared to the auroral zone sites ( Swarnalingam et al., 2009b; Singer et al., 2010 ). Complications arise with PMSEs in which the echo strength is controlled by the electrons, which are, in turn, influenced by heavily charged ice particles as well as the variability in the D-region plasma. In recent years, PMSE experiments were conducted inside the polar cap utilizing a 51 MHz radar located at Eureka. In this paper, we investigate calibrated observations, conducted during 2009–2015. Seasonal and diurnal variations of the backscatter signal strengths are discussed and compared to previously published results from the ALOMAR radar, which is a radar of similar design located in the auroral zone at Andenes, Norway (69°N, 16°E). At Eureka, while PMSEs are present with a daily occurrence rate which is comparable to the rate observed at the auroral zone site for at least two seasons, they show a great level of inter-annual variability. The occurrence rate for the strong echoes tends to be low. Furthermore, comparison of the absolute backscatter signal strengths at these two sites clearly indicates that the PMSE backscatter signal strength at Eureka is weak. Although this difference could be caused by several factors, we investigate the intensity of the neutral air turbulence at Eureka from the measurements of the Doppler spectrum of the PMSE backscatter signals. We found that the level of the turbulence intensity at Eureka is weak relative to previously reported results from three high latitude sites. [ABSTRACT FROM AUTHOR]

The mesospheric metal layers are formed by the vaporization of high-speed cosmic dust particles as they enter the Earth's upper atmosphere. We show that the downward fluxes of these metal atoms, induced locally by waves and turbulence, are related in a straightforward way to the meteoric influxes of the metals, their chemical losses and their advective transport by the large-scale vertical and horizontal motions associated with the meridional circulation system. Above the peak of the metal layers where chemical losses and large-scale vertical motions are small, the wave-induced flux is insensitive to changes in local wave activity. If the downward transport velocity increases, because wave activity increases, then in response, the metal densities will decrease to maintain a constant vertical flux. By fitting the theoretical Na flux profile to the annual mean vertical flux profile measured during the night at the Starfire Optical Range, NM, we derive improved estimates for the global influxes of both Na and cosmic dust. The mean Na influx is 22,500±1050 atoms/cm 2 /s, which equals 389±18 kg/d for the global input of Na vapor. If the Na composition of the dust particles is identical to CI chondritic meteorites (4990 ppm by mass), then the global influx of cosmic dust is 176±38 t/d. If the composition is identical to ordinary chondrites (7680 ppm), the global dust influx is 107±22 t/d. [ABSTRACT FROM AUTHOR]

Polar Mesospheric Cloud (PMC) observations from the Solar Occultation for Ice Experiment (SOFIE) and the Cloud Imaging and Particle Size (CIPS) experiment are used to investigate the response of PMCs to forcing associated with the 27-day solar rotation. We quantify the PMC response in terms of sensitivity values. Analysis of PMC data from 14 seasons indicate a large seasonal variability in sensitivity with both correlation and anti-correlation between PMC properties and Lyman-alpha irradiance for individual seasons. However, a superposed epoch analysis reveals the expected anti-correlation between variations in solar Lyman-alpha and variations in PMC ice water content, albedo, and frequency of occurrence. The PMC height is found to significantly correlate with 27-day variations in solar Lyman-alpha in the Southern Hemisphere (SH), but not in the Northern hemisphere (NH). Depending on instrument and property, the time lag between variations in PMC properties and solar Lyman-alpha ranges from 0 to 3 days in the NH and from 6 to 7 days in the SH. These hemispheric differences in PMC height and time lag are not understood, but it is speculated that they result from dynamical forcing that is controlled by the 27-day solar cycle. [ABSTRACT FROM AUTHOR]

Observations by the Cloud Imaging and Particle Size (CIPS) instrument on the Aeronomy of Ice in the Mesosphere (AIM) satellite have demonstrated the existence of Polar Mesospheric Cloud (PMC) regions populated by particles whose mean sizes range between 60 and 100 nm (radii of equivalent volume spheres). It is known from numerous satellite experiments that typical mean PMC particle sizes are of the order of 40–50 nm. Determination of particle size by CIPS is accomplished by measuring the scattering of solar radiation at various scattering angles at a spatial resolution of 25 km 2 . In this size range we find a robust anti-correlation between mean particle size and albedo. These very-large particle-low-ice (VLP-LI) clouds occur over spatially coherent areas. The surprising result is that VLP-LI are frequently present either in the troughs of gravity wave-like features or at the edges of PMC voids. We postulate that an association with gravity waves exists in the low-temperature summertime mesopause region, and illustrate the mechanism by a gravity wave simulation through use of the 2D Community Aerosol and Radiation Model for Atmospheres (CARMA). The model results are consistent with a VLP-LI population in the cold troughs of monochromatic gravity waves. In addition, we find such events in Whole Earth Community Climate Model/CARMA simulations, suggesting the possible importance of sporadic downward winds in heating the upper cloud regions. This newly-discovered association enhances our understanding of the interaction of ice microphysics with dynamical processes in the upper mesosphere. [ABSTRACT FROM AUTHOR]

