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Atomic oxygen (AO) is the most abundant species in the terrestrial thermosphere at altitudes of 180 to 700 km and plays a crucial role in numerous upper atmospheric processes. The drag encountered by a spacecraft is directly influenced by the background atmospheric density, making AO a notable contributor in this regard. The objective of this study is to analyze the variability of the density of AO in the near-Earth space environment under different solar activity conditions, as well as during severe space weather events. The analysis is primarily focused on the altitude range of 300 to 500 km in the Earth's thermosphere across the globe, which is a crucial operating region for a large number of Low Earth Orbiting satellites. AO exerts significant drag on the Low Earth Orbit (LEO) satellites and reacts with the materials of the satellite body and in situ probes mounted therein, causing considerable degradation to both. Additionally, AO interferes with measurements made by optical instruments through phenomena such as 'shuttle glow'. Analysis based on NRLMSIS 2.0 model revealed that the concentration of AO displays significant seasonal variations primarily due to the asymmetry in solar insolation. The highest concentration of AO was observed during the spring equinoctial month of the solar maximum year, while the lowest concentration was noted during the winter solstitial month of the solar minimum year. The enhancement seen in AO density contours show an asymmetry in the zonal direction at approximately 50 ∘ N in June and 50 ∘ S in December. Similarly, during geomagnetically disturbed days, the AO density shows enhancements along various longitudes, as indicated by the elongated contours. During geomagnetic storms, the heightened Joule heating in the polar region modifies the meridional circulation, which in turn affects the global distribution of AO density. Similarly, an analysis of thermospheric neutral winds obtained from the HWM93 model reveals that the zonal winds exhibit a strong westward acceleration over the region where AO density is enhanced. Furthermore, such an increase in AO density during geomagnetic storms has been found to cause orbital degradation ranging from a few tens of meters to a few hundred meters, depending on the strength and geo-effectiveness of the storms. This study emphasizes the critical contribution of AO density to satellite drag force, necessitating in situ measurements for an accurate estimation of the altitude decay of the spacecraft. [ABSTRACT FROM AUTHOR]

Satellite drag modeling remains the largest source of uncertainty affecting space operations in low Earth orbit. The uncertainty stems from inaccurate models for mass density and drag coefficient. Drag coefficient modeling also impacts scientific knowledge on the physics and dynamics of the upper atmosphere through the estimation of high-fidelity mass density from measurements of acceleration on-board satellites. Efforts over the last decade have pushed drag coefficient modeling in the right direction, however, have resulted in multiple methods and tools. We provide a comprehensive review of the drag coefficient modeling methods and tools. Current scale differences between thermospheric data sets mostly originate from errors in the aerodynamic modeling, specifically in the modeling of the satellite outer surface geometry and the gas-surface interactions. Enhancing these models' accuracy is intrinsically connected to the satellite drag fidelity for science and operations. A team of invested scientists recently met under the community-driven International Space Weather Action Teams (ISWAT) initiative to discuss and consolidate the efforts towards a drag coefficient modeling baseline for consistency in science and operations. In this paper, we compare the available methods for drag coefficient modeling, their impact on the derived density estimates, and make recommendations for adoption of baseline methods for science and operations. Results show that the differences in derived densities estimates can reach tens of percent at altitudes above 4̃50 km during solar minimum conditions resulting mainly from differences in the modeling of gas-surface interactions. As a result, we conclude and recommend that robust uncertainty quantification be an integral part of any modeling efforts that employ the high-fidelity accelerometer derived density estimates. We also conclude and recommend that gas-surface interaction models that account for impact of altitude and solar variations be employed moving forward. Finally, we recommend future science missions to improve our understanding of gas-surface interactions and eventually the upper thermosphere variability. [ABSTRACT FROM AUTHOR]

The idea that a tropospheric heat source generating waves can be replaced with a surface source of pressure oscillations on the Earth's surface is proposed and analyzed. In the surface source, atmospheric pressure fluctuations caused by a tropospheric source are used. The discrepancies due to the replacement of the tropospheric heat source by the surface one are estimated using experiments with a high-resilution numerical model of acoustic-gravity waves. It is shown that at frequencies of infrasound, the wave amplitudes in the upper atmosphere are very close for both tropospheric heat and surface pressure sources. At frequencies of internal gravity waves, the time-spatial structure of the wave fields are close in the upper atmosphere, but wave amplitudes for the surface pressure generation could be larger (up to two times) than those for the respective tropospheric heat sources. This discrepancy is explained and some corrections to the surface pressure source are proposed, which provide better agreement and may be applied for modeling of waves from tropospheric sources, based on the data of atmospheric microbarometer measurements. [ABSTRACT FROM AUTHOR]

