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We analyze periodic features in the F10.7 cm solar radio flux and the Ap geomagnetic index to examine solar-terrestrial coupling in Earth's atmosphere above 100 km. The coupling is indicated by the same periodic features observed in the global infrared power radiated by Nitric Oxide (NO) and Carbon Dioxide (CO 2). The infrared power data have been measured by the SABER instrument on the NASA TIMED satellite since January 2002. A strong dominant feature for the length of the solar cycle (≈11 years) can be observed in all the datasets. The length of this periodicity can vary from 9.95 years to 12.25 years for different solar cycles. Periodic features larger than the length of the solar cycle are also observed in the F10.7 and Ap index (≈22 years and ≈44 years). A strong 180-day periodic feature can be observed in the Ap index which corresponds to the well-known Russell-McPherron effect. Additionally, the NO power and the Ap index exhibit statistically significant periodic features for shorter periods (e.g., 27, 13.5 and 9 days). This similarity indicates a strong link between the thermosphere's infrared energy budget and the geomagnetic conditions of the upper atmosphere. • LNP analysis has been applied to detect periodic features of solar activities. • F10.7 index, Ap index and SABER infrared measurements have been investigated. • A strong periodic feature for the solar cycle (≈11 years) exists in the datasets. • Larger (≥22 years) and shorter (≤365 days) periodic features are also present. • Earth's infrared radiated energy and geomagnetic behavior have a strong coupling. [ABSTRACT FROM AUTHOR]

Tropical cyclones are one of the potential sources of gravity waves, which can transport energy and momentum to higher heights through their vertical propagation and interaction with the background flow and thereby influence the dynamics of the upper atmosphere. High-resolution wind data acquired by the Medium Frequency (MF) radar at Kolhapur (16.69°N, 74.24°E) are utilized to study the high frequency gravity waves (20–60 min) associated with the tropical cyclone named "Vayu" formed in the Indian Arabian Sea in June 2019. An enhancement of the gravity wave (GW) activities in the meridional wind is observed during 13–15 June 2019. The source of the gravity wave is the tropical Vayu cyclone of category 2 storm. We employed the gravity wave variance data from NASA's Atmospheric Infrared Sounder (AIRS) satellite for our analysis over stratospheric heights. The enhancement of gravity wave variance in both the stratosphere mesosphere heights and the low values of OLR indicate the strong convection associated with cyclone, the source of observed gravity waves. In the present study we analyzed the horizontal propagation direction of the cyclone generated gravity waves using perturbation ellipse, it is found to be in the north–south plane. The temperature profiles obtained from the SABER instrument on board TIMED satellite indicate the presence of mesospheric inversion layer with an amplitude of nearly 40 K on the day of large gravity wave variance (June 13, 2019). These results indicate the cyclone can generate gravity waves which can propagate to higher heights and modify the MLT thermal structure, providing evidence for the vertical coupling between lower and upper atmosphere. • High-frequency GW activity in meridional wind associated with Tropical Cyclone "Vayu". • Studied cyclone generated high-frequency gravity waves (GWs) over Kolhapur. • Anticorrelation of GW activity with OLR reveals tropospheric convection associated with cyclone as a source. [ABSTRACT FROM AUTHOR]

Anthropogenic polluting substances, mainly CO 2 , affect the lower atmosphere where they excite the well-known greenhouse warming. However, they also affect the upper atmosphere (mesosphere, thermosphere and embedded ionosphere), where they cause long-term trends stronger than those in the troposphere. Calculations of long-term trends in the upper atmosphere suffer from various problems, which may be divided into three groups: (1) natural variability, (2) data problems, and (3) methodology. These problems have often been underestimated in trend calculations, which lead to controversial trend results. Here we briefly treat various problems of long-term trend calculations in the upper atmosphere and some ways how to deal with these problems are suggested. • Calculations of long-term trends in the upper atmosphere suffer from various problems. • Problems caused by natural variability, serious data problems and problems in methodology. • Some guidance is given how to minimize the impact of these problems. [ABSTRACT FROM AUTHOR]

Abstract We present the results of studying the behavior of temperature and of the atomic oxygen concentration in the mesopause region during the 2013 January major Sudden Stratospheric Warming (SSW). The data on the hydroxyl molecule OH(6-2), 834.0 nm emission intensity and rotational temperature were analyzed. These data were obtained through spectrometric measurements at the Geophysical Observatory of the Institute of Solar-Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences (51.8°N, 103.1°E, Tory), and at the Zvenigorod Station at the Obukhov Institute of Atmospheric Physics of the Russian Academy of Sciences (55.7°N, 36.8 E°). We calculated the concentration of atomic oxygen and its variations by using the data of the OH emission measurements. We revealed, that the response of the mesopause characteristics for two longitudinally spaced mid-latitude regions essentially differs. Thus, from the Tory Station data, the maximal increase in the OH emission intensity (by a factor of ∼2) and in the concentration [O] (by a factor of ∼3) occurred during the sudden stratospheric warming (SSW) evolution, whereas, from the Zvenigorod Station data, the OH emission intensity increase (by a factor of ∼3) and the concentration [O] increase (by a factor of ∼3.5) was observed at the SSW recovery phase. As a result of numerical modeling using the Middle and Upper Atmosphere Model (MUAM), it was shown, that the cause for the revealed effect may probably be longitudinal differences in the diurnal variation in the vertical wind at the mesopause heights over the indicated stations during the SSW. One may elucidate these differences through the generation of non-migrating tides due to a non-linear interaction between the intensified stationary planetary wave 1 (SPW1) and migrating tides and forcing a set of high-frequency PWs at the stratospheric heights. All these waves are capably of propagating into the MLT region and produce observed changes in behavior of the OH emission intensity, temperature and atomic oxygen concentration over Tory and Zvenigorod. Highlights • Effects of 2013 SSW on the MLT for two longitudinally spaced mid-latitude sites are investigated. • Longitudinal differences in the behavior of the mesopause temperature and composition were found. • It may be caused by longitudinal differences in the diurnal variation of vertical wind. [ABSTRACT FROM AUTHOR]

Abstract Geomorphologies and mechanisms of thermospheric mass density variations caused by geomagnetic storms are still challenging due to limited observations and imprecise models. Recently, precise orbit determination (POD) of Gravity Recovery and Climate Experiment (GRACE) and Swarm satellites is able to estimate thermospheric mass density variations, which may provide data to study thermospheric mass density variations following the storm. In this paper, the thermospheric mass density is estimated from GRACE-A and Swarm-A POD and the distinct thermospheric mass density variations are investigated as function of latitude during the September 2017 geomagnetic storm. Different enhancements in mass density response to the geomagnetic storm are presented for the Northern (GRACE) and the Southern (Swarm) Hemispheres. Swarm observations show symmetric mass density variations between two hemispheres and a slightly stronger enhancement in Southern Hemisphere. GRACE POD and accelerometer observations both show a very strong enhancement in Northern Hemisphere while no corresponding enhancement in Southern Hemisphere. This anomaly may attribute to the effects of vertical winds in high latitude region and plasma drift considering the similar solar zenith angles in both hemispheres. Highlights • Distinct thermospheric variations during the September 2017 storm are observed from GRACE and Swarm. • Swarm observations show symmetric variations with a slightly stronger density enhancement in Southern Hemisphere. • GRACE observations show a strong enhancement in Northern Hemisphere while no enhancement in Southern Hemisphere. • GRACE-derived asymmetric anomaly may be due to the effects of vertical winds and plasma drift in both hemispheres. [ABSTRACT FROM AUTHOR]