Recent high resolution temperature measurements by resonance lidar occasionally showed a sudden mesopause altitude increase by ∼5 km and an associated mesopause temperature decrease by ∼10 K at Davis (69°S). In this paper we present further observations which are closely related to this ‘mesopause jump’, namely the increase of mean height of polar mesospheric summer echoes (PMSE) observed by a VHF radar, very strong westward winds in the upper mesosphere measured by an MF radar, and relatively large eastward winds in the stratosphere taken from reanalysis. We present a detailed explanation of mesopause jumps. They occur only when stratospheric winds are moderately eastward and mesospheric winds are strongly westward. Under these conditions, gravity waves with comparatively large eastward phase speeds can pass the stratosphere and propagate to the lower thermosphere because their vertical wavelengths in the mesosphere are rather large which implies enhanced dynamical stability. When finally breaking in the lower thermosphere, these waves drive an enhanced residual circulation that causes a cold and high-altitude mesopause. The conditions for a mesopause jump occur only in the Southern Hemisphere (SH) and are associated with the late breakdown of the polar vortex. Mesopause jumps are primarily, but not only, observed prior and close to solstice. Our study also shows that during the onset of PMSE in the SH, stratospheric zonal winds are still eastward (up to 30 m/s), and that the onset is not closely related to the transition of the stratospheric circulation. Unlike previously published results with polar mesospheric clouds, we find an overall poor correlation between PMSE onset and the date of the vortex breakdown. [ABSTRACT FROM AUTHOR]

We report measurements of atomic sodium, iron and temperature in the mesosphere and lower thermosphere (MLT) made by ground-based lidars at the ALOMAR observatory (69°N, 16°E) during a major sudden stratospheric warming (SSW) event that occurred in January 2009. The high resolution temporal observations allow the responses of the Na and Fe layers to the SSW at high northern latitudes to be investigated. A significant cooling with temperatures as low as 136 K around 90 km was observed on 22–23 January 2009, along with substantial depletions of the Na and Fe layers (an ~80% decrease in the column abundance with respect to the mean over the observation period). The Whole Atmosphere Community Climate Model (WACCM) incorporating the chemistry of Na, Fe, Mg and K, and nudged with reanalysis data below 60 km, captures well the timing of the SSW, although the extent of the cooling and consequently the depletion in the Na and Fe layers is slightly underestimated. The model also predicts that the perturbations to the metal layers would have been observable even at equatorial latitudes. The modelled Mg layer responds in a very similar way to Na and Fe, whereas the K layer is barely affected by the SSW because of the enhanced conversion of K + ions to K atoms at the very low temperatures. [ABSTRACT FROM AUTHOR]

In this letter, a high pulse energy and high beam quality 819.7 nm Ti:sapphire laser pumped by a frequency-doubled Nd:YAG laser is demonstrated. At incident pump energy of 774 mJ, the maximum output energy of 89 mJ at 819.7 nm with a pulse width of 100 μs is achieved at a repetition rate of 5 Hz. To the best of our knowledge, this is the highest pulse energy at 819.7 nm with pulse width of hundred microseconds for a Ti:sapphire laser. The beam quality factor M is measured to be 1.18. This specific wavelength with the high pulse energy and high beam quality at 819.7 nm is a promising light source to create a polychromatic laser guide star together with a home-made 589 nm laser via exciting the sodium atoms in the mesospheric atmosphere. [ABSTRACT FROM AUTHOR]