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]

• F 10.7 cannot describe the hmF 2 behavior in the 24th SA cycle. • hmF 2 behaves during the " v ague" period similarly to foF 2. • The reduction of atomic oxygen in the thermosphere is concluded. The measurements of the F 2-layer height, hmF 2, at Juliusruh station during 1958–2018 are analyzed by the method developed by the authors earlier and used to determine long-term trends in parameters of the F 2 layer. It is found that, in the same way as for the behavior of the critical frequency described by the authors in the previous publication, peculiarities in the hmF 2 behavior are observed after 2002–2003. If the solar activity index F 10.7 is used, an increase in the Δ hmF 2 value is observed and provides a positive trend in hmF 2 that contradicts to the available ideas on the continuing process of cooling and contraction of the upper atmosphere. If the F 10.7 index corrected by other SA indices (the sunspot number Rz or the Lyman-alpha line intensity) is used, as it has been done for the foF 2 trends in the previous publications by the authors, the picture of changes in the hmF 2 trends is obtained similarly to the changes in the foF 2 trends. A well pronounced negative trend in hmF 2 is observed to 2002–2003, and then it is substituted by a chaotic change in Δ hmF 2 during the "vague" period, whereas during the last analyzed years it becomes negative again. Analysis of the slope s of the foF 2(hmF 2) dependence shows that there is a systematic decrease in s with time. That confirms the conclusion of the authors on a decrease in the oxygen amount in the thermosphere. [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]

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]

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]

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]

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]

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]

At the 10th Thermospheric Ionospheric GEospheric Research (TIGER/COSPAR) symposium held in Moscow in 2014 the achievements from the start of TIGER in 1998 were summarized. During that period, great progress was made in measuring, understanding, and modeling the highly variable UV-Soft X-ray (XUV) solar spectral irradiance (SSI), and its effects on the upper atmosphere. However, after more than 50 years of work the radiometric accuracy of SSI observation is still an issue and requires further improvement. Based on the extreme ultraviolet (EUV) data from the SOLAR/SolACES, and SDO/EVE instruments, we present a combined data set for the spectral range from 16.5 to 105.5 nm covering a period of 3.5 years from 2011 through mid of 2014. This data set is used in ionospheric modeling of the global Total Electron Content (TEC), and in validating EUV SSI modeling. For further investigations the period of 3.5 years is being extended to about 12 years by including data from SOHO/SEM and TIMED/SEE instruments. Similarly, UV data are used in modeling activities. After summarizing the results, concepts are proposed for future real-time SSI measurements with in-flight calibration as experienced with the ISS SOLAR payload, for the development of a space weather camera for observing and investigating space weather phenomena in real-time, and for providing data sets for SSI and climate modeling. Other planned topics are the investigation of the relationship between solar EUV/UV and visible/near-infrared emissions, the impact of X-rays on the upper atmosphere, the development of solar EUV/UV indices for different applications, and establishing a shared TIGER data system for EUV/UV SSI data distribution and real-time streaming, also taking into account the achievements of the FP7 SOLID (First European SOLar Irradiance Data Exploitation) project. For further progress it is imperative that coordinating activities in this special field of solar–terrestrial relations and solar physics is emphasized. [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]

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 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: 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: 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: Given the ability of global models of the upper atmosphere to utilize high-resolution solar spectra to specify the solar soft X-ray and EUV irradiance, researchers now have the ability to perform detailed theoretical analysis of the response of the upper atmosphere to dynamic solar forcing more completely than ever before. Therefore, it is possible to develop a more quantitative understanding of the response of the thermosphere to solar flares. In this study, the effect of different characteristics of solar flares on the thermosphere is investigated. This is done in a theoretical manner, using synthetic solar irradiance data that is based on observations as input to the global ionosphere-thermosphere model (GITM). Specifically, the neutral response to the total incident energy, peak flare magnitude, background irradiance magnitude, duration of the impulsive phase, and decay time is investigated. It is found that the density response at 400km altitude is linearly dependent on the total integrated energy above the background level being deposited into the atmosphere, and that the day-side response is strongly dependent on both the total incident energy into the system and the peak flare magnitude. Also, the decay time of the flare is important in determining the time at which the maximum global response occurs. Finally, the duration of the impulsive phase is found to have little effect on the global response of the system. [Copyright &y& Elsevier]