Abstract This study investigated variation in the upwelling radiation contribution to typical remote sensing channels of temperature (50–60 GHz, 118 GHz, and 424 GHz) and water vapor (22 GHz, 183 GHz, and 380 GHz) with altitude to evaluate their sounding abilities and obtain channel sets for sounding of lower atmosphere with high resolution. The gaseous attenuation described in Recommendation ITU-R P.676–11 and atmosphere profiles illustrated in Recommendation ITU-R P.835–6 were applied to calculate the upwelling radiation based on radiative transfer theory. The variation of brightness temperature contribution (T b contribution vs. altitude) with both channels and profiles was analyzed in detail. The T b contribution altitude at microwave channels was lower than that at submillimeter channels, making higher frequency channels suitable for remote sensing of the middle and upper atmosphere. Offset frequencies were selected to extend the sounding range: offsets of up to 1 GHz for the 118 GHz channel and up to 10 GHz for the 183 GHz channel enable sounding of the Earth's atmosphere. Differences in the T b contribution altitudes to different profiles were more significant for water vapor than temperature channels because of the variable distribution of water vapor in the atmosphere. Therefore, the sounding ability of one channel varies for different profiles. An offset frequency of 10 GHz for the 380 GHz channel is sufficient for sounding the Earth's atmosphere in high latitude winter. Sea surface radiation calculations indicated that it can be neglected for the 380 GHz and 424 GHz channels even for the high latitude winter. Highlights • Altitudinal variations in radiation contribution to remote sensing channels studied. • 1 and 10 GHz offsets for 118 and 183 GHz channels enable sounding of the lower atmosphere. • Offset frequency of 10 GHz for 380 GHz channels can realize the sounding of earth surface only for high latitude winter due to high attenuation of other profiles. • Performance of the same channels is higher and coverage is wider during winter. • Sea surface impact on 50–60 GHz, 118 GHz, 22 GHz, and 118 GHz channels cannot be ignored for high latitude winter. [ABSTRACT FROM AUTHOR]

Numerical modeling of changes in the global atmospheric circulation and characteristics of stationary planetary waves with zonal wavenumbers 1–4 is performed, taking into account the effects of changes in solar activity at altitudes above 100 km. The middle and upper atmosphere model (MUAM) is utilized to simulate the general circulation and planetary waves at altitudes 0 - 300 km, averaged over 12-member ensemble of model runs for values of the solar radio flux at the wavelength of 10.7 cm corresponding to the high and low levels of solar activity. The ionospheric conductivities and their latitudinal, longitude and temporal dependences are taken into account in the MUAM. Calculations for January-February in the thermosphere showed larger eastward wind velocity at altitudes above 130 km at high solar activity level. In the thermosphere, the amplitudes of planetary waves decrease at most latitudes at high solar activity level. Simulated changes in the atmospheric refractivity index and the Eliassen-Palm flux correspond to the obtained changes in planetary waves amplitudes. Changes in the conditions of propagation and reflection of stationary planetary waves caused by impacts of solar activity on the thermosphere can influence atmospheric circulation in a broad altitude range including the middle atmosphere. [ABSTRACT FROM AUTHOR]

Enhanced anthropogenic activities in the recent decades are ensuing in the large-scale accumulation of carbon dioxide (CO 2) concentration in the Earth's atmosphere. CO 2 in the atmosphere induces warming in the troposphere and cooling in the middle atmosphere. A systematic analysis of the middle atmospheric thermal structure and its variability at decadal scale is need of the hour to perceive the consequences of increased anthropogenic activity in totality. In this regard, long-term variations of temperature, CO 2 concentration and associated cooling rates in the middle and upper atmosphere (30–110 km) are investigated using Sounding of Atmosphere by Broadband Emission Radiometry (SABER) on board Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) satellite observations during 2002–2021 and Specified Dynamics-Whole Atmosphere Community Climate Model (SD-WACCM) simulations during 2002–2017 over 50 N–50 S latitudes. The results show that CO 2 concentration measured by SABER is significantly underestimated by the WACCM simulations, especially above 90 km altitude. In order to find the long-term trends in each variable, Multivariate Linear Regression (MLR) technique is employed to the residual data obtained after removing seasonal variations. SABER observations show decreasing trend in temperature with a rate of 0.5–2.5 K/decade and an increase in CO 2 vmr of 16–18 ppmv/decade. The effect of Solar Cycle, QBO and ENSO on each of the variables is also investigated. The significance of the present study lies in evaluating the SD-WACCM simulations of long-term variabilities in the thermal structure, CO 2 vmr and associated cooling rates using two decades of observations. • Long term trends in middle atmospheric temperature, CO2 vmr and cooling rates from SD-WACCM are evaluated using SABER observations. • Response of middle atmospheric temperature, CO2 vmr and associated cooling rates to solar cycle, QBO, ENSO is investigated. • Observations show a decreasing trend in temperature at a rate of 0.5–2.5 K/decade in the middle atmosphere. [ABSTRACT FROM AUTHOR]

The dynamical coupling of the lower and upper atmosphere by planetary waves (PWs) is studied. Numerical simulations of planetary wave (PW) amplitudes during composite sudden stratospheric warming (SSW) events in January-February are made using a model of general circulation of the middle and upper atmosphere with initial and boundary conditions typical for the westerly and easterly phases of quasi-biennial oscillation (QBO). The changes in PW amplitudes in the middle atmosphere before, during and after SSW event for the different QBO phases are considered. Near the North Pole, the increase in the mean temperature during SSW reaches 10–30 K at altitudes 30–50 km for four pairs of the model runs with the eQBO and wQBO, which is characteristic for the sudden stratospheric warming event. Amplitudes of stationary PWs in the middle atmosphere of the Northern hemisphere may differ up to 30% during wQBO and eQBO before and during the SSW. After the SSW event SPW amplitudes are substantially larger during wQBO phase. PW refractivity indices and Eliassen-Palm flux vectors are calculated. The largest EP-fluxes in the middle atmosphere correspond to PWs with zonal wavenumber m =1. Simulated changes in PW amplitudes correspond to inhomogeneities of the global circulation, refractivity index and EP-flux produced by the changes in QBO phases. Comparisons of differences in PW characteristics and circulation between the wQBO and eQBO show that PWs could provide effective coupling mechanism and transport dynamical changes from local regions of the lower atmosphere to distant regions of the upper atmosphere of both hemispheres. [ABSTRACT FROM AUTHOR]

Combining limb and nadir satellite observations of Polar Mesospheric Clouds (PMCs) has long been recognized as problematic due to differences in observation geometry, scattering conditions, and retrieval approaches. This study offers a method of comparing PMC brightness observations from the nadir-viewing Aeronomy of Ice in the Mesosphere (AIM) Cloud Imaging and Particle Size (CIPS) instrument and the limb-viewing Odin Optical Spectrograph and InfraRed Imaging System (OSIRIS). OSIRIS and CIPS measurements are made comparable by defining a common volume for overlapping OSIRIS and CIPS observations for two northern hemisphere (NH) PMC seasons: NH08 and NH09. We define a scattering intensity quantity that is suitable for either nadir or limb observations and for different scattering conditions. A known CIPS bias is applied, differences in instrument sensitivity are analyzed and taken into account, and effects of cloud inhomogeneity and common volume definition on the comparison are discussed. Not accounting for instrument sensitivity differences or inhomogeneities in the PMC field, the mean relative difference in cloud brightness (CIPS - OSIRIS) is −102 ± 55%. The differences are largest for coincidences with very inhomogeneous clouds that are dominated by pixels that CIPS reports as non-cloud points. Removing these coincidences, the mean relative difference in cloud brightness reduces to −6 ± 14%. The correlation coefficient between the CIPS and OSIRIS measurements of PMC brightness variations in space and time is remarkably high, at 0.94. Overall, the comparison shows excellent agreement despite different retrieval approaches and observation geometries. [ABSTRACT FROM AUTHOR]