We compare simulations from three high-top (with upper lid above 120 km) and five medium-top (with upper lid around 80 km) atmospheric models with observations of odd nitrogen (NOx = NO+NO2), temperature, and carbon monoxide from seven satellite instruments (ACE-FTS on SciSat, GOMOS, MIPAS, and SCIAMACHY on Envisat, MLS on Aura, SABER on TIMED, and SMR on Odin) during the Northern Hemisphere (NH) polar winter 2008/2009. The models included in the comparison are the 3-D chemistry transport model 3dCTM, the ECHAM5/MESSy Atmospheric Chemistry (EMAC) model, FinROSE, the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA), the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA), the modelling tools for SOlar Climate Ozone Links studies (SOCOL and CAO-SOCOL), and the Whole Atmosphere Community Climate Model (WACCM4). The comparison focuses on the energetic particle precipitation (EPP) indirect effect, that is, the polar winter descent of NOx largely produced by EPP in the mesosphere and lower thermosphere. A particular emphasis is given to the impact of the sudden stratospheric warming (SSW) in January 2009 and the subsequent elevated stratopause (ES) event associated with enhanced descent of mesospheric air. The chemistry climate model simulations have been nudged toward reanalysis data in the troposphere and stratosphere while being unconstrained above. An odd nitrogen upper boundary condition obtained from MIPAS observations has further been applied to medium-top models. Most models provide a good representation of the mesospheric tracer descent in general, and the EPP indirect effect in particular, during the unperturbed (pre-SSW) period of the NH winter 2008/2009. The observed NOx descent into the lower mesosphere and stratosphere is generally reproduced within 20 %. Larger discrepancies of a few model simulations could be traced back either to the impact of the models' gravity wave drag scheme on the polar wintertime meridional circulation or to a combination of prescribed NOx mixing ratio at the uppermost model layer and low vertical resolution. In March-April, after the ES event, however, modelled mesospheric and stratospheric NOx distributions deviate significantly from the observations. The too-fast and early downward propagation of the NOx tongue, encountered in most simulations, coincides with a temperature high bias in the lower mesosphere (0.2-0.05 hPa), likely caused by an overestimation of descent velocities. In contrast, upper-mesospheric temperatures (at 0.05-0.001 hPa) are generally underestimated by the high-top models after the onset of the ES event, being indicative for too-slow descent and hence too-low NOx fluxes. As a consequence, the magnitude of the simulated NOx tongue is generally underestimated by these models. Descending NOx amounts simulated with medium-top models are on average closer to the observations but show a large spread of up to several hundred percent. This is primarily attributed to the different vertical model domains in which the NOx upper boundary condition is applied. In general, the intercomparison demonstrates the ability of state of- the-art atmospheric models to reproduce the EPP indirect effect in dynamically and geomagnetically quiescent NH winter conditions. The encountered differences between observed and simulated NOx, CO, and temperature distributions during the perturbed phase of the 2009 NH winter, however, emphasize the need for model improvements in the dynamical representation of elevated stratopause events in order to allow for a better description of the EPP indirect effect under these particular conditions. [ABSTRACT FROM AUTHOR]

Using 3 years (2013-2015) of O2(0-1) and OH(6-2) band nightglow emission intensities and corresponding rotational temperatures as tracers of mesospheric dynamics, we have investigated large- and small-timescale variations in the mesosphere over a low-latitude location, Gurushikhar, Mount Abu (24.6° N, 72.8° E), in India. Both O2 and OH intensities show variations similar to those of the number of sunspots and F10.7 cm radio flux with coherent periodicities of 150 ± 2.1, 195 ± 3.6, 270 ± 6.4, and 420 ± 14.8 days, indicating a strong solar influence on mesospheric dynamics. In addition, both mesospheric airglow intensities also showed periodicities of 84 ± 0.6, 95 ± 0.9, and 122 ± 1.3 days which are of atmospheric origin. With regard to the variability of the order of a few days, O2 and OH intensities were found to be correlated, in general, except when altitude-dependent atmospheric processes were operative. To understand mesospheric gravity wave behavior over the long term, we have carried out a statistical study using the periodicities derived from the nocturnal variations in all four parameters (O2 and OH intensities and their respective temperatures). It was found that the major wave periodicity of around 2 h duration is present in all the four parameters. Our analyses also reveal that the range of periods in O2 and OH intensities and temperatures is 11 to 24 and 20 to 60 min, respectively. Periods less than 15 min were not present in the temperatures but were prevalent in both emission intensities. No seasonal dependence was found in either the wave periodicities or the number of their occurrence. [ABSTRACT FROM AUTHOR]

An intense activity of ripples during the nighttime was observed in airglow images over São João do Cariri (36.5° W, 7.4° S) on 10 October 2004 which lasted for two hours. Those ripples appeared simultaneously with the crossing of a mesospheric front and medium scale gravity waves. The ripples occurred ahead of the mesospheric front and their phase front were almost parallel to the phase of the mesospheric front and were almost perpendicular to the phase front of the gravity wave. Using wind measurements from a meteor radar located at São João do Cariri and simultaneous vertical temperature profiles from the TIMED/SABER satellite, on the night of the events and within the imager field of view, the atmospheric background environment in the mesosphere and lower thermosphere (MLT) was investigated in order to understand the instability process that caused the appearance of the ripples. Dynamic and convective instabilities have been pointed out as responsible for creation of ripples in the MLT. The observed ripples were advected by the neutral wind, they occurred into a region with negative lapse rate of the potential temperature and the Richardson number was negative as well. According to these characteristics, the ripple structures could be generated in the MLT region due to the predominance of convective instability. [ABSTRACT FROM AUTHOR]