Abstract: This paper examines the response of the high latitude ionosphere–thermosphere system during two intense geomagnetic storms. For that, data taken by instruments on board Dynamic Explorer 2 taken at heights of the F2-layer are used. These results represent a comparison of simultaneous measurements of storm disturbances in gas composition, electron density and temperature in common local time sectors. Documented are an increase in electron temperature and a decrease in electron density; increases both in electron temperature and electron density; and the correlation between electron density decreases and increases in the ratio N2/O. It is noticed that the decrease in electron density is sometimes due to an increase in the molecular nitrogen density N2 and not always is attributed both to the increase in N2 density and the simultaneous decrease in the atomic oxygen density. [Copyright &y& Elsevier]

Abstract: We have used two different General Circulation Models to study the thermal and wind structure of the Martian upper atmosphere (mesosphere and thermosphere). Both models take into account the effects of waves propagating from the lower atmosphere, although they use different methods for this purpose. We present the results of three simulations that allow us to take into account the seasonal variability of the Martian atmosphere. Simplified dust scenarios and a common set of input parameters are used. The temperatures and winds predicted by both models show an overall good agreement. However, some differences have been identified, generally of a local nature. The magnitude of these differences tends to increase with the amount of dust in the lower atmosphere. The different heating terms of the upper atmosphere predicted by both models are in good agreement, which suggests that the differences between the models have their origin in the propagation of waves from the lower atmosphere. This study has allowed us to confirm the important role of the redistribution of the energy by the winds in producing the longitude–latitude structure of the temperatures. Both models predict also a thermospheric polar warming during the Southern summer solstice, although its intensity seems to be model-dependent and connected to lower atmosphere dust loading. [ABSTRACT FROM AUTHOR]

Abstract: A differential emission measure technique is used to determine flare spectra using solar observations from the soft X-ray instruments aboard the Thermosphere Ionosphere Mesosphere Energetics Dynamics and Solar Radiation and Climate Experiment satellites. We examine the effect of the solar flare soft X-ray energy input on the nitric oxide (NO) density in the lower thermosphere. The retrieved spectrum of the 28 October 2003 X18 flare is input to a photochemical thermospheric NO model to calculate the predicted flare NO enhancements. Model results are compared to Student Nitric Oxide Explorer Ultraviolet Spectrometer observations of this flare. We present results of this comparison and show that the model and data are in agreement. In addition, the NO density enhancements due to several flares are studied. We present results that show large solar flares can deposit the same amount of 0.1–2 and 0.1–7nm energy to the thermosphere during a relatively short time as the Sun normally deposits in one day. The NO column density nearly doubles when the daily integrated energy above 5Jm−2 is doubled. [Copyright &y& Elsevier]

Abstract: The global pattern of long-term trends and changes in the upper atmosphere and ionosphere has been presented by Laštovička et al. [2006a. Global change in the upper atmosphere. Science 314 (5803), 1253–1254]. Trends in the mesospheric temperature, electron concentration in the lower ionosphere, electron concentration and height of its maximum in the E-region, electron concentration in the F1-region maximum, thermospheric neutral density and F-region ion temperature qualitatively agree with consequences of the enhanced greenhouse effect and form a consistent pattern of global change in the upper atmosphere. Three groups of parameters were identified as not-fitting this global pattern, the F2-region ionosphere, mesospheric water vapour, and the mesosphere/upper thermosphere dynamics. The paper reports progress in development of the global pattern of trends with emphasis to these three open problems. There are several other factors contributing to long-term trends, namely the stratospheric ozone depletion, mesospheric water vapour concentration changes, long-term changes of geomagnetic activity and of the Earth''s magnetic field. [Copyright &y& Elsevier]