The atmospheric sodium layer normally occurs in the mesopause (80–105 km) region, but rarely in the lower thermosphere region (>105 km) at low latitude. We observed a kind of peculiar sodium layer in lower thermosphere at Haikou (19.99°N, 110.34°E)—the thermospheric convective sodium layer (TCSL) in a lidargram. The TCSL's sodium density unstably developed over time and appeared as several discontinuous convective shapes vertically. It is the first time convective sodium layer observed in the lower thermosphere region (105–120 km). Based on Haikou lidar data, we obtained 14 TCSL events during 180 nights from March 2010 to August 2012. Most of the apogees of the TCSL events are higher than 108 km. A TCSL event lasts several hours and is composed of several convective structures, with each veitical shape lasting ∼5–30 min. All TCSL events occurred during spring and summer, and generally appear near midnight (22:00–00:00 LT). The TCSL has potential regional feature and appears to be related to the thermospheric sporadic E (Es) layers, winds, and field-aligned ionospheric irregularities (FAI). [ABSTRACT FROM AUTHOR]

Knowledge of the state of the upper atmosphere, and in particular of the ionosphere, is essential in several applications such as systems used in radio frequency communications, satellite positioning and navigation. In general, these systems depend on the state and evolution of the ionosphere. In all applications involving the ionosphere an essential task is to determine the path and modifications of ray propagation through the ionospheric plasma. The ionospheric refractive index and the maximum usable frequency (MUF) that can be received over a given distance are some key parameters that are crucial for such technological applications. However, currently the representation of these parameters are in general simplified, neglecting the effects of Earth’s magnetic field. The value of M(3000)F2, related to the MUF that can be received over 3000 km is routinely scaled from ionograms using a technique which also neglects the geomagnetic field effects assuming a standard simplified propagation model. M(3000)F2 is expected to be affected by a systematic trend linked to the secular variations of Earth’s magnetic field. On the other hand, among the upper atmospheric effects expected from increasing greenhouse gases concentration is the lowering of the F2-layer peak density height, hmF2. This ionospheric parameter is usually estimated using the M(3000)F2 factor, so it would also carry this “systematic trend”. In this study, the geomagnetic field effect on MUF estimations is analyzed as well as its impact on hmF2 long-term trend estimations. We find that M(3000)F2 increases when the geomagnetic field is included in its calculation, and hence hmF2, estimated using existing methods involving no magnetic field for M(3000)F2 scaling, would present a weak but steady trend linked to these variations which would increase or compensate the few kilometers decrease (~2 km per decade) expected from greenhouse gases effect. [ABSTRACT FROM AUTHOR]

The anthropogenic emissions of greenhouse gases affect not only the weather and climate in the troposphere; they affect also long-term trends in the mesosphere-thermosphere-ionosphere system, where the amplitudes of anthropogenic changes are substantially larger than in the troposphere. The last four years have seen significant progress in investigating these trends but also some new puzzles have been created. Observations of the CO 2 trend in the lower thermosphere appeared but their interpretation is a matter of debate. The role of ozone in mesospheric temperatures and E-region ionosphere has been confirmed and quantified. Agreement between observational and simulated trends in the thermospheric density, supported by satellite observations of radiative cooling was reached but the most recent result re-opened the problem. Much new partial information about trends in the ionospheric F region was reported. Also new information on other experimental trends helped to improve our understanding of long-term trends in the upper atmosphere. Significant progress has been reached in modelling the long-term trends; in a few parameters the agreement with observed trends is now not only quantitative but also qualitative. Several attempts to explain ionospheric trends without CO 2 appeared but they are shown not to be correct; CO 2 remains to be the primary (although not the only) trend driver. On the other hand, many open questions or puzzles, listed in Concluding remarks, remain to be investigated. [ABSTRACT FROM AUTHOR]

A photometric technique to obtain daytime optical airglow emissions is described. Airglow emissions occur naturally due to photochemical and chemiluminescent processes taking place in the upper atmosphere. Airglow emissions that occur in the daytime are referred to as dayglow. It is a challenge to detect these dayglow emissions as the solar scattered background against which they occur is extremely bright. Thus, it is required that high spectral resolution measurements are carried out to delineate the difference between the dayglow signal and the solar scattered background brightness. In the method being described here, a low-resolution Fabry-Perot etalon is used as a high spectral resolution filter to obtain the dayglow signal. The spectral resolution achieved by this instrument is 0.026 nm at 630.0 nm, which is sufficient to separate the signal and neighbouring spectral regions. The present technique builds up on the success of employing such methods in the past and brings in innovations that make it reliable, rugged, and suitable for unattended field operations. The details of the technique and the new elements brought in are described in this paper. • Innovative technique for measurement of dayglow emissions. • Fabry-Perot etalon based photometer. • A portable, rugged, and reliable optical instrument for unattended field operation. [ABSTRACT FROM AUTHOR]

Observations from the Solar Occultation For Ice Experiment (SOFIE) on the Aeronomy of Ice in the Mesosphere (AIM) satellite are used to examine noctilucent clouds (NLC) and their environment at middle latitudes (~56°N and ~52°S). Because SOFIE is uniquely capable of measuring NLC, water vapor, and temperature simultaneously, the local cloud environment can be specified to examine what controls their formation at mid-latitudes. Compared to higher latitudes, mid-latitude NLCs are less frequent and have lower ice mass density, by roughly a factor of five. Compared to higher latitudes at NLC heights, mid-latitude water vapor is only ~12% lower while temperatures are more than 10 K higher. As a result the reduced NLC mass and frequency at mid-latitudes can be attributed primarily to temperature. Middle and high latitude NLCs contain a similar amount of meteoric smoke, which was not anticipated because smoke abundance increases towards the equator in summer. SOFIE indicates that mid-latitude NLCs may or may not be associated with supersaturation with respect to ice. It is speculated that this situation is due in part to SOFIE uncertainties related to the limb measurement geometry combined with the non-uniform nature of NLCs. SOFIE is compared with concurrent NLC, temperature, and wind observations from Kühlungsborn, Germany (54°N) during the 2015 summer. The results indicate good agreement in temperature and NLC occurrence frequency, backscatter, and height. SOFIE indicates that NLCs were less frequent over Europe during 2015 compared to other longitudes, in contrast to previous years at higher latitudes that showed no clear longitude dependence. Comparisons of SOFIE and the Solar Backscatter Ultraviolet (SBUV) indicate good agreement in average ice water column (IWC), although differences in occurrence frequency were often large. [ABSTRACT FROM AUTHOR]

Particle precipitation causes atmospheric ionization. Modeled ionization rates are widely used in atmospheric chemistry/climate simulations of the upper atmosphere. As ionization rates are based on particle measurements some assumptions concerning the energy spectrum are required. While detectors measure particles binned into certain energy ranges only, the calculation of a ionization profile needs a fit for the whole energy spectrum. Therefore the following assumptions are needed: (a) fit function (e.g. power-law or Maxwellian), (b) energy range, (c) amount of segments in the spectral fit, (d) fixed or variable positions of intersections between these segments. The aim of this paper is to quantify the impact of different assumptions on ionization rates as well as their consequences for atmospheric chemistry modeling. As the assumptions about the particle spectrum are independent from the ionization model itself the results of this paper are not restricted to a single ionization model, even though the Atmospheric Ionization Module OSnabrück ( Aimos , Wissing and Kallenrode, 2009 ) is used here. We include protons only as this allows us to trace changes in the chemistry model directly back to the different assumptions without the need to interpret superposed ionization profiles. However, since every particle species requires a particle spectrum fit with the mentioned assumptions the results are generally applicable to all precipitating particles. The reader may argue that the selection of assumptions of the particle fit is of minor interest, but we would like to emphasize on this topic as it is a major, if not the main, source of discrepancies between different ionization models (and reality). Depending on the assumptions single ionization profiles may vary by a factor of 5, long-term calculations may show systematic over- or underestimation in specific altitudes and even for ideal setups the definition of the energy-range involves an intrinsic 25% uncertainty for the ionization rates. The effects on atmospheric chemistry (HO x , NO x and Ozone) have been calculated by 3dCTM, showing that the spectrum fit is responsible for a 8% variation in Ozone between setups, and even up to 50% for extreme setups. [ABSTRACT FROM AUTHOR]

