Lower thermosphere

During recent years, special attention has been paid to the background circulation of the middle atmosphere and, including a variety of new measurements particularly using radar especially in the mesosphere/lower thermosphere (MLT) region, to the comparison of existing empirical middle atmosphere wind models as CIRA-86 and HWM with new data. This leads to the construction of empirical models of MLT winds as the Global Empirical Wind Model (GEWM). Further investigations aim at the construction of new empirical and semi-empirical wind models of the whole middle atmosphere including the new experimental results. Results of a new wind climatology (0-100 km) are presented based upon the GEWM, stratospheric reanalysis data, and a numerical model to fill the gap between stratospheric and MLT data. Recently direct wind observations from the wind imaging interferometer (WINDII, Wang et al., 1997) and the high-resolution Doppler imager (HRDI, Fleming et al., 1996) on board the Upper Atmosphere Research Satellite (UARS) have provided a new global wind data set for the upper mesosphere/lower thermosphere region. From these data, also a vertical wind model (Fauliot et al., 1997) has been constructed. Fleming et al. (1996) and Portnyagin et al. (1999) concluded that, in general, the space-based zonal wind models exhibited significant differences relative to the ground-based models. In contrast, however, Fauliot et al. (1997) have stated that the WINDII-based prevailing meridional model winds have cellular structure, similar to those from the ground-based Portnyagin et al.

In order to enhance our understanding of the possible influence of meteor ablation on the enrichment in OH and O 2 of the lower thermosphere we studied intense Leonid meteor activity by using the SATI (Spectral Airglow Temperature Imager) interferometer of the Instituto de Astrofıśica de Andalucıá. We measured the emission rate and rotational temperature of OH and O 2 airglow emission layers during two observation periods of high meteoric activity: the 1998 Leonid outburst and the 2002 Leonid storm. The results show that there is not a clear relation of O 2 and OH airglow emission and rotational temperature with meteoric activity.

Reviewer: The paper presents a topic of sure interest that can stimulate the curiosity of a number of scientists because it poses questions still unsolved and because the analysis is based on measurements taken in a region scarcely reported in the open literature. This is the reason why I would be in favour of the paper publication.

The AURA-MLS daily mean temperatures and zonal wind from NASA-MERRA reanalysis for latitudes between 60 • N and 80 • N are used to investigate the planetary wave (PW) characteristics in the stratosphere and lower mesosphere during sudden stratospheric warming (SSW) (November 2008 to March 2009). Here, we used a novel method called empirical mode decomposition (EMD) to extract the PWs from the temperature data. The EMD is an interesting approach to decompose signals into locally periodic components, the intrinsic mode functions (IMFs), and will easily identify the embedded structures, even those with small amplitudes. The spectral analysis reveals prevailing planetary wave periods of ∼6-day, ∼8-day, ∼15-day, and ∼21-23-day in IMFs 1, 2, 3, and 4, respectively. Clear upward propagation of these waves (20-30 days) is observed, suggesting that sources for these oscillations are in the troposphere.

A number of studies have shown that 5-day planetary waves modulate noctilucent clouds and the closely related Polar Mesosphere Summer Echoes (PMSE) at the summer mesopause. Summer stratospheric winds should inhibit wave propagation through the stratosphere and, although some numerical models (Geisler and Dickinson, 1976) do show a possibility for upward wave propagation, it has also been suggested that the upward propagation may in practice be confined to the winter hemisphere with horizontal propagation of the wave from the winter to the summer hemisphere at mesosphere heights causing the effects observed at the summer mesopause. It has further been proposed (Garcia et al., 2005) that 5-day planetary waves observed in the summer mesosphere could be excited in-situ by baroclinic instability in the upper mesosphere. In this study, we first extract and analyze 5-day planetary wave characteristics on a global scale in the middle atmosphere (up to 54 km in temperature, and up to 68 km in ozone concentration) using measurements by the Odin satellite for selected days during northern hemisphere summer from 2003, 2004, 2005 and 2007. Second, we show that 5-day temperature fluctuations consistent with westward-traveling 5-day waves are present at the summer mesopause, using local ground-based meteor-radar observations. Finally we examine whether any of three possible sources of the detected temperature fluctuations at the summer mesopause can be excluded: upward propagation from the stratosphere in the summer-hemisphere, horizontal propagation from the winter-hemisphere or in-situ excitation as a result of the baroclinic instability. We find that in one case, far from solstice, the baroclinic instability is unlikely to be involved.