Simultaneous imaging observations of a noctilucent cloud (NLC) from five sites in Hokkaido, Japan (43.17-45.36°N), were successfully carried out using digital cameras in the early hours of the morning (around 02:00 LST) on June 21, 2015. This is the first NLC event that has been captured from multiple sites in Japan. The simultaneous images obtained from multiple sites made it possible to calculate the exact altitude (=83.9 ± 0.1 km) and spatial distribution (47.5-50.0°N and 143.0-147.5°E) of the NLC by triangulation and image correlation methods. Based on a comparison of atmospheric parameters of the upper mesosphere provided by satellites and a middle-frequency (MF) radar in northern Hokkaido (Wakkanai) with the cloud distribution obtained from the Aeronomy of Ice in the Mesosphere satellite, this particular event is considered to be the result of southward advection of the NLC from a higher-latitude (i.e., colder) region. Anomalies in the upper mesospheric temperature of the northern hemispheric summer in 2015 were examined using AURA satellite data, because this is the first NLC event that has been identified in Japan. However, no remarkable temperature variations relative to other years were found in upper mesosphere. Based on a comparison between the NLC period and the record of sky conditions archived by the Japan Meteorological Agency, a high percentage of cloud (especially low-level) cover during the summer in Hokkaido cannot be ruled out as a possible reason why the NLC had not previously been sighted in Hokkaido. [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. 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 investigate 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 (1.8 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, possibly 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 an indication of strong vertical transport. [ABSTRACT FROM AUTHOR]

The article discusses polar mesospheric clouds, or noctilucent clouds. It includes information on the thermodynamics and weather conditions that are involved in their formation in the mesophere, as seen from countries in the northern hemisphere. It contains charts of sightings of these clouds in the northern countries, and shows that there were more sightings in 2013-14 than 2011-12.

Meteorological and atmospheric models are being extended up to 80 km altitude but there are very few observing techniques that can measure stratospheric–mesospheric winds at altitudes between 20 and 80 km to verify model datasets. Here we demonstrate the feasibility of horizontal wind profile measurements using ground-based passive millimetre-wave spectroradiometric observations of ozone lines centred at 231.28, 249.79, and 249.96 GHz. Vertical profiles of horizontal winds are retrieved from forward and inverse modelling simulations of the line-of-sight Doppler-shifted atmospheric emission lines above Halley station (75°37′ S, 26°14′ W), Antarctica. For a radiometer with a system temperature of 1400 K and 30 kHz spectral resolution observing the ozone 231.28 GHz line we estimate that 12 h zonal and meridional wind profiles could be determined over the altitude range 25–74 km in winter, and 28–66 km in summer. Height-dependent measurement uncertainties are in the range 3–8 m s−1 and vertical resolution  ∼  8–16 km. Under optimum observing conditions at Halley a temporal resolution of 1.5 h for measuring either zonal or meridional winds is possible, reducing to 0.5 h for a radiometer with a 700 K system temperature. Combining observations of the 231.28 GHz ozone line and the 230.54 GHz carbon monoxide line gives additional altitude coverage at 85 ± 12 km. The effects of clear-sky seasonal mean winter/summer conditions, zenith angle of the received atmospheric emission, and spectrometer frequency resolution on the altitude coverage, measurement uncertainty, and height and time resolution of the retrieved wind profiles have been determined. [ABSTRACT FROM AUTHOR]

Since December 2013 the new imaging system FAIM (Fast Airglow IMager) for the study of smaller-scale features (both in space and time) is in routine operation at the NDMC (Network for the Detection of Mesospheric Change) station at DLR (German Aerospace Center) in Oberpfaffenhofen (48.1° N, 11.3° E). Covering the brightest OH vibrational bands between 1 and 1.7 µm, this imaging system can acquire two frames per second. The field of view is approximately 55 km times 60 km at the mesopause heights. A mean spatial resolution of 200m at a zenith angle of 45° and up to 120m for zenith conditions are achieved. The observations show a large variety of atmospheric waves. This paper introduces the instrument and compares the FAIM data with spectrally resolved GRIPS (GRound-based Infrared P-branch Spectrometer) data. In addition, a case study of a breaking gravity wave event, which we assume to be associated with Kelvin-Helmholtz instabilities, is discussed. [ABSTRACT FROM AUTHOR]

During the last decades, ground-based microwave radiometry has matured into an established remote sensing technique for measuring vertical profiles of a number of gases in the stratosphere and the mesosphere. Microwave radiometry is the only ground-based technique that can provide vertical profiles of gases in the upper stratosphere and mesosphere both day and night, and even during cloudy conditions. Except for microwave instruments placed at high-altitude sites, or at sites with dry atmospheric conditions, only molecules with significant emission lines below 150 GHz, such as CO, H2O, and O3, can be observed. Vertical profiles of these molecules can give important information about chemistry and dynamics in the middle atmosphere. Today these measurements are performed at relatively few sites; more simple and reliable instrument solutions are required to make the measurement technique more widely spread. This need is urgent today as the number of satellite sensors observing the middle atmosphere is about to decrease drastically. In this study a compact double-sideband frequency-switched radiometer system for simultaneous observations of mesospheric CO at 115.27 GHz and O3 at 110.84 GHz is presented. The radiometer, its calibration scheme, and its observation method are presented. The retrieval procedure, including compensation of the different tropospheric attenuations at the two frequencies and error characterization, are also described. The first measurement series from October 2014 until April 2015 taken at the Onsala Space Observatory, OSO (57° N, 12° E), is analysed. The retrieved vertical profiles are compared with co-located CO and O3 data from the MLS instrument on the Aura satellite. The data sets from the instruments agree well with each other. The main differences are the higher OSO volume mixing ratios of O3 in the upper mesosphere during the winter nights and the higher OSO volume mixing ratios of CO in the mesosphere during the winter. The low bias of mesospheric winter values of CO from MLS compared to ground-based instruments was reported earlier. [ABSTRACT FROM AUTHOR]