This paper presents the first direct observational evidence for the possible role of meteoric activity in the generation of the equatorial Counter Electrojets (CEJ), an enigmatic daytime electrodynamical process over the geomagnetic equatorial upper atmosphere. The investigation carried out using the data from Proton Precession Magnetometer and Meteor Wind Radar over a geomagnetic dip equatorial station, Trivandrum (8.5°N, 77°E, 0.5°N dip lat.) in India, revealed that the occurrence of the afternoon CEJ events during a month is directly proportional to the average monthly meteor counts over this location. The observation is found to be very consistent during the considered period of study, i.e the years 2006 and 2007. The study vindicates that the meteor showers play a major role in setting up the background condition conducive for the generation of CEJ by reducing the strength of the upward polarization field. [ABSTRACT FROM AUTHOR]

The interaction between the resolved waves and parameterized gravity wave (GW) drag in the upper atmosphere extension of the ICON model (UA-ICON) are investigated. To this end, we performed a set of 30-year long time slice UA-ICON runs with two sensitivity simulations where either the subgrid-scale orographic (SSO) or non-orographic GW drag parameterizations are switched off (noSSO and noGWD, respectively), and compare them against a control (CTL) simulation where both GWs parameterizations are active. The stratospheric polar vortex accelerates, cools and shifts poleward in both sensitivity runs. The frequency of sudden stratospheric warmings in the CTL simulation is 5.7 events per decade and drops to 1.7 and 4 events per decade in the noSSO and noGWD, respectively. In both sensitivity runs (particularly in noGWD), an enhancement in the resolved wave amplitude is found in the high latitude stratosphere and in the mesosphere and lower thermosphere (MLT) region in all latitudes. The magnitudes of the resolved waves responses are generally larger for noGWD than noSSO. Our results confirm the compensation mechanism in the UA-ICON model, whereby the perturbed forcings in the GW parameterization drag are often canceled or partly compensated by a resolved large-scale wave driving of opposite sign. In addition, evidence is presented that the compensation mechanism is not always operative everywhere. For example, we found a suppression of the upward propagation of the resolved waves in the cold seasons in the noSSO experiment, which is likely linked to a significant reduction of the favorable propagation conditions in the upper stratosphere. • Gravity waves play an important role in determining the frequency of SSWs. • Enhancement in the resolved wave amplitude due to the absence of gravity waves. • Evidence is presented that the compensation mechanism is not always operative. [ABSTRACT FROM AUTHOR]

Airglow emissions from the thermosphere act as tracers of the behaviour of the upper atmosphere. OI 630 nm nightglow emissions that originate from an altitude of around 250 km in the thermosphere have been investigated using a large field-of-view ground-based optical spectrograph from Mt. Abu (24.6°N, 72.7°E, 19°N Mag.), a low-latitudinal location in India. The latitudinal movement of the crest of the equatorial ionization anomaly (EIA) in both poleward in the evening and equatorward in the night has been reported using the OI 630 nm nightglow emissions. The EIA crest is found to shift away from the equator after the sunset, which has been shown to be directly related to the strength of the twilighttime equatorial electrodynamics. Later in the night, after 20 LT, a clear movement of the crest back towards the equator, known as the reversal of EIA, has been observed. The speeds of the EIA reversals for forty nights in the years 2014 and 2015 have been obtained and are found to be in the range of 10–55 ms−1. As the global equatorial electric fields ( E ) are irrotational in nature (∇ × E = 0), simultaneous variations in the daytime electrojet strength over Jicamarca in the American sector have been compared with the nighttime reversal speeds of the EIA over the Indian sector, which show a remarkable relationship with each other. Thus, it is hereby demonstrated that the reversal in EIA as inferred by the OI 630 nm nightglow emissions in our measurements is due to the westward equatorial electric field. It is hereby proposed that the reversal speed derived from optical nightglow measurements can serve as a proxy for the determination of westward electric field over equator for that longitude sector. • Post-sunset poleward movement of EIA crest correlates well with the strength of PRE. • The reversal in the EIA is shown to be due to the nighttime westward electric fields. • Equatorward movement in redline nightglow a potent tool for westward electric field. [ABSTRACT FROM AUTHOR]

According to known experiments both the zonal and meridional components of the wind demonstrate a clear increase in amplitude, starting from heights of 80–85 km, and the maximum amplitudes, which are reached at altitudes of 100–110 km. With a further increase in altitude, a gradual decrease in amplitudes begins. The wind maximum amplitudes turn out to be much larger than predicted by the theory of tidal movements in the ionosphere or the empirical Horizontal Wind Model (HWM). We propose a simple two-dimensional model to indicate the cause of abnormal amplitudes of wind motion in the lower ionosphere. The model is based on the existence of a well-established Ekman wind profile at the heights of the mesosphere-thermosphere. • Experiments demonstrate clear maximum in amplitudes of both the zonal and meridional components of the wind at altitudes of 100–110 km with gradual decrease in amplitudes both above and below these altitudes. • The wind maximum amplitudes turn out to be much larger than predicted by the theory. • The negative turbulent helicity of the Ekman flow in the upper atmosphere leads to an increase in the amplitude of the wind. [ABSTRACT FROM AUTHOR]

Trapped atmosphere waves, such as IGW waveguide modes and Lamb modes, are described using dissipative solution above source (DSAS) ( Dmitrienko and Rudenko, 2016 ). According to this description, the modes are disturbances penetrating without limit in the upper atmosphere and dissipating their energy throughout the atmosphere; leakage from a trapping region to the upper atmosphere is taken into consideration. The DSAS results are compared to those based on both accurate and WKB approximated dissipationless equations. It is shown that the spatial and frequency characteristics of modes in the upper atmosphere calculated by any of the methods are close to each other and are in good agreement with the observed characteristics of traveling ionospheric disturbances. [ABSTRACT FROM AUTHOR]

A method of construction of solution for acoustic-gravity waves (AGW) above a wave source, taking dissipation throughout the atmosphere into account (dissipative solution above source, DSAS), is proposed. The method is to combine three solutions for three parts of the atmosphere: an analytical solution for the upper isothermal part and numerical solutions for the real non-isothermal dissipative atmosphere in the middle part and for the real non-isothermal small dissipation atmosphere in the lower one. In this paper the method has been carried out for the atmosphere with thermal conductivity but without viscosity. The heights of strong dissipation and the total absorption index in the regions of weak and average dissipation are found. For internal gravity waves the results of test calculations for an isothermal atmosphere and calculations for a real non-isothermal atmosphere are shown in graphical form. An algorithm and appropriate code to calculate DSAS, taking dissipation due to finite thermal conductivity into account throughout the atmosphere, are developed. The results of test DSAS calculations for an everywhere isothermal atmosphere are given. The calculation results for DSAS for the real non-isothermal atmosphere are also presented. A method for construction of the 2×2 Green's matrix fully taking dissipation into account and allowing us to find disturbance from some source of AGW in the atmosphere is proposed. [ABSTRACT FROM AUTHOR]

During a 10-day URSI World Day observational campaign beginning on August 1, 2011, an isolated, major geomagnetic storm occurred. On August 5, Kp reached values of 8 − and Dst dropped to −113 nT. The occurrence of this isolated storm in the middle of a 10-day URSI World Day campaign provides and unprecedented opportunity to observe the coupling of solar wind energy into the magnetosphere and to evaluate the varied effects that occur in the coupled magnetosphere–ionosphere–thermosphere system. Dramatic changes in the ionosphere are seen at every one of the active radar stations, extending from Greenland down to equatorial Peru in the American sector and at middle latitudes in Ukraine. Data from TIMED and THEMIS are shown to support initial interpretations of the observations, where we focus on processes in the middle latitude afternoon sector during main phase, and the formation of a dense equatorial ionosphere during storm recovery. The combined measurements strongly suggest that the changes in ionospheric conditions observed after the main storm phase can be attributed in large part to changes in the stormtime thermosphere. This is through the generation of disturbance dynamo winds and also global neutral composition changes that either reduce or enhance plasma densities in a manner that depends mainly upon latitude. Unlike larger storms with possibly more sustained forcing, this storm exhibits minimal effects of persistent meridional stormtime wind drag, and little penetration of solar wind electric potentials to low latitudes. It is, therefore, an outstanding example of an impulsive event that exhibits longer-term effects through modification of the background atmosphere. [ABSTRACT FROM AUTHOR]

