Thermospheric General Circulation Model Research Articles

A study into the effect of the diurnal tide on the structure of the background mesosphere and thermosphere using the new coupled middle atmosphere and thermosphere (CMAT) general circulation model

Abstract. A new coupled middle atmosphere and thermosphere general circulation model has been developed, and some first results are presented. An investigation into the effects of the diurnal tide upon the mean composition, dynamics and energetics was carried out for equinox conditions. Previous studies have shown that tides deplete mean atomic oxygen in the upper mesosphere-lower thermosphere due to an increased recombination in the tidal displaced air parcels. The model runs presented suggest that the mean residual circulation associated with the tidal dissipation also plays an important role. Stronger lower boundary tidal forcing was seen to increase the equatorial local diurnal maximum of atomic oxygen and the associated 0(1S) 557.7 nm green line volume emission rates. The changes in the mean background temperature structure were found to correspond to changes in the mean circulation and exothermic chemical heating.Key words. Atmospheric composition and structure (middle atmosphere – composition and chemistry) Meterology and atmospheric dynamics (middle atmosphere dynamics; waves and tides)

Mars-GRAM 2000: A Mars atmospheric model for engineering applications

This paper presents Mars Global Reference Atmospheric Model 2000 Version (Mars-GRAM 2000) and its new features. All parameterizations for temperature, pressure, density, and winds versus height, latitude, longitude, time of day, and Ls have been replaced by input data tables from NASA Ames Mars General Circulation Model, for the surface through 80-km altitude, and the University of Arizona Mars Thermospheric General Circulation Model for 80 to 170 km. A modified Stewart thermospheric model is still used for higher altitudes and for dependence on solar activity. “Climate factors” to tune for agreement with general circulation model data are no longer needed. Adjustment of exospheric temperature is still an option. Consistent with observations from Mars Global Surveyor, a new longitude-dependent wave model is included, with user input to specify waves having one to three wavelengths around the planet. A simplified perturbation model has been substituted for the earlier one. An input switch allows users to select either East or West longitude positive. The paper includes instructions on obtaining Mars-GRAM source code, data files, and a users guide for running the program. The Mars-GRAM users guide provides sample input and output and gives an example for incorporating Mars-GRAM as an atmospheric subroutine in a trajectory code.

The global-scale effect of small-scale thermospheric disturbances

Thermospheric behavior at high latitudes is well described at large spatial and long temporal scales by current thermospheric general circulation models. On the other hand, many small-scale horizontal and vertical motions observed from space and the ground do not appear in simulations by these models or in versions with a nested grid. This paper reviews small-scale motions which include strong vertical components and small-scale structures in the horizontal wind field. Synthesizing from available data sources, some general patterns of vertical wind disturbance are derived. These suggest that the variability often seen in observations of neutral wind and temperature at high latitudes can frequently be due to waves downstream of strong vertical disturbances. It is also concluded that since the general circulation models do not include the small-scale activity and its expected upward transport of molecular species, they must be underestimating the consequent ionospheric and radiant heat losses at F-region altitudes.

Comparative terrestrial planet thermospheres: 3. Solar cycle variation of global structure and winds at solstices