During the last decades, ground-based microwave radiometry has matured into an established remote sensing technique for measuring vertical profiles of a number of gases in the stratosphere and the mesosphere. Microwave radiometry is the only ground-based technique that can provide vertical profiles of gases in the upper stratosphere and mesosphere both day and night, and even during cloudy conditions. Except for microwave instruments placed at high-altitude sites, or at sites with dry atmospheric conditions, only molecules with significant emission lines below 150 GHz, such as CO, H2O, and O3, can be observed. Vertical profiles of these molecules can give important information about chemistry and dynamics in the middle atmosphere. Today these measurements are performed at relatively few sites; more simple and reliable instrument solutions are required to make the measurement technique more widely spread. This need is urgent today as the number of satellite sensors observing the middle atmosphere is about to decrease drastically. In this study a compact double-sideband frequency-switched radiometer system for simultaneous observations of mesospheric CO at 115.27 GHz and O3 at 110.84 GHz is presented. The radiometer, its calibration scheme, and its observation method are presented. The retrieval procedure, including compensation of the different tropospheric attenuations at the two frequencies and error characterization, are also described. The first measurement series from October 2014 until April 2015 taken at the Onsala Space Observatory, OSO (57° N, 12° E), is analysed. The retrieved vertical profiles are compared with co-located CO and O3 data from the MLS instrument on the Aura satellite. The data sets from the instruments agree well with each other. The main differences are the higher OSO volume mixing ratios of O3 in the upper mesosphere during the winter nights and the higher OSO volume mixing ratios of CO in the mesosphere during the winter. The low bias of mesospheric winter values of CO from MLS compared to ground-based instruments was reported earlier. [ABSTRACT FROM AUTHOR]

Gravity waves (GWs) greatly influence the background state of the middle atmosphere by imposing their momentum on the mean flow upon breaking and by thus driving, e.g., the upper mesospheric summer zonal wind reversal. In this situation momentum is conserved by a balance between the vertical divergence of GW momentum flux (the so-called GW drag) and the Coriolis acceleration of the mean meridional wind. In this study, we present first quantitative mean annual cycles of these two balancing quantities from the medium frequency Doppler radar at the polar site Saura (SMF radar, 69° N, 16° E). Three-year means for 2009 through 2011 clearly show that the observed zonal momentum balance between 70 and 100 km with contributions from GWs only is fulfilled during summer when GW activity is strongest and more stable than in winter. During winter, the balance between GW drag and Coriolis acceleration of the mean meridional wind is not existent, which is likely due to the additional contribution from planetary waves, which are not considered by the present investigation. The differences in the momentum balance between summer and winter conditions are additionally clarified by 3-month mean vertical profiles for summer 2010 and winter 2010/2011. [ABSTRACT FROM AUTHOR]

We report on the polar mesosphere summer echoes (PMSE) occurrence probability over SANAE (South African National Antarctic Expedition) IV, for the first time. A matching coincidence method is described and implemented for PMSE extraction from SuperDARN (Super Dual Auroral Radar Network) HF radar. Several SuperDARN-PMSE characteristics are studied during the summer period from years 2005 - 2007. The seasonal and interannual SuperDARN-PMSE variations in relation to the mesospheric neutral winds are studied and presented in this paper. The occurrence probability of SuperDARN-PMSE on the day-to-day scale show, predominantly, diurnal variation, with a broader peak between 12 - 14 LT and distinct minimum of 22 LT. The SuperDARN-PMSE occurrence probability rate is high in the summer solstice. Seasonal variations show a connection between the SuperDARN-PMSE occurrence probability rate and mesospheric temperature from SABER (Sounding of the Atmosphere using Broadband Emission Radiometry). The seasonal trend for both meridional and zonal winds is very stable year-to-year. Analysis of the neutral wind variations indicates the importance of pole-to-pole circulations in SuperDARN-PMSE generation. [ABSTRACT FROM AUTHOR]

The image observations of mesospheric O(1S) 558nm have been performed from a low latitude Indian station, Gadanki (13.5° N; 79.2°E) using a CCD based all sky camera system. Based on three years (from year 2012 to the year 2014) of image data during March-April, we characterize the small scale gravity wave properties. We noted 50 strong gravity wave event and 19 ripple events to occur. The horizontal wavelengths of the gravity waves are found to vary from 12 to 42 km with the phase velocity ranging from 20 to 90 km. In most cases, these waves were propagating towards north with only a few occasions of southward propagation. The outgoing longwave radiation data suggest that lower atmospheric convection was most possible reason for the generation of the waves observed in the airglow data. [ABSTRACT FROM AUTHOR]