Icy particles containing a variety of Fe compounds are present in the upper atmospheres of planets such as the Earth and Saturn. In order to explore the role of ice sublimation and energetic ion bombardment in releasing Fe species into the gas phase, Fe-dosed ice films were prepared under UHV conditions in the laboratory. Temperature-programmed desorption studies of Fe/H 2 O films revealed that no Fe atoms or Fe-containing species co-desorbed along with the H 2 O molecules. This implies that when noctilucent ice cloud particles sublimate in the terrestrial mesosphere, the metallic species embedded in them will coalesce to form residual particles. Sputtering of the Fe-ice films by energetic Ar + ions was shown to be an efficient mechanism for releasing Fe into the gas phase, with a yield of 0.08 (Ar + energy=600 eV). Extrapolating with a semi-empirical sputtering model to the conditions of a proton aurora indicates that sputtering by energetic protons (>100 keV) should also be efficient. However, the proton flux in even an intense aurora will be too low for the resulting injection of Fe species into the gas phase to compete with that from meteoric ablation. In contrast, sputtering of the icy particles in the main rings of Saturn by energetic O + ions may be the source of recently observed Fe + in the Saturnian magnetosphere. Electron sputtering (9.5 keV) produced no detectable Fe atoms or Fe-containing species. Finally, it was observed that Fe(OH) 2 was produced when Fe was dosed onto an ice film at 140 K (but not at 95 K). Electronic structure theory shows that the reaction which forms this hydroxide from adsorbed Fe has a large barrier of about 0.7 eV, from which we conclude that the reaction requires both translationally hot Fe atoms and mobile H 2 O molecules on the ice surface. [ABSTRACT FROM AUTHOR]

The NASA Aeronomy of Ice in the Mesosphere (AIM) satellite is the first spaceborne mission dedicated to studying high-altitude (~83 km) Polar Mesospheric Clouds (PMCs). Since its launch in 2007, the Cloud Imaging and Particle Size (CIPS) instrument onboard AIM has obtained large-field, high resolution (25 km 2 /pixel) images of the PMCs, enabling a unique investigation of mesospheric gravity wave activity in the summer polar mesosphere where previous measurements have been sparse. In this study, we have analyzed 12 consecutive seasons of AIM/CIPS PMC albedo data to determine the statistical properties of medium and large horizontal scale (>100 km) gravity waves present in the PMC data. Over 60,000 wave events with horizontal scale-sizes ranging up to >2000 km have been identified and measured, revealing a wealth of wave events particularly in the ~300–800 km range where our analysis sensitivity is largest. These data are ideal for investigating the intra-seasonal, inter-annual and hemispheric variability of these waves as observed over the whole summer polar cap regions. Throughout this 6 year study, the wave activity in the southern hemisphere was found to be consistently 10–15% higher than in the northern hemisphere and both the northern and southern hemisphere wave activity was determined to decrease systematically (by ~15%) during the course of each summer season. This decrease agrees well with previous seasonal stratospheric studies of variations in the wave energy, suggesting a direct influence of the lower atmospheric sources on polar mesospheric dynamics. Very similar and consistent results were also found from season to season in both hemispheres providing new information for gravity wave modeling and dynamical studies of the high-latitude summer-time mesosphere. [ABSTRACT FROM AUTHOR]

The fractal perimeter dimension is a fundamental property of clouds. It describes the cloud shape and is used to improve the understanding of atmospheric processes responsible for cloud shapes. von Savigny et al. (2011) determined the fractal perimeter dimension of noctilucent clouds (or polar mesospheric clouds) for the first time based on a limited data set of cloud images observed with the CIPS (Cloud Imaging and Particle Size) instrument on board the AIM (Aeronomy of Ice in the Mesosphere) satellite. This paper builds on von Savigny et al. (2011) by first presenting a sensitivity analysis of the determination of the fractal perimeter dimension, and secondly presenting results on the seasonal and interhemispheric differences of the perimeter dimension of noctilucent clouds (NLCs). The same method as in the earlier study is applied to an extended data set of satellite images of noctilucent cloud fields taken with the CIPS experiment. The sensitivity studies reveal that cloud holes play an important role for the area–perimeter method, since excluding clouds with holes reduces the dimension value by up to 3%. The results on the fractal perimeter dimension over six NLC seasons from 2007 to 2009 demonstrate that the dimension values of the NLCs neither show significant differences between the seasons nor between the hemispheres. [ABSTRACT FROM AUTHOR]

Many physics-based models are used to study and monitor the terrestrial upper atmosphere. Each of these models has internal parameterizations that introduce bias if they are not tuned for a specific set of run conditions. This study uses Retrospective Cost Model Refinement (RCMR) to remove internal model bias in the Global Ionosphere Thermosphere Model (GITM) through parameter estimation. RCMR is a low-cost method that uses the error between truth data and a biased estimate to improve the biased model. Neutral mass density measurements are used to estimate an appropriate photoelectron heating efficiency, which is shown to drive the modeled thermosphere closer to the real thermosphere. Observations from the Challenging Mini-Payload (CHAMP) satellite taken under active and quiet solar conditions show that RCMR successfully drives the GITM thermospheric mass density to the observed values, removing model bias and appropriately accounting for missing physical processes in the thermospheric heating through the photoelectron heating efficiency. [ABSTRACT FROM AUTHOR]

Using time series of vertical data collected from middle and upper atmospheric (MU) radar, the adaptive Capon beamforming technique is used to synthesize beams in the desired pointing directions within the radar beamwidth. Beam synthesis has been performed at the tilt angle of 1.5° with different beam configurations (4 beams, 8 beams, 16 beams, 32 beams, 48 beams and 64 beams), which are equally separated azimuth plane. The Power Spectral Density (PSD) is obtained from the synthesized beam using the eigenvector (EV) sub-space based spectral estimation method. The first moment is derived from the EV produced spectrum using the adaptive moment estimation method. From the first order moments derived along equally spaced pointing directions, radial wind velocities are readily obtained in the corresponding directions. From radial velocity obtained in various pointing directions, momentum flux of short duration (<2 h) is estimated using the symmetric beam method (SBM) which requires 4 symmetric beams separated by 90° in the azimuth plane. A comparative study has been performed to study the optimum beam configuration, required for the flux estimation, which shows that 32 beam configuration is sufficient to estimate the flux with least error. Study with 32 beam configuration, gives 8 sets of symmetric beams (8×4). Using 8 set of beams, the flux is estimated for each set of beams and averaged. The averaged flux is further integrated over different lengths of time up to 14 h. This systematic method for the estimation of momentum flux reveals that spatial averaging of beams in azimuth and integration over different lengths of time have reduced the time of integration from 15 to 16 h for the conventional approach to 8–9 h for the new approach using Postset Beam Steering (PBS) technique. The flux estimated with spatial-averaging of beam using PBS technique has been compared with other standard methods. [ABSTRACT FROM AUTHOR]

The question about the survival possibility and the life duration of the topside ionosphere equatorial spread F (ESF) plasma bubbles observed in the separate ion component (He + ) is investigated. For this aim the main aeronomy processes, in which plasma bubbles and their He + ions are involved, were under consideration. It was obtained that the main competition takes place between the He + loss reactions (He + –N 2 reaction) and the uplift during linear growth phase (~10 min) of the Rayleigh–Taylor (R–T) instability, when the plasma bubbles are forming. It was revealed that the ambipolar diffusion of the He + ions inside the plasma bubble is the fastest (~1–2 min) in the altitude region up to 500 km and becomes slower (~1 h) above 500 km. On the other hand, the plasma bubbles seen in He + density are pretty stable structures against the cross-field (Bohm) diffusive collapse. It was concluded that the ESF plasma bubbles, reaching the “ceiling” heights, can exist for a night and several morning hours (~10–13 h) and that there is a principal opportunity to observe them in the separate ion component (He + ). [ABSTRACT FROM AUTHOR]