Abstract: We have studied the temperature response to changes in the CO2 concentration in the middle and upper atmosphere using the Coupled Middle Atmosphere–Thermosphere Model 2 (CMAT2). We have performed simulations with a range of CO2 concentrations and three different ways of accounting for the effects of gravity waves, to allow for comparison with previous studies and sensitivity analyses. We initially find that the response of the model to the changes in CO2 concentration which took place between 1965 and 1995 (320–360ppm) is strongly dependent on the gravity wave parameterization that is used, but this is to a large degree due to steps or kinks in an otherwise nearly linear curve describing the temperature as a function of CO2 concentration. We have not been able to identify the cause of these steps as part of the present study, which is a limitation and must be studied in future work. Here we treated the steps as model noise and rather focused on correcting for their effects by fitting straight lines to the temperature–CO2 curves to estimate the overall slope of the curves. From these slopes we were able to obtain more robust trend estimates than can be obtained by comparing only two model simulations, as is normally done in other, similar studies. The corrected temperature responses to a 40ppm change in CO2 concentration still show up to 15–17% sensitivity to the gravity wave parameterization in the mesosphere and thermosphere. This remaining sensitivity is likely to be related to the fundamental differences in the way a change in temperature modifies the propagation and dissipation characteristics of gravity waves in each parameterization, which is particularly different for linear and non-linear schemes. The corrected trends we find are largely in agreement with other modelling studies, and therefore do not fully explain observed trends, which are typically larger than those predicted by modelling studies. However, modelling results could be similarly sensitive to other model parameters and settings, for example to gravity wave characteristics or solar activity level, and this should be further investigated as well. [Copyright &y& Elsevier]

Abstract: In the dayside polar region—loosely referred to as the cleft region—particle precipitation and Joule heating cause significant perturbations of the upper atmosphere. Here Dynamics Explorer-2 satellite data are used to present a synopsis of these disturbance effects. Documented are an increase in electron temperature and a decrease in electron density; increases in ion drift speed and ion temperature; an increase in the upward-directed ion velocity; increases in zonal wind speed and neutral gas temperature; and changes in the neutral gas composition and mass density. It is suggested that the increase in electron temperature is partly controlled by the decrease in electron density; that the ion upflow velocity mainly depends on the electron temperature, less frequently on the ion temperature; and that the observed decrease in thermospheric mass density is due to a decrease in the atomic oxygen density, which in turn is caused by diverging wind flows. [Copyright &y& Elsevier]

Abstract: The processes leading to enhancements in mid latitude nitric oxide (NO) densities following geomagnetic storms have been investigated using the University College London (UCL) Coupled Middle Atmosphere and Thermosphere (CMAT) general circulation model. A comparison of calculated storm time and quiet time NO densities at 110km altitude reveals the presence of aurorally produced NO at both high and mid latitudes for several days after subsidence of activity. At 150km, the NO enhancements are shorter lived and remain for up to approximately 2 days after the storm. By separating the contribution of chemical production and loss, horizontal and vertical advection, and molecular and eddy diffusion in the calculation of NO densities, we show that at 150km altitude, horizontal transport must be taken into consideration if post-storm mid latitude enhancements are to be reproduced. Chemical production of NO at high latitudes continues for up to 2 days after subsidence of a storm at altitudes of around 150km. We show that equatorward winds at this altitude are sufficiently strong to transport the aurorally produced NO to mid latitudes. Vertical diffusion transports NO from altitudes of 150km and above, to lower altitudes where it is longer lived. At 110km altitude, chemical, diffusive and advective terms must all be included in the calculation of NO density in order to simulate realistic mid latitude enhancements. We propose that it is the combined effects of increased chemical production, downward diffusion from altitudes of 150km and above, and transport by winds that lead to increases in mid latitude NO density at altitudes of around 110km. This is the first detailed study of the causes of post-storm mid latitude NO enhancements to use a three-dimensional general circulation model. [Copyright &y& Elsevier]