Sudden stratospheric warmings (SSWs) are the most prominent vertical coupling process in the middle atmosphere, which occur during winter and are caused by the interaction of planetary waves (PWs) with the zonal mean flow. Vertical coupling has also been identified during the equinox transitions, and is similarly associated with PWs. We argue that there is a characteristic aspect of the autumn transition in northern high latitudes, which we call the "hiccup", and which acts like a "mini SSW", i.e. like a small minor warming. We study the average characteristics of the hiccup based on a superimposed epoch analysis using a nudged version of the Canadian Middle Atmosphere Model, representing 30 years of historical data. Hiccups can be identified in about half the years studied. The mesospheric zonal wind results are compared to radar observations over Andenes (69° N, 16° E) for the years 2000-2013. A comparison of the average characteristics of hiccups and SSWs shows both similarities and differences between the two vertical coupling processes. [ABSTRACT FROM AUTHOR]

We compare the nitric oxide measurements in the mesosphere and lower thermosphere (60 to 150 km) from four instruments: ACE-FTS, MIPAS, SCIAMACHY, and SMR. We use the daily zonal mean data in that altitude range for the years 2004-2010 (ACE-FTS), 2005-2012 (MIPAS), 2008-2012 (SCIAMACHY), and 2003-2012 (SMR). We first compare the data qualitatively with respect to the morphology, focussing on the major features, and then compare the time series directly and quantitatively. In three geographical regions, we compare the vertical density profiles on coincident measurement days. Since none of the instruments delivers continuous daily measurements in this altitude region, we carried out a multi-linear regression analysis. This regression analysis considers annual and semi-annual variability in form of harmonic terms and inter-annual variability by responding linearly to the solar Lyman- radiation index and the geomagnetic Kp index. This analysis helps to find similarities and differences in the individual data sets with respect to the inter-annual variations caused by geomagnetic and solar variability. We find that the data sets are consistent and that they only disagree on minor aspects. SMR and ACE-FTS deliver the longest time series in the mesosphere and they both agree remarkably well. The shorter time series from MIPAS and SCIAMACHY also agree with them where they overlap. The data agree within ten to twenty percent when the number densities are large, but they can differ by 50 to 100% in some cases. [ABSTRACT FROM AUTHOR]

The airglow hydroxyl temperature record from Longyearbyen, Svalbard, is updated with data from the last seven seasons (2005/2006-2011/2012). The temperatures are derived from ground-based spectral measurements of the hydroxyl airglow layer, which ranges from 76 to 90 km height. The overall daily average mesospheric temperature for the whole temperature record is 206 K. This is by 3 K less than what Dyrland and Sigernes (2007) reported in their last update on the temperature series. This temperature difference is due to cold winter seasons from 2008 to 2010. 2009/2010 was the coldest winter season ever recorded over Longyearbyen, with a seasonal average of 185 K. Temperature variability within the winter seasons is investigated, and the temperature difference between late December (local minimum) and late January (local maximum) is approximately 8 K. [ABSTRACT FROM AUTHOR]

By combining electrostatic measurements of lightning-induced electrostatic field changes with radio frequency lightning location, some field changes from exceptionally distant lightning events are apparent which are inconsistent with the usual inverse cube of distance. Furthermore, by using two measurement sites, a transition zone can be identified beyond which the electric field response reverses polarity. For these severe lightning events, we infer a horizontally extensive charge sheet above a thunderstorm, consistent with a mesospheric halo of several hundred kilometers' extent. [ABSTRACT FROM AUTHOR]

The article presents a study which examines the quality of the mesospheric temperatures of Optical Spectrograph and InfraRed Imaging System (OSIRIS) on the Odin satellites using satellite and ground-based measurements. The mesospheric temperatures of OSIRIS were in good agreement with the coincident temperature profiles during the study period. results of the study shows that the OSIRIS temperatures displayed a cold bias of 10-15 K.

The present study for the first time reports the global gravity wave activity in terms of their potential energy derived from TIMED/SABER observations right from the stratosphere to the mesosphere lower thermosphere (MLT) region. The potential energy profiles obtained from SABER temperature are validated by comparing them with ground based LIDAR observations over a low latitude site, Gadanki (13.5° N, 79.2° E). The stratospheric and mesospheric global maps of gravity wave energy showed pronounced maxima over high and polar latitudes of the winter hemisphere. The interannual variability of the stratospheric gravity wave activity exhibited prominent annual oscillation over mid-latitudes. The equatorial gravity wave activity exhibited quasi-biennial oscillation in the lower stratosphere and semi-annual oscillation in the upper stratosphere. The MLT region maps revealed summer hemispheric maxima over polar latitudes and secondary maxima over the equatorial region. The results are discussed in the light of present understanding of global gravity wave observations. The significance of the present study lies in emphasizing the importance of satellite measurements in elucidating gravity waves, which is envisaged to have profound impact on parameterizing these waves. [ABSTRACT FROM AUTHOR]