We tried to find with the ULF/ELF electric field sensor onboard Chibis-M microsatellite signatures of the IAR (Ionospheric Alfvén Resonator) and Schumann resonance (SR) in the upper ionosphere. Whereas observations of the IAR and SR multi-band emissions with ground magnetometers are ubiquitous, in-situ satellite observations of their signatures are very rare. ULF events detected by the Chibis-M show a possibility of triggered excitation of IAR. In contrast to dominating view, IAR has been found to be effectively excited on the dayside, too. Chibis-M observations also supported the possibility of the SR leakage into the upper ionosphere. [ABSTRACT FROM AUTHOR]

Past observations have shown that the effects of planetary waves can travel beyond their dissipation region and into the thermosphere and ionosphere. One possible mechanism through which this event can occur is the modulation of turbulent mixing near the turbopause. Because turbulent mixing in the mesosphere-lower thermosphere is strongly dependent on the amount of wave dissipation, planetary wave filtering of gravity waves from below can indirectly modulate the amount of turbulent mixing. This study uses the NCAR TIE-GCM to investigate how this mechanism induces variability in upper atmospheric composition and density. It is shown that varying the amount of turbulent mixing at discrete planetary wave periods induces similar periodicities in neutral and electron density. This process is driven by species diffusive flux through isobaric (constant pressure) levels, resulting in atmospheric contraction or expansion. Results also indicate that the mechanism studied has a wave frequency dependence and is most effective for long period planetary waves. [ABSTRACT FROM AUTHOR]

Abstract: The wavelength dependence of solar irradiance enhancement during flare events is one of the important factors in determining how the Thermosphere–Ionosphere (T–I) system responds to flares. To investigate the wavelength dependence of flare enhancement, the Flare Irradiance Spectral Model (FISM) was run for 61 X-class flares. The absolute and the percentage increases of solar irradiance at flare peaks, compared to pre-flare conditions, have clear wavelength dependences. The 0–14nm irradiance increases much more (~680% on average) than that in the 14–25nm waveband (~65% on average), except at 24nm (~220%). The average percentage increases for the 25–105nm and 122–190nm wavebands are ~120% and ~35%, respectively. The influence of 6 different wavebands (0–14nm, 14–25nm, 25–105nm, 105–120nm, 121.56nm, and 122–175nm) on the thermosphere was examined for the October 28th, 2003 flare (X17-class) event by coupling FISM with the National Center for Atmospheric Research (NCAR) Thermosphere–Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) under geomagnetically quiet conditions (Kp=1). While the enhancement in the 0–14nm waveband caused the largest enhancement of the globally integrated solar heating, the impact of solar irradiance enhancement on the thermosphere at 400km is largest for the 25–105nm waveband (EUV), which accounts for about 33K of the total 45K temperature enhancement, and ~7.4% of the total ~11.5% neutral density enhancement. The effect of 122–175nm flare radiation on the thermosphere is rather small. The study also illustrates that the high-altitude thermospheric response to the flare radiation at 0–175nm is almost a linear combination of the responses to the individual wavebands. The upper thermospheric temperature and density enhancements peaked 3–5h after the maximum flare radiation. [Copyright &y& Elsevier]

Abstract: The objective of this research effort is to investigate the use of a high power-aperture Rayleigh lidar to measure neutral density and temperature in the upper atmosphere, into the lower thermosphere (>100km). The scientific interest with this system is to obtain measurements of atmospheric tides, planetary and gravity wave amplitudes and phase in the lower thermosphere. Measurement of waves into the lower thermosphere is accomplished using Rayleigh lidar methods with high power-aperture (PA) products using new technology lasers and large collecting apertures, resulting in PA of 50–700Wm2. This paper describes the simulation of high PA Rayleigh lidar systems and estimates their capability to measure gravity waves and tides in the mesosphere and lower thermosphere. Both traditional backscatter and bistatic (imaging) methods are simulated. Simulations show that substantive measurements can be achieved for characterizing gravity waves at altitudes >90km and atmospheric tides at altitudes >100km for this lidar system. Measurements of density and temperature to 10% precision are possible up to 120km. [Copyright &y& Elsevier]

Abstract: The Cloud Imaging and Particle Size (CIPS) instrument aboard the Aeronomy of Ice in the Mesosphere (AIM) mission produces detailed images of Polar Mesospheric Clouds (PMCs) from space and has revealed several distinct types of cloud structures. One such commonly observed structure is an ‘ice void’—a nearly circular ice-free region with diameter varying from tens to thousands of kilometers. We document the morphology of large ice voids (diameter of ∼500–1000km) as seen by CIPS in the northern hemisphere 2007 PMC season. The meteorology of the mesosphere when these voids occur is described by the Navy Operational Global Atmospheric Prediction System–Advanced Level Physics High Altitude (NOGAPS–ALPHA) forecast/assimilation system. A comprehensive study of dynamics and transport in the mesosphere using NOGAPS–ALPHA during the occurrence of a large ice void observed by CIPS on 19 July 2007 indicates that warmer temperatures (warmer than the frost point temperature of ∼144K) at the location of the void may be related to increased tidal activity and transport of warm air from low latitudes. However, a detailed analysis of all identified voids is necessary to generalize the factors that could be responsible for the formation of an ice void. [Copyright &y& Elsevier]

Abstract: This paper presents a framework for understanding and quantifying the leverage available for inverting the radiance signal in order to retrieve particle size distribution mode radius and albedo from polar mesospheric clouds (PMC) observations using the cloud imaging and particle size (CIPS) instrument on board the aeronomy of ice in the mesosphere (AIM) satellite. The observed signal is a superposition of the scattering angle dependence of the cloud albedo and the Rayleigh scattered albedo controlled by ozone at the stratopause. The leverage as is defined in the paper is a way to quantify how much the net scattering angle dependence changes as a function of mode radius. The leverage is determined by decomposing the observed signal into orthogonal components which isolate the parts of the signal that are unique to changes in mode radius from those that could be due to changes in the background. This leverage is considered along with instrument noise performance to determine retrieval uncertainties and to understand minimum thresholds in the cloud retrieval parameters. [Copyright &y& Elsevier]

A high-spectral resolution multiwavelength imaging slit spectrograph using echelle grating (MISE) that is capable of measuring daytime optical emissions at multiple wavelengths simultaneously over a large (140°) field-of-view is presented. Optical emissions during daytime (either dayglow or daytime aurora) are buried in the strong daytime solar scattered background continuum and therefore very high spectral resolution measurements are required to obtain their contributions. MISE measures the emission intensities of OI 557.7nm, OI 630.0nm, and OI 777.4nm that originate in the upper atmosphere. The dispersion achieved by the spectrograph at these three spectral regions, respectively, is 0.004, 0.0049, and 0.0059nmpixel−1. By using an echelle grating as the dispersing element, multiple spectral regions of aeronomic interest are made to fall in the same diffraction angle range so that rotation of grating is avoided. This instrument is immune to ambient temperature fluctuations and vibrations and is suitable for long and continuous field operations. The spectral and intensity calibration of this instrument along with the data analysis methodology are discussed. Sample data of emission intensities from all the three wavelengths mentioned above for a couple of days obtained from Hyderabad (17°N, 80°E; 8.7°N Mag. Lat.) are presented which show a good similarity when compared with those of empirical and photochemical model results. The OI 557.7nm daytime emissions are measured sparsely from ground-based techniques and ground-based OI 777.4nm daytime emissions from ground are presented for the first time, to the best of our knowledge. The variability is highlighted and the potential of such measurements to derive information on vertical coupling of atmospheric regions and wave dynamics during daytime are discussed. [ABSTRACT FROM AUTHOR]