Abstract: Airglow volume emission rates of the O(1D) red line at 630.0nm and the O(1S) green line at 557.7nm were measured by the Wind Imaging Interferometer (WINDII) on the Upper Atmospheric Research Satellite (UARS) during 1991–1997. Focus of this study is on the peak volume emission rates of the two airglows after removing the direct solar effect, which are referred to as the ‘dark’ peak emission rates. The main results are as follows. For the red line emission, at low and mid-latitudes the daytime variation does not have a clear pattern except an enhancement at dusk; during nighttime there is an enhancement in the equatorial region at 20–03h, which has a semiannual variation with maxima at equinoxes; at solstices the daytime O(1D) dark emission rate is stronger in winter than in summer. For both the green line E-and F-layers the distribution of the dark peak volume emission rate is symmetric about noon in all seasons, symmetric about the equator at equinoxes, and stronger in summer than in winter. The O(1S) E-layer is profoundly affected by tides. For the first time the diurnal and semidiurnal amplitudes for the emission rates are derived using 24-h zero-sun data. The amplitude of the diurnal tide can be as large as 20% of the mean peak volume emission rate, and has maxima at the equator and about 40°N/S, and minima at about 20°N/S. The daily diurnal maximum is at noon at the equator but at midnight at 40°N/S. There is a clear semiannual variation of the diurnal amplitude in the equatorial region with maxima at equinoxes. The amplitude of the semidiurnal tide is mostly less than 10% of the mean peak volume emission rate with maximum amplitudes at noon and midnight. There is an annual variation of the semidiurnal amplitude at mid-latitudes peaking in summer. Aurorae appear in all three emission layers day and night. The green aurorae are brighter than the red aurorae, and the green E-layer aurorae are 2–3 times stronger than the F-layer aurorae. The green aurora has a clear midday gap in the F-layer and an afternoon gap in the E-layer. The red aurorae are particularly strong in the so-called cusp region at equinoxes. [Copyright &y& Elsevier]

Abstract: The Accelerometer Experiment (ACC) onboard Mars Global Surveyor (MGS) measured 1600 density profiles in the upper atmosphere of Mars during aerobraking. These measurements reveal large-scale and small-scale structure in the thermosphere of Mars. Here, the measurements of mass density for 115 orbits (#P0670–P0789) from November 1 to 30, 1998, under spring equinox and medium solar activity conditions (average F 10.7 ∼137) during phase 2 of the aerobraking in the thermosphere of Mars at different altitudes and longitudes are presented for northern mid-latitude (17–42°N) in the dayside atmosphere using ACC onboard MGS. From these mass densities, the neutral densities of different gases are derived from their mixing ratios. Using these neutral densities, the longitudinal distribution of photoionization rates and photoelectron impact ionization rates are calculated at wavelength range 1–102.57nm due to EUV and soft X-ray radiation under photochemical controlled region using Analytical Yield Spectrum approach (AYS). These conditions are appropriate for MGS Phase 2 aerobraking period from which the accelerometer data is used. Under the photochemical equilibrium condition, the electron density near the peak varies as the square root of the total peak ionization rate. Using this fact, an attempt is being made to estimate the mean primary and secondary peak electron density by averaging the longitudinal variations of total peak ionization rates in the northern mid-latitude (17–42°N) ionosphere of Mars, as there is no radio science measurement at this latitude region by MGS. [Copyright &y& Elsevier]

Abstract: New solar indices have been developed to improve thermospheric density modeling for research and operational purposes. Out of 11 new and 4 legacy indices and proxies, we have selected 3 (F 10.7, S 10.7, and M 10.7) for use in the new JB2006 empirical thermospheric density model. In this work, we report on the development of these solar irradiance indices. The rationale for their use, their definitions, and their characteristics, including the IS 21348:2007 spectral category and sub-category, wavelength range, solar source temperature region, solar source feature, altitude region of terrestrial atmosphere absorption at unit optical depth, and terrestrial atmosphere thermal processes in the region of maximum energy absorption, are described. We also summarize for each solar index the facility and instrument(s) used to observe the solar emission, the time frame over which the data exist, the measurement cadence, the data latency, and the research as well as operational availability. The new solar indices are provided in forecast as well as real time and historical time frames (http://SpaceWx.com JB2006 Quicklink). We describe the forecast methodology, compare results with actual data for active and quiet solar conditions, and compare improvements in F 10.7 forecasting with legacy HASDM and NOAA SWPC forecasts. [Copyright &y& Elsevier]

Abstract: Recent measurements by the accelerometer on the CHAMP Satellite have confirmed that there are prominent dayside density enhancements at high latitudes at all times. The magnetosphere is the source of the energy which produces these density features. We present a historical review of the observations and a semi-empirical global model which reproduces the observed density increases during geomagnetically quiet times. The parameters in the model were determined over 30 years ago. There is now the opportunity to improve those parameters by using densities measured by the modern accelerometers, as well as by using the latest improvements in describing the UV radiation and the semi-annual density variation. Recent progress in determining physical drag coefficients can enable the model to provide absolute densities. [Copyright &y& Elsevier]