During recent years, special attention has been paid to the background circulation of the middle atmosphere and, including a variety of new measurements particularly using radar especially in the mesosphere/lower thermosphere (MLT) region, to the comparison of existing empirical middle atmosphere wind models as CIRA-86 and HWM with new data. This leads to the construction of empirical models of MLT winds as the Global Empirical Wind Model (GEWM). Further investigations aim at the construction of new empirical and semi-empirical wind models of the whole middle atmosphere including the new experimental results. Results of a new wind climatology (0-100 km) are presented based upon the GEWM, stratospheric reanalysis data, and a numerical model to fill the gap between stratospheric and MLT data. Recently direct wind observations from the wind imaging interferometer (WINDII, Wang et al., 1997) and the high-resolution Doppler imager (HRDI, Fleming et al., 1996) on board the Upper Atmosphere Research Satellite (UARS) have provided a new global wind data set for the upper mesosphere/lower thermosphere region. From these data, also a vertical wind model (Fauliot et al., 1997) has been constructed. Fleming et al. (1996) and Portnyagin et al. (1999) concluded that, in general, the space-based zonal wind models exhibited significant differences relative to the ground-based models. In contrast, however, Fauliot et al. (1997) have stated that the WINDII-based prevailing meridional model winds have cellular structure, similar to those from the ground-based Portnyagin et al.

The vertical fine structures and the time evolution of plasma irregularities in the sporadic E (E s) layer were observed via calcium ion (Ca +) density measurements using a resonance scattering lidar with a high time-height resolution (5 s and 15 m) at Tachikawa (35.7°N, 139.4°E) on December 24, 2014. The observation successfully provided clearer fine structures of plasma irregularities, such as quasi-sinusoidal height variation, localized clumps, "cats-eye" structures, and twist structures, in the sporadic Ca + (Ca + s) layers at around 100 km altitude. These fine structures suggested that the Kelvin-Helmholtz instabilities occurred in the neutral atmosphere whose density changed temporarily or spatially. The maximum Ca + density in the Ca + s layer was two orders of magnitude smaller than the maximum electron density estimated from the critical frequency (f o E s) simultaneously observed by the ionosonde at Kokubunji (35.7°N, 139.5°E). A strong positive correlation with a coefficient of 0.91 suggests that Ca + contributes forming the E s layer as well as major metallic ions Fe + and Mg + in the lower thermosphere. Moreover, the formation of a new Ca + s layer at 110 km and the upward motions of the Ca + s layers at 100 km and 110 km were observed before the local sunrise and just after the sunrise time at the conjugation point. Although the presence or absence of a causal relationship with the sunrise time was not clear, a possible explanation for the formation and the upward motions of the Ca + s layers was the occurrence of strong horizontal wind, rather than the enhancement of the eastward electric field.

Tristatic Fabry-Perot Interferometer (FPI) measurements of the upper thermosphere were co-located with tristatic EISCAT radar measurements of the ionosphere. Tristatic measurements should remove assumptions of uniform wind fields and ion drifts, and zero vertical winds. The FPIs are located close to the 3 radars of the EISCAT configuration in northern Scandinavia. Initial studies indicate that the thermosphere is more dynamic and responsive to ionospheric forcing than expected. Mesoscale variations are observed on the scales of tens of kilometers and minutes. The magnitude of the upper thermosphere neutral wind dynamo field is on average 50% of the magnetospheric electric field and contributes an average magnitude of 41% of in-situ Joule heating. The relative orientations of the 2 dynamo field vectors produce a standard deviation of ±65% in the contribution of the neutral wind dynamo.

First results are presented from a Scanning Doppler Imager (SCANDI) installed at the Nordlysstasjonen optical observatory near Longyearbyen, Svalbard (78.2 • N, 15.8 • E). Observations of the atomic oxygen 630 nm red line emission, originating in the upper thermosphere at around 250 km, have been used to determine neutral winds and temperatures from multiple zones within an extended spatial field. The instrument utilises all-sky optics to achieve multiple simultaneous measurements, compared to the standard Fabry-Perot Interferometer (FPI) procedure of separate lineof-sight samples within a sequence of narrow angle look directions. SCANDI is colocated with such a standard FPI and comparison of neutral wind velocities between the instruments on the night of 15 March 2007 has revealed detailed and consistent structure in the wind field. Southward meridional wind enhancements of several hundred m/s are observed simultaneously with both instruments, revealing structure on scales not currently considered in thermospheric general circulation models (GCMs). The data from this night also demonstrate the influence of discrete auroral events on thermospheric behaviour. High intensities observed by SCANDI in the presence of auroral arcs coincide with a drop in measured neutral temperatures. This is interpreted as a result of the effective altitude of the 630 nm emission being lowered under conditions of soft auroral precipitation. The optical instruments as a consequence sample a region of lower temperature. This effect has been observed previously with lower thermospheric atomic oxygen emissions at 557.7 nm. The EISCAT Svalbard Radar (ESR) provides ion temperatures and electron densities for the night which confirm the influence of precipitation and heating during the auroral events. The minima of ion temperatures through the pre-midnight period provide a good match to the neutral tem