Abstract: The Super Dual Auroral Radar Network (SuperDARN) is a network of HF radars used to study phenomena in the Earth's magnetosphere, ionosphere, and upper atmosphere. Phenomena in the upper mesosphere and lower thermosphere (MLT) can be studied as the SuperDARN radars act effectively as meteor radars at near ranges. However, SuperDARN meteor echo measurements from all heights have typically been combined together to give a height-averaged picture of large-scale characteristics and dynamics of the MLT. This is in part due to the uncertainty in the measurement of individual meteor echo heights, which is in turn partly due to the lack of reliable (and for some radars, the lack of any) interferometric information. Here, we present a method for calibrating SuperDARN interferometer data which reduces the systematic offsets in meteor echo height estimations. Using 9 years of SuperDARN data we then determine occurrence distributions of SuperDARN meteor echo heights. The distributions are approximately Gaussian with height, extending from to and peaking around . In addition, we investigate whether the Doppler spectral width measured by the SuperDARN radars, which is related to the ambipolar diffusion coefficient for meteor echoes, can be used as a proxy measurement for meteor echo height. Due to the large spread of spectral width measurements at any one height we conclude that this proxy measurement is not practical and that the height of individual SuperDARN meteor echoes cannot be estimated without interferometric information. We also discuss how more accurate height information could be used to study the height variation of neutral wind velocities and the ambipolar diffusion coefficient across the MLT altitude range, and conclude that SuperDARN meteor echo observations have the potential to complement, and significantly extend the altitude range of, meteor echo observations from standard VHF meteor radars. [Copyright &y& Elsevier]

Abstract: It is well known that the 630.0nm nightglow emission intensity at mid latitudes increased more than twofold in periods of maximum sunspot activity. It is assumed that this phenomenon is caused by variations in solar ultraviolet radiation intensity in solar activity cycles (Fishkova, 1983). In this paper, we present the results of photometric measurements of the nightglow emission intensity at 63°N and 130°E (Yakutsk) in 1990–2007. Moreover, we show the dependence of the 630.0nm emission intensity in magnetically quiet days on solar activity in the 22nd and 23rd cycles. It was established that there is a close relationship between the 630.0nm line nightglow intensity and the solar extreme ultraviolet (EUV) intensity based on SOHO/SEM data, obtained in 1997–2007 with a correlation coefficient of 0.8–0.9. The changes of ionization level of nighttime F2 layer at the Yakutsk station and neutral composition of the upper atmosphere (thermosphere) during the photometric observations have been analyzed. It is shown that the long-term variations of the red line emission intensity are caused by the change of nighttime F2 layer electron density. The nighttime ionization level is related to the electron density of the daytime F2 region which depends on the EUV flux intensity during a solar activity cycle. [Copyright &y& Elsevier]

Abstract: Saturation in the dependence of foF2 on solar flux is a phenomenon more pronounced in the equatorial ionization anomaly region. The phenomenon was observed in the relationship between the F2 critical frequency (foF2) and any solar indices. Using a two-segmented regression fit on data from an African low latitude station (Geographical Latitude 12.4°N; Geomagnetic latitude 3.5°N), saturation features from the dependence of foF2 on solar radio flux (F10.7) was studied. Diurnal and seasonal variation were studied for the first time in this low latitude region of the African sector. Significant variations were observed, especially in the solstices. It was observed that saturation effect is closely related to the hourly F2 critical frequency and these results were compared with those from Asian, Australian and the American sectors. The diurnal and seasonal variations find their explanations in the photo-ionization process, the fountain effect, and the pre-reversal enhancement while the seasonal variation was attributed to both the ion drift and thermospheric circulation. Future work with larger volume of data is expected to validate the observations from this study. [Copyright &y& Elsevier]

In 2018 a band of strong westward zonal wind (400–600 m s−1) was found in both daytime polar thermospheres, at 65° – 70° geographic latitude. It was imbedded in the eastward zonal wind background and because of its narrow longitudinal structure was called a "wind wall". The observations were made with the Wind Imaging Interferometer (WINDII) on the NASA Upper Atmosphere Research Satellite (UARS), through Doppler wavelength shifts of the O(1S) atomic oxygen dayglow emission at 557.7 nm. This wind wall was not always present, but when it was it always occurred at the same geographic longitude, around 300°E in the northern hemisphere and 150°E in the southern hemisphere. A subsequent study showed that these wind walls occurred during times of enhanced geomagnetic disturbance, indicated by the a p index, and that the wind fields corresponded closely with those predicted by the WACCM-X model. Additional information on the wind wall and its temporal behavior could be provided by simultaneous observations from a ground-based instrument, located at the appropriate latitude and longitude. It proved possible to obtain a few WINDII observations made early in the UARS mission coincident with a Super Dual Auroral Radar Network (SuperDARN) station, and the results of these comparisons, together with data provided by the WACCM-X model are presented and discussed. The comparison showed that for significant geomagnetic disturbances the neutral wind/plasma drift reversals occurred at the same longitude and the same Universal Time. • A high-latitude wind reversal occurs at specific longitudes in both hemispheres. • This neutral wind reversal is persistently seen by the Wind Imaging Interferometer. • A plasma flow reversal is seen by SuperDARN, although at somewhat lower latitudes. • Winds are shown as a function of longitude and the plasma flow as a function of time. • The reversals occur during geomagnetic disturbances, identified by increased a p. [ABSTRACT FROM AUTHOR]

Abstract: To investigate the vertical propagation of gravity waves from the lower to the upper atmosphere, combined measurements with an airglow imager and lidars were carried out at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) station (69.3°N, 16.0°E) in northern Norway. Airglow imaging reveals the two-dimensional structure of gravity waves in the mesopause region, while the ALOMAR Rayleigh/Mie/Raman (RMR) lidar and sodium lidar provide the vertical structures between the stratosphere and the lower thermosphere. On 26 November 2010, the imager identified a mesoscale gravity wave structure in the sodium airglow that had a horizontal wavelength of 277km, a wave period of 59min, and propagated northeastward at a phase speed of 78m s−1. Simultaneous lidar measurements also showed upward wave signatures with a similar wave period in the temperature perturbations; the vertical wavelength of the upward wave seen in the temperature data is consistent with the dispersion relation for gravity waves. Based on the combined measurements with the imager and sodium lidar, the momentum flux of this gravity wave was estimated to be 1.0m2 s−2 at the sodium airglow height. Ray-tracing analysis suggested that the observed gravity wave was generated by a distortion of the polar jet at the tropopause via a geostrophic adjustment process. [Copyright &y& Elsevier]

Abstract: Interaction of atmospheric non-stationary stream with obstacles on the Earth''s terrestrial surface causes disturbances which are a source of various wave processes. A study of such processes and their influence on the upper atmosphere temperature regime was carried out at Kislovodsk high-mountainous scientific station of the Institute of Atmospheric Physics Russian Academy of Sciences (IAP RAS) by measuring the mesopause temperature in the northern leeward area of the Caucasian ridge. Using the data of the spectrophotometric measurements of the upper atmosphere hydroxyl emission characteristics over almost two decades, information about the orographic disturbances at the mesopause altitudes were obtained and features of their generation in the surface atmosphere were studied. It was found that the atmospheric temperature at altitudes around 90km in the lee of mountains increased by 10K (the mean value is about 200K) at a distance of about 150km from the ridge. The 300-km width of the observed airglow disturbances is from the observations near the Ural and Caucasian mountains. The sources of wave disturbances are shown to be concentrated near the mountainous irregularities of the Caucasian ridge. These sources appear in the troposphere at altitudes of about 4km. The process of generating waves with periods from 7 to 20min were believed to be caused by wind gusts. The spatial distribution of energy flow in the lee of the mountains was calculated and amounts to about 3ergcm−2 s−1. [Copyright &y& Elsevier]