Abstract: A new empirical atmospheric density model is developed using the CIRA72 (Jacchia 71) model as the basis for the diffusion equations. New solar indices based on orbit-based sensor data are used for the solar irradiances in the extreme and far ultraviolet wavelengths. New exospheric temperature and semiannual density equations are employed to represent the major thermospheric density variations. Temperature correction equations are also developed for diurnal and latitudinal effects, and finally density correction factors are used for model corrections required at high altitude (1500–4000km). The new model, Jacchia–Bowman 2006, is validated through comparisons of accurate daily density drag data previously computed for numerous satellites. For 400km altitude the standard deviation of 16% for the standard Jacchia model is reduced to 10% for the new JB2006 model for periods of low geomagnetic storm activity. [Copyright &y& Elsevier]

Abstract: The purpose of the paper is to answer the question if planetary waves (PW) are capable of propagating into the thermosphere. First the simplest vertical structure equation of the classic tidal theory accounting for a realistic vertical temperature profile is considered. Analysis and simulation show that the well-known normal atmospheric modes (NM), which are trapped in the lower and middle atmosphere, exhibit a wave-like vertical structure with a large vertical wavelength in the thermosphere. Moreover, the reflection of these modes from the vertical temperature gradient in the lower thermosphere causes appearance of the wave-energy upward flux in the middle atmosphere, and in a linearized formulation this flux is constant above the source region. To investigate a possibility of the NM forcing by stratospheric vacillations and to consider the propagation of different PW up to the heights of the upper thermosphere, a set of runs with a mechanistic Middle and Upper Atmosphere Model has been performed. The results of the simulation show that quasi-stationary and longer-period PW are not able to penetrate into the thermosphere. The shorter-period NM and ultra-fast Kelvin wave propagate up to the heights of the lower thermosphere. However, above about 150km they are strongly suppressed by dissipative processes. The role of the secondary waves (nonmigrating tides) arising from nonlinear interaction between the primary migrating tides and quasi-stationary PW is discussed. We conclude that PW are not capable of propagating directly up to the heights of the ionospheric F2 region. It is suggested that other physical processes (for instance, the electrostatic field perturbations) have to be taken into account to explain the observed PW-like structures in ionospheric parameters. [Copyright &y& Elsevier]

Abstract: Recent observations from the CHAMP satellite indicate that neutral density enhancements are common in the northern dayside cusp. The neutral density in this region can be nearly a factor of 2 larger than in adjacent regions of the thermosphere on the poleward and equatorward sides of the cusp. The presence of density enhancements implies that the neutral atmosphere is being heated in the cusp region causing upwelling. A high-resolution model of the global thermosphere is used to study the thermospheric response to heating in the northern dayside cusp. It is found that heating in the cusp results in the creation of a neutral fountain. Specifically, upward drift of the thermosphere within the cusp region is followed at higher altitudes by poleward and equatorward movement out of the cusp region and the gradual subsidence of the neutral gas. Density enhancements of the magnitude observed by the CHAMP satellite occur in the model results for sufficiently strong heating in the cusp. Neutral temperature enhancements also occur and are strongest near the poleward and equatorward boundaries of the cusp region. [Copyright &y& Elsevier]

Abstract: The greenhouse effect, commonly associated with lower-atmospheric warming, manifests as cooling in the middle and upper atmosphere. Carbon dioxide is the main cooler and its continuing rise has been demonstrated to result in dramatic temperature reductions, particularly in the thermosphere. In a hydrostatic atmosphere, the cooling is associated with a density decrease at a given height. The stratospheric ozone depletion documented in satellite observations since 1979 and a steady increase of water vapor are also expected to introduce a net cooling in the middle atmosphere primarily via a reduced solar heating and increased emissions in the infrared, respectively. These effects are simulated with the global spectral mesosphere/lower thermosphere model (SMLTM) extending approximately from the tropopause to over 200km. Climatological distributions of the radiatively active gases are prescribed in the model, which makes it suitable for studies with imposed realistic trends in , , and approximately corresponding to the period 1980–2000. Although confined to the stratosphere, the ozone depletion has a profound cooling effect on mesospheric temperatures, which is comparable to or exceeding that of the forcing. The water vapor cooling appears to play a secondary but non-negligible role, especially in the overall density reduction in the lower thermosphere. The additional hydrostatic contraction of the colder middle atmosphere is predicted to result in a local maximum of the density decline near 110km of up to per decade over the twenty-year period. [Copyright &y& Elsevier]