The mesosphere and lower thermosphere (MLT) wind structure over Wuhan (30° N, 114° E) in 2000/2001 winter and over Langfang (39.4° N, 116.6° E) in 2009/2010 winter are examined to reveal the effects of stratospheric sudden warming (SSW) in mid-low-latitude MLT region. The result shows that the MLT daily zonal wind over these two sites reversed from eastward wind to westward wind for several days during the SSW events. The reversals were almost coincident with the polar stratospheric temperature reaching its maximum at lOhPa, 90°N and were about ten days prior to the reversal of high latitude stratospheric zonal wind at lOhPa, 60° N. The temporal variations of tides, gravity waves and 2-day planetary waves in the mid-latitude MLT showed different behavior during the two SSW events. During the 2001 SSW event, MLT diurnal tide reached its maximum when the MLT zonal wind decreased rapidly and SSW event began in polar stratosphere; the activity of 2-day waves decreased after the onset of the 2001 SSW, while the gravity wave increased when the 2001 SSW developed into a major warming. However, in the 2009/2010 winter, the semidiurnal tide and 2-day wave in MLT over Langfang reached a peak about two days earlier than zonal wind reversal at 10 hPa, 60°N; no significant features were found in diurnal tides, terdiurnal tides and gravity waves related to the 2010 SSW event. [ABSTRACT FROM AUTHOR]

In November 2008 and in March and April 2009 the Indian MST radar (53 MHz) at (Gadanki was operated during the daytime in a special experiment, with 600 m altitude resolution, for understanding the characteristics of low-latitude mesospheric echoes (LLME). The data of three days when the echoes were strongest have been analysed in terms of spectral widths and radar volume reflectivities. Spectral widths of LLME show some decrease with altitude, with median values of 4-6ms-1 at 69-72km and of 2-4ms-1 at 73-78km. This corresponds to 20-200mW kg-1 turbulent energy dissipation rates. It has been shown that stronger echoes have broader spectra consistent with a turbulent scattering mechanism. For the first time, the volume reflectivities for the strong LLME for Gadanki have also been calculated. They are in the range of 10-17-10-15m-1, so LLME at Gadanki are somewhat stronger than those reported so far from Jicamarca, Peru (Lehmacher et al., 2009). [ABSTRACT FROM AUTHOR]

Abstract: We report nighttime mesospheric OH and O2 temperature measurements during 21–29 April, 2009, over Gadanki (13.5°N, 79.2°E), India. Large night-to-night variability is observed, which is dominated by large amplitudes of short period gravity wave features. It is noted that on 21 and 23 April 2009, the mean nocturnal temperature values for OH emissions altitudes were higher than the O2 emissions, which reversed on 27 April 2009. Further, the wavelet analysis of nighttime mesospehric temperature shows a persistent 35–50min wave for all the durations of observations. In this paper, we discuss the observed variability in the mesospheric OH and O2 temperatures in connection with the passage of convective activity noted over South India along the west coast. [Copyright &y& Elsevier]

Abstract: This paper reports the variation of the daytime zonal wind and OH temperature at upper mesospheric altitudes during the afternoon equatorial counter electrojet (CEJ) events over a geomagnetic dip equatorial station, Trivandrum (8.5°N, 77°E, 0.5°N diplat.), in India. The OH temperature is estimated using a unique Multiwavelength Dayglow Photometer and the zonal wind is obtained from a co-located Meteor Wind Radar. The CEJ events are identified by taking the difference between the surface magnetic field measurements of Trivandrum and Alibag (18.3°N, 72.6°E, 10.2°N diplat.), obtained using two Proton Precession Magnetometers. Two major features were observed associated with the CEJ events: (i) an eastward increase in the zonal wind at 98km altitudes and (ii) a clear-cut cooling in the mesopause, which in turn is proportional to the extent of the magnetic field reversal. These results are discussed in terms of the role played by the neutral winds, upward propagating gravity waves and the changes in the mesospheric chemistry. [Copyright &y& Elsevier]

Very low and low radio frequency (VLF/LF) propagation responds sensitively to the electron density distribution in the lower ionosphere (upper mesosphere). Whereas propagation paths crossing subpolar and polar regions are frequently affected by forcing from above by particle precipitations, mid- and lowlatitude paths let forcing from below be more prominent. Our observations (2009-2011) show, that the low frequency propagation conditions along the midlatitude path from Sicily to Germany (52° N 8° E) using the NSY 45.9 kHz transmitter (37° N 14° E) prove to be a good proxy of mesosphere planetary wave activity along the propagation path. High absorption events with VLF/LF propagation correlate to the well known winter time D-layer anomaly observed with high frequency (HF) radio waves. VLF/LF propagation calculations are presented which show that the radio signal amplitude variations can be modeled by planetary wave modulated collison frequency and electron density profiles. The other way around wave pressure amplitudes can be inferred from the VLF/LF data. [ABSTRACT FROM AUTHOR]