Abstract: We investigate the X ray and UV flux changes during flare events and the corresponding ionospheric response. The study reveals that UV flux enhancement depends on both flare intensity and position on solar disk while X-ray flux enhancement depends only on the former. The study brings out a new result that the E region response to flares directly relates to the X-ray flux enhancement and that it does not exhibit limb effect. Further, the F region response shows limb effect indicating the UV flux control on the same and it also shows seasonal variability due to solar zenith angle variability. [Copyright &y& Elsevier]

Abstract: Geomagnetic storms at solar minimum are driven by the interaction between high speed streams and low speed streams (Corotating Interactions regions/High Speed Streams—CIR/HSSs- this includes both the CIR part of the storm and the HSS part as both has effects on the thermosphere and ionosphere), rather than by Coronal Mass Ejections (CMEs). Solar minimum storms are generally of smaller amplitude, but they also have other characteristics that affect the response of the thermosphere/ionosphere (TI) system to them. We explore both the background upper atmosphere and the characteristics of these CIR/HSS events in 2008 using both models and data. The model data comparisons were good, showing mid-latitude positive storm effects on NmF2 on the day of the storm and long, extended periods of storm induced changes on the following days. Generally, the version of the NCAR-TIEGCM (National Center for Atmospheric Research-Thermosphere Ionosphere Electrodynamics Model) run with the Heelis convection pattern was in better agreement with the data than the version run with the Weimer convection pattern. We attribute this difference in the response of the two versions of the NCAR-TIEGCM to the fact that the precipitation we associate with the Heelis model has been tuned to better represent observed precipitation over many years, whereas such tuning has just begun for the version of the NCAR-TIEGCM that uses the Weimer model. Physically, the most important conclusion that we draw is the CIR/HSS events can have significant effects on the ionosphere and thermosphere for several days after the CIR has ended. While CIR/HSS events are normally weaker than CME events, the effects of the latter normally disappear in a day or two. Thus the effects of CIR/HSS events may be comparable to those of CME events for some fields, notably neutral density changes. [Copyright &y& Elsevier]

Abstract: Long term variability of cold point tropopause (CPT) height (CPTh) and cold point tropopause temperature (CPTt) using 21 years (1985–2005) of radiosonde observations over Banqiao (25°N, 121.3°E), Taiwan is reported. The time-frequency analysis of CPTh using the Morlet wavelet revealed a pronounced annual cycle (AC) over the entire observational period, whereas the CPTt showed a quasi-biennial oscillation (QBO) signal during some specific time period (1988–1998). It is observed that the CPTh and CPTt vary at different time scales. To examine the origin of observed CPT oscillations, the outgoing longwave radiation (OLR) observations from the National Centers for Environmental Prediction and the National Center for Atmospheric Research (NCEP/NCAR) have been used for the same time period (1985–2005) over the study site. The ozone observations from ozonesonde, which is launched from the Banqiao during the period from 1992 to 2005, are also used. The wavelet spectra of the CPTh, CPTt, OLR and stratospheric ozone obtained in the present analysis reveal that the CPTh and OLR have similar kind of variability, whereas the CPTt and the stratospheric ozone shows correlative interannual variations. [Copyright &y& Elsevier]

Abstract: A long and continuous temperature data set from ground to mesopause was obtained in merging lidar and radiosonde data at mid-latitude over south of France (44°N). The analyses using Empirical Orthogonal Functions has been applied on vertical temperature profiles to investigate the variability differently than it has been done in previous investigations. This study reveals as the first mode in winter, a strong anti-correlation between upper stratosphere and mesosphere that is most probably link with planetary waves propagation and associated stratospheric warmings. While in summer the variability is located in the mesosphere and associated with mesospheric inversions that are probably generated by gravity waves breaking. This study shows that even if the daily temperature variability appears to be complex, a large part (30%) can be modeled, each season, using the first EOF. These vertical patterns exhibit some similarities with solar-atmospheric responses, suggesting a potential feedback of the dynamic. This is already observed for winter response, but during summer the contribution of gravity waves on the mesospheric solar response suggests future investigations to explore the role of this potential mechanism in solar-atmospheric connections. [Copyright &y& Elsevier]

Abstract: Meteoric ablation injects similar quantities of Fe, Mg and Si vapours into the upper mesosphere; these vapours recondense to form meteoric smoke particles. Electronic structure theory is used to argue that the metal silicate molecules, FeSiO3 and MgSiO3, should form in the upper atmosphere. These molecules, which may be regarded as the smallest smoke particles, have extremely large electric dipole moments of 9.5 and 12.2 Debye, respectively. H2O molecules therefore bind very effectively to them, with free energy changes in excess of −100kJmol−1. Indeed, the addition of up to 8H2Os occurs more favourably than the sublimation of H2O to bulk ice. FeSiO3 and MgSiO3 should therefore nucleate ice particles under polar mesospheric conditions at temperatures around 140K. These silicates should form readily and are shown to be stable in the mesosphere. Their daily production should be between 2000 and 8000cm−3 around 85km, which is sufficient to compete with transport out of the polar mesosphere by the meridional circulation. [Copyright &y& Elsevier]

Abstract: Polar Mesospheric Cloud (PMC) observations from the Aeronomy of Ice in the Mesosphere (AIM) Cloud Imaging and Particle Size (CIPS) experiment are compared to results from the Solar Backscatter UltraViolet (SBUV/2) instrument for two PMC seasons in each hemisphere, from 2007–2009. For these comparisons, a CIPS cloud retrieval algorithm based on a technique similar to that used in the operational SBUV PMC retrievals was developed. CIPS nadir data analyzed this way agree with SBUV data to better than 1% (2%) for PMC occurrence frequency (albedo). Non-nadir, forward scattering CIPS data are also analyzed with the SBUV-type algorithm, enabling an indirect verification of the CIPS operational retrievals. CIPS operational PMC frequencies agree with these CIPS–SBUV-type retrievals to within 0.6% at 80–85° latitude, and are lower by 9.8% at 70–75° latitude. Overall, the comparisons show that the v3.20 CIPS operational cloud retrievals are valid for scientific research. [Copyright &y& Elsevier]

Abstract: A synchronized observation of a noctilucent cloud (NLC) using an airborne camera and a polar mesospheric cloud (PMC) using spaceborne cameras was obtained on 6 July 2009. Two Canon EOS-30D cameras were integrated to a Mooney M20K research aircraft and flown to a location of 58.32°N, 112.30OW in Northern Alberta, Canada to coordinate with the overpass of the Aeronomy of Ice in the Mesosphere (AIM) satellite at 06:48 GMT. The aircraft was positioned so that imagery would be taken while the sun was at a solar depression angle (SDA) of 8.5° and then oriented so that the imager would be pointed along the AIM groundtrack. The common-volume imagery obtained from both the cloud imaging and particle size (CIPS) experiment and the aircraft at the time of the AIM overpass concurrently detected clouds identified by similar band structures. The imagery from the aircraft also shows fine transverse cloud structures not discernable within the CIPS image, implying that greater spatial resolution is achievable through the airborne imagery. Subsequent analysis of the CIPS imagery from the AIM satellite at this location shows a correlation in orientation of the prominent structures in both sets of imagery, showing overlay of prominent features and coincidence of band orientation. [Copyright &y& Elsevier]

Abstract: The paper presents for the first time the global spatial (latitude and altitude) structure of the mean ionospheric response to sudden stratospheric warming (SSW) events in winters of 2007/2008 and 2008/2009. To elucidate the effect of the SSWs on the ionosphere the COSMIC foF2, hmF2, and electron density data at fixed altitudes are analyzed. Both the mean foF2 and hmF2 parameters and the mean electron density at fixed heights indicate regular negative responses to the SSW temperature pulses at high latitudes. Similar response is found for the diurnal variability of the COSMIC electron density. The response is confined mainly to low and middle latitudes. A possible mechanism causing the observed negative ionospheric response is suggested. [Copyright &y& Elsevier]