Abstract: The forthcoming observations by Venus Express provide an ideal opportunity to comprehensively study the atmosphere of Venus for the first time since Pioneer Venus (1978–1992), and for the first time ever in detail at polar latitudes. This article reviews some of our current knowledge from space and ground-based observations about the upper atmosphere of Venus, such as its thermal structure, the global distribution of gases and dynamics. We discuss the processes most likely responsible for phenomena such as the cold nightside cryosphere, the cloud top superrotation and waves, and highlight outstanding scientific challenges for Venus Express measurements. In particular, we describe an experiment to measure atmospheric drag using the on-board accelerometers. [Copyright &y& Elsevier]

Abstract: In November 1994 and August 1997 the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) instrument performed a global measurement (–) of infrared limb radiance from 4 to in the altitude range 6–180km. Using a non-local thermodynamic equilibrium (non-LTE) model that couples the collisional, radiative and photochemical processes, the kinetic temperature was recovered from emission for CRISTA-1 and -2 missions by direct assimilation of the integrated limb radiances between 72 and 120km. Falling sphere and airglow validation measurements at NASA/GSFC Wallops Flight Facility (WFF) and the Mesosphere–Thermosphere Emissions for Ozone Remote Sensing (METEORS) experiment at White Sands Missile Range were coordinated with CRISTA during the field campaign. The derived temperatures show good agreement with these rocket soundings, partial climatology obtained from Rayleigh scattering observations by Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite (UARS) and NRLMSISE-00 model. [Copyright &y& Elsevier]

Abstract: Passive ground-based optical techniques have been used for over 15 years to study the effects of auroral energy deposition on the composition of the lower thermosphere. These techniques typically involve measuring bright auroral emissions that depend on the number density of atomic oxygen [O] and molecular nitrogen [N2] as well as the energy flux and energy spectrum of the precipitating auroral particles. Here we review the application of these techniques to the composition problem. In particular, we present data obtained from photometric observations in Alaska and Greenland in the 1980s and 1990s. While some of these results are available in the existing literature, some are new and have not been presented before. Recent results from Alaska obtained in support of the TIMED satellite are also presented. The data as a whole suggest that the ratio of column integrated [O] to [N2] above a given site decreases as the total energy fluence from both auroral particle and Joule heating increases. However, the dependence is complicated by effects such as the prior history of disturbance and by horizontal transport due to neutral winds; it is not yet determined if composition responds linearly to energy fluence even in an average sense. [Copyright &y& Elsevier]

Abstract: The solar wind–magnetosphere–ionosphere–thermosphere system has two ways to transfer the force that the solar wind exerts at the magnetopause to the Earth: (1) a dipole interaction mode and (2) a thermospheric drag mode. This paper uses MHD simulations to discuss both modes comparatively. The dipole interaction mode is relatively well known and so is used as a model against which to contrast the less well known thermospheric drag mode. The thermospheric drag mode itself consists of two distinct mechanisms, a direct mechanism in which the solar wind pulls on the thermosphere and (here discussed for the first time) an indirect mechanism involving an interaction between the region 1 current system and the geomagnetic dipole which increases the drag on the thermosphere nearly an order of magnitude over the direct drag mechanism. This additional force appears to be necessary to account for observed high-latitude thermospheric winds that correlate with IMF Bz. [Copyright &y& Elsevier]

Abstract: For the first time, using many years of measurements of lower-thermospheric parameters (the horizontal wind velocity for 1976–1995, and minimum reflection frequency for 1983–1991) over Irkutsk (52°N, 105°E), the dynamics of their large-scale variations was investigated. The study used a sliding multiple correlation periodogram analysis. Regular seasonal, semi-annual and annual variations were detected. Quasi-biennial variations are regarded as irregular variations with a constantly changing over a rather wide range period. Variations with similar periods were also detected in the planetary index of geomagnetic activity A p, and in the solar radio flux index F 10.7. Our correlation analysis indicates that large-scale variations of the wind velocity and f min in the lower thermosphere are correlated with those in the geomagnetic field. The large-scale variations under investigation characterize a regular long-term variability of lower-thermospheric parameters and can serve as climatic characteristics for the description of the climate of this atmospheric region. [Copyright &y& Elsevier]