Regular and systematic measurements of mesospheric temperatures have been carried out during March and April 2007 to determine planetary-scale wave activities in the tropical region, utilizing ground-based Rayleigh Light Detection and Ranging (LiDAR) and satellite-based Microwave Limb Sounder (MLS) data from Gadanki, India (13.5° N, 79.2° E) and MLS data over a site in North America (23.5° N, 100° W). A dominant component of the 3-5 day period wave is revealed at four altitudes (80, 70, 60 and 50 km) over the two observation stations. The estimated vertical wavelength (∼40 km), zonal phase speed (∼140 m s-1), longitudinal and latitudinal extensions have suggested the wave to be an ultra-fast Kelvin (UFK) wave with zonal wave number 1. In addition to the UFK wave, a probable quasi 2 day Rossby gravity wave is also found to be present most of the time. Another 5-7 day wave component, observed at some altitudes with significant prominence, is surmised to be a manifestation of a 6.5 day wave. [ABSTRACT FROM AUTHOR]

The paper is devoted to optical remote-sensing techniques successfully applied to investigations of the mesospheric aerosol layer, starting with a short history of the development of optical sounding experiments. In the following part, a theoretical basis of scattering measurements is discussed. A brief description of the theoretical principles of optical analysis of aerosol properties, ending with a survey of current activity and, as yet, unresolved problems is presented. It can be concluded that there is a consensus regarding the size range of mesospheric aerosol particles. The contribution of the smallest particles cannot be assessed by current optical methods. The combination of the scattering measurement with an electromechanical impact detector is a promising technique. The true size distribution, more correctly a statistical average of many possible distributions, must rely on numerical modelling. Regarding the shape of the particles, the noctilucent-cloud particles are not perfect spheres. New insight into this problem can be expected from the ongoing observational program of the Atmospheric Chemistry Experiment (ACE) and Aeronomy of Ice in the Mesosphere (AIM). [ABSTRACT FROM AUTHOR]

The occurrence of a sudden stratospheric warming (SSW) excites disturbances in the mesosphere-lower thermospheric (MLT) wind and temperature. Here, we have examined the high frequency (HF) radar wind data from the South African National Antarctic Expedition, SANAE (72° S, 3° W), a radar which is part of the Super Dual Auroral Radar Network (SuperDARN). Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) on board the Thermosphere-Ionosphere-Mesosphere-Energetics and Dynamics (TIMED) satellite temperature data and National Centre for Environmental Prediction (NCEP) temperature and wind data are used to investigate the dynamical effects of the unprecedented September 2002 SSW in the Antarctica stratosphere and MLT. The mean zonal wind (from SANAE HF radar) at the MLT shows reversal approximately 7 days before the reversal at 10 hPa (from NCEP). This indicates that there was a downwards propagation of circulation disturbance. Westerly zonal winds dominate the winter MLT, but during the 2002 winter there are many periods of westward winds observed compared to other years. The normalised power spectrums of both meridional and zonal winds show presence of planetary waves (of ~14-day period) before the occurrence of the SSW. The SABER vertical temperature profiles indicated the cooling of the MLT region before the SSW event. [ABSTRACT FROM AUTHOR]

Abstract: The thermosphere–ionosphere–mesosphere-electrodynamics coupled model TIME-GCM, coupled to NCEP lower atmosphere data, is used to simulate the noontime ionospheric peak electron density NmF2 at low latitudes for year 2002. Model output are compared with observations a three ionosonde stations: Jicamarca, Ascension Island and Darwin, stations at geomagnetic latitudes of 3°S, 10°S and 22°S, respectively. The modeled electron density at the peak of the F2-layer (NmF2) matches the general trend of the data fairly well at noon throughout the year. The shapes of the diurnal curves of NmF2 vs. local time are not well produced in the model, and particularly so at the two stations away from the geomagnetic equator. At all sites the day-to-day variability of NmF2, assessed using the percent standard deviation about the monthly mean, is about twice the modeled variability. Possible sources of this shortfall in the model may be due to the under-representation of coupling from below and/or from auroral sources. [Copyright &y& Elsevier]

This paper investigates the relationship between high latitude upper mesospheric winds and the state of the stratospheric polar vortex in the absence of major sudden stratospheric warmings. A ground based Michelson Interferometer stationed at Resolute Bay (74°43′ N, 94°58′ W) in the Canadian High Arctic is used to measure mesopause region neutral winds using the hydroxyl (OH) Meinel-band airglow emission (central altitude of ~85 km). These observed winds are compared to analysis winds in the upper stratosphere during November and December of 1995 and 1996; years characterized as cold, stable polar vortex periods. Correlation of mesopause wind speeds with those from the upper stratosphere is found to be significant for the 1996 season when the polar vortex is subtly displaced off its initial location by a strong Aleutian High. These mesopause winds are observed to lead stratospheric winds by approximately two days with increasing (decreasing) mesospheric winds predictive of decreasing (increasing) stratospheric winds. No statistically significant correlations are found for the 1995 season when there is no such displacement of the polar vortex. [ABSTRACT FROM AUTHOR]