The response of thermospheric neutral parameters such as winds and temperatures to rapid changes in geophysical conditions has usually been considered to be relatively slow, on the order of hours, and steady, representing an integration of more rapid ionospheric changes. Quantifying the relevant ion-neutral coupling has proved difficult due to a lack of relevant laboratory data for the most important collisions, namely between neutral atomic oxygen and its first ion. As a result the representation of ion-neutral coupling in numerical models of the upper atmosphere has often produced poor comparison to experimental data. Using a unique combination of spatially extended ion and neutral thermospheric parameters we show that the neutral response can be very rapid, within 15 min, to imposed forcing via ion-neutral coupling. The array of complementary instrumentation measuring the thermosphere above Svalbard in the Northern Hemisphere allows detailed study of the causes and effects from both the ion and neutral perspectives. The implications for development and testing of the thermospheric numerical models is discussed.

It is generally assumed that horizontal wind velocities are independent of height above the F 1 region (> 300 km) due to the large molecular viscosity of the upper thermosphere. This assumption is used to compare two completely different methods of thermospheric neutral wind observation, using two distinct locations in the high-latitude Northern Hemisphere. The measurements are from groundbased Fabry-Perot interferometers (FPI) and from in situ accelerometer measurements onboard the challenging minisatellite payload (CHAMP) satellite, which was in a nearpolar orbit. The University College London (UCL) Kiruna Esrange Optical Platform Site (KEOPS) FPI is located in the vicinity of the auroral oval at the ESRANGE site near Kiruna, Sweden (67.8 • N, 20.4 • E). The UCL Longyearbyen FPI is a polar cap site, located at the Kjell Henriksen Observatory on Svalbard (78.1 • N, 16.0 • E). The comparison is carried out in a statistical sense, comparing a longer time series obtained during night-time hours in the winter months (DOY 300-65) with overflights of the CHAMP satellite between 2001 and 2007 over the observational sites, within ±2 • latitude (±230 km horizontal range). The FPI is assumed to measure the line-of-sight winds at a height of ∼ 240 km, i.e. the peak emission height of the atomic oxygen 630.0 nm emission. The cross-track winds are derived from state-of-the-art precision accelerometer measurements at altitudes between ∼ 450 km (in 2001) and ∼ 350 km (in 2007), i.e. 100-200 km above the FPI wind observations. We show that CHAMP wind values at high latitudes are typically 1.5 to 2 times larger than FPI winds. In addition to testing the consistency of the different measurement approaches, the study aims to clarify the effects of viscosity on the height dependence of thermospheric winds.

It is generally assumed that horizontal wind velocities are independent of height above the F 1-region (> 300 km) due to the large viscosity of the upper thermosphere. This assumption is used to compare two completely different methods of thermospheric neutral wind observation, using two distinct locations in the high-latitude Northern Hemisphere. The measurements are from ground-based Fabry-Perot Interferometers (FPI), and from in-situ accelerometer measurements onboard the CHAMP satellite, which was in a near polar orbit. The UCL KEOPS FPI is located in the vicinity of the auroral oval at the ESRANGE site near Kiruna, Sweden (67.8°N, 20.4°E). The UCL Longyearbyen FPI is a polar cap site, located at the Kjell Henriksen Observatory on Svalbard (78.1°N, 16.0°E). The comparison is done in a statistical sense, comparing a longer time series obtained during nighttime hours in the winter months (November to January); with overflights of the CHAMP satellite between 2001 and 2007 over the observational sites, within ±2° (±220 km horizontal range). The FPI is assumed to measure the line-of-sight winds at ~240 km height, i.e. the peak emission height of the atomic oxygen 630.0 nm emission. The cross-track winds are derived from state-of-the-art precision accelerometer measurements at altitudes between 450 km (in 2001) to 330 km (in 2007); i.e. 100-200 km above the FPI wind observations. We show that CHAMP winds at high latitudes are systematically 1.5-2 times larger than FPI winds. In addition to testing the consistency of the different measurement approaches, the study aims to clarify the effects of viscosity on the height dependence of thermospheric winds. 1 Introduction Global circulation models (GCM) of the upper atmosphere (80-600 km altitude) appear in two forms: climatologies based on empirical measurements, and theoretical models that calculate the atmospheric conditions using the principles of physics and chemistry. These models are important for space weather studies and are also applied in understanding and predicting drag on low-altitude satellites, space debris, and the study of re-entry of near Earth objects. The theoretical and empirical models rely on observations from ground-based instruments around the world and global observations by satellites to provide constraints and boundary conditions. In particular, models must account for energy from sources external to the upper atmosphere (i.e., direct solar radiation, particle precipitation and heat flow from above; radiative, conductive and convective