Abstract: Winds from a meteor radar at Wuhan (30.6°N, 114.5°E) and a MF radar at Adelaide (35°S, 138°E) are used to study the 16-day waves in the mesosphere and lower thermosphere (MLT). The height range is 78–98km at Wuhan and 70–98km at Adelaide. By comparison, it is found that the zonal components at both sites are generally larger than the meridional ones, and eastward motion of the zonal background winds is favorable for the 16-day waves penetration to the MLT region. The zonal maximum amplitude appears in the autumn (September–October) around 86–98km at Wuhan and in the winter months and early spring (July–October) around 72–82km at Adelaide. Differences are found in wave amplitudes and time of appearance between the two years of 2002 and 2003. In 2003, the intensity of the wave amplitudes is relatively smaller than that for 2002 at both sites. The summer 16-day waves are comparatively weaker at Adelaide in both years, but stronger in 2002 at Wuhan near the mesopause and the lower thermosphere (86–98km). The strong summer waves at Wuhan may come from the winter southern hemisphere. [Copyright &y& Elsevier]

Abstract: TREND2004, the third IAGA/ICMA Workshop Long Term Changes and Trends in the Atmosphere, presented a lot of new information and in some way summarized progress over the last few years in the field of long-term trends in the atmosphere–ionosphere system with emphasis on the middle atmosphere and ionosphere. The paper briefly summarizes the results of the Workshop with focus on the mesosphere, ionosphere and thermosphere and presents some new findings. Progress in recent years was substantial in trends in thermospheric density, mesospheric and mesopause region temperature, and in the lower ionosphere (ionized component). On the other hand, principal contradictions remain in trends in the F2 region of the ionosphere, particularly in interpretation of the observed trends. [Copyright &y& Elsevier]

Abstract: The equatorial ionosphere and thermosphere constitute a coupled system, with its electro dynamical and plasma physical processes being responsible for a variety of ionospheric phenomena peculiar to the equatorial region. The most important of these phenomena are: the equatorial electrojet (EEJ) current system and its instabilities, the equatorial ionization anomaly (EIA), and the plasma instabilities/irregularities of the night ionosphere (associated with the plasma bubble events – ESF). They constitute the major topics of investigations having both scientific and practical objectives. The tidal wind interaction with the geomagnetic field is responsible for the atmospheric dynamo electric fields, that together with the wind system, drives the major phenomena, under quiet conditions. Drastic modifications of these phenomena can occur due to magnetospheric forcing under solar-, interplanetary- and magnetospheric disturbances. They can also undergo significant modifications due to forcing by atmospheric waves (such as planetary- and atmospheric gravity waves) propagating upward or from extra tropics. This article will focus on the ambient conditions of the ionosphere–thermosphere system and the electro dynamics and plasma instability processes that govern the plasma irregularity generation. Major emphasis is given to problems related to the structuring of the equatorial night ionosphere through plasma bubble/ESF irregularity processes. Specific topics to be covered will include: equatorial electric fields, thermospheric winds, sunset electrodynamic processes, plasma drifts, EEJ plasma instability/irregularity generation, nighttime/post sunset plasma bubble irregularity generation, and very briefly, disturbance electric fields and winds and their effect on the ionization anomaly, the TEC and ESF/plasma bubble irregularities. [Copyright &y& Elsevier]

Abstract: The thermosphere–ionosphere nested grid (TING) model has been successfully coupled with the Lyon–Fedder–Mobarry (LFM) global magnetosphere MHD code. The coupling between these models is two-way: the LFM provides the TING model with global electric fields and precipitating electron energy fluxes, and the TING model feeds ionospheric conductances back to the LFM. This code coupling enables studies of the global energy budget of the magnetosphere–ionosphere–thermosphere system. In this paper, we present simulation results from the coupled magnetosphere–ionosphere–thermosphere (CMIT) model under solar minimum, northern hemisphere summer conditions. The IMF input to the CMIT model changed its direction every 4h. Comparisons are made between the simulated results of the CMIT model and those of the stand-alone TING model. It is found that the CMIT model predicted higher cross polar cap potential drops than the empirical model used by the stand-alone TING model. The energy input to the upper atmosphere by precipitating electrons, however, was much lower in the CMIT model during the southward IMF interval. The simulated responses of the thermosphere and ionosphere were also significantly different. As a result of the greater Joule heating calculated in the CMIT model, neutral temperatures and winds were significantly enhanced in the CMIT model in comparison with the stand-alone TING model. [Copyright &y& Elsevier]