We present a climatology of quiet time thermospheric winds and temperatures estimated from high-resolution Fabry-Perot interferometer measurements of the 630.0 nm airglow emission spectral line shape. Three locations are examined in this long-term study:

The ionosphere responds almost immediately to magneto- spheric forcing, however, the neutral atmosphere has inertia owing to its far greater mass density. As a consequence the neutral atmosphere can significantly modify the ionosphere- magnetosphere coupling processes such as the energy trans- ferred between them. Thermospheric neutral temperatures measured by Fabry-Perot Interferometers (FPI) from airglow are presented from both auroral and polar cap sites. These are compared to both Incoherent Scatter Radar (ISR) derived values and modelled temperatures from MSIS. The large dif- ferences evident in the comparison to model results are dis- cussed and the reaction of the measured values to geomag- netic conditions are demonstrated from both sites. We also demonstrate the importance of accurate neutral temperatures in monitoring the dissipation of energy and in the derivation of ionospheric parameters.

Gravity waves are an important feature of mesosphere -lower thermosphere (MLT) dynamics, observed using many techniques and providing an important mechanism for energy transfer between atmospheric regions. It is known that some gravity waves may propagate through the mesopause and reach greater altitudes before eventually "breaking" and depositing energy. The generation, propagation, and breaking of upper thermospheric gravity waves have not been studied directly often. However, their ionospheric counterparts, travelling ionospheric disturbances (TIDs), have been extensively studied in, for example, radar data. At high latitudes, it is believed localised auroral activity may generate gravity waves in-situ. Increases in sensor efficiency of Fabry-Perot Interferometers (FPIs) located in northern Scandinavia have provided higher time resolution measurements of the auroral oval and polar cap atomic oxygen red line emission at 630.0 nm. A Lomb-Scargle analysis of this data has shown evidence of gravity wave activity with periods ranging from a few tens of minutes to several hours. Oscillations are seen in the intensity of the line as well as the temperatures and line of sight winds. Instruments are located in Sodankylä, Finland; Kiruna, Sweden; Skibotn, Norway, and Svalbard in the Arctic Ocean. A case study is presented here, where a wave of 1.8 h period has a phase speed of 250 ms −1 with a propagation angle of 302 • , and a horizontal wavelength of 1600 km. All the FPIs are colocated with EISCAT radars, as well as being supplemented by a range of other instrumentation. This allows the waves found in the FPI data to be put in context with the ionosphere and atmosphere system. Consequently, the source region of the gravity waves can be determined.

The Japan Aerospace Exploration Agency (JAXA) DELTA rocket experiment, successfully launched from Andøya at 0033 UT on December 13, 2004, supported by ground based optical instruments, primarily 2 Fabry-Perot Interferometers (FPIs) located at Skibotn, Norway (69.3 • N, 20.4 • E) and the KEOPS Site, Esrange, Kiruna, Sweden (67.8 • N, 20.4 • E). Both these instruments sampled the 557.7 nm lower thermosphere atomic oxygen emission and provided neutral temperatures and line-of-sight wind velocities, with deduced vector wind patterns over each site. All sky cameras allow contextual auroral information to be acquired. The proximity of the sites provided overlapping fields of view, adjacent to the trajectory of the DELTA rocket. This allowed independent verification of the absolute temperatures in the relatively quiet conditions early in the night, especially important given the context provided by co-located EISCAT ion temperature measurements which allow investigation of the likely emission altitude of the passive FPI measurements. The results demonstrate that this altitude changes from 120 km pre-midnight to 115 km post-midnight. Within this large scale context the results from the FPIs also demonstrate smaller scale structure in neutral temperatures, winds and intensities consistent with localised heating. These results present a challenge to the representation of thermospheric variability for the existing models of the region.