The comparison of planetary upper atmospheres using global databases has entered a new era with the advent of recent aerobraking measurements of the Mars thermosphere [e.g., Keating, et al., 1998a]. The present maturity of available modeling capabilities also permits us to contrast the Earth and Mars thermosphere structures, winds, and controlling processes using global three‐dimensional models [e.g., Bougher et al., 1999b]. This present effort focuses upon the comparison of the combined seasonal‐solar cycle responses of the thermospheres of Earth and Mars using the National Center for Atmospheric Research (NCAR) Thermospheric General Circulation Model (TGCM) utility to address the coupled energetics, dynamics, and neutral‐ion composition above ∼100 km. Extreme thermospheric conditions are expected at solstices, thereby revealing the changing importance of fundamental physical processes controlling the Earth and Mars thermospheric structures and winds. Seasonal‐solar cycle extremes in Mars exobase temperatures are calculated to range from 200 to 380 K, giving rise to maximum horizontal winds of nearly 215 to 400 m/s. Corresponding extremes in Earth exobase temperatures are 700 to 1600 K, with rather small variations in global winds. The orbital eccentricities of Earth and Mars are also shown to drive substantial variations in their thermospheric temperatures. For Mars, dayside exobase temperatures vary by ∼60 K (18%) from aphelion to perihelion during solar maximum conditions. Such large temperature variations strongly impact thermospheric densities and global winds. The corresponding Earth dayside temperatures also vary by 60–80 K between solstices. However, the percent temperature variation (5%) over the Earth's orbit and its overall impact on the thermospheric structure and winds are much smaller. Auroral activity may in fact obscure these orbital variations. Changing dust conditions throughout the Martian year modulate the aerosol heating of its lower atmosphere, yielding considerable variability in the height of the subsolar ionospheric peak about its observed seasonal trend (∼115–130 km). Significant further progress in the comparison of Earth and Mars thermospheric features and underlying processes must await expanded Mars global databases expected from Planet‐B and Mars Express (2004–2005).

Velocity distributions of C atoms in CO+ dissociative recombination: Implications for photochemical escape of C from Mars

We have carried out Monte Carlo calculations to determine the velocity distributions of C atoms produced by dissociative recombination of CO+ using recent data for the branching ratios of various allowed channels and ion and electron temperatures appropriate to the Martian thermosphere. We find that the fractions of 12C atoms with velocities greater than the escape velocity are ∼ 0.66 and ∼ 0.62, and those for 13C are ∼ 0.47 and ∼ 0.48 at the plasma temperatures characteristic of the exobases at low and high solar activities, respectively. The ratio of the escape fractions of 13C and 12C, which is a measure of the isotope effect inherent in the CO+ dissociative recombination mechanism, is thus in the range 0.72–0.77. Using model thermospheres and ionospheres for low solar activity from Viking, and for high solar activity from the Mars Thermospheric General Circulation Model, we have computed the global escape flux of C due to dissociative recombination of CO+ as (3 − 5) × 105 cm−2 s−1. This value is of the same order as current estimates for the escape flux of C in all forms due to sputtering by O+ pickup ions at the current epoch but may be much less than that due to sputtering at earlier times in the history of the planet.

The Mars thermosphere-ionosphere: Predictions for the arrival of Planet-B

The primary science objective of the Planet-B mission to Mars is to study the Martian upper atmosphere-ionosphere system and its interaction with the solar wind. An improved knowledge of the Martian magnetic field (whether it is induced or intrinsic) is needed, and will be provided by Planet-B. In addition, a proper characterization of the neutral thermosphere structure is essential to place the various plasma observations in context. The Neutral Mass Spectrometer (NMS) onboard Planet-B will provide the required neutral density information over the altitude range of 150–500 km. Much can be learned in advance of Planet-B data taking as multi-dimensional thermosphere-ionosphere and MHD models are exercised to predict the Mars near-space environment that might be expected during the solar maximum conditions of Cycle 23 (1999–2001). Global model simulations of the Mars thermosphere-ionosphere system are presented and analyzed in this paper. These Mars predictions pertain to the time of Planet-B arrival in October 1999 (F10.7∼200; Ls∼220). In particular, the National Center for Atmospheric Research (NCAR) Mars Thermosphere General Circulation Model (MTGCM) is exercised to calculate thermospheric neutral densities (CO2, CO, N2, O, Ar, O2), photochemical ions (CO+2, O+2, O+ below 200 km), neutral temperatures, and 3-components winds over 70–300 km. Cases are run with and without dust loading of the lower atmosphere in order to examine the potential impacts of dust storms on the thermosphere-ionosphere structure. Significant dust-driven impacts are predicted in the lower thermosphere (100–120 km), but are less pronounced above 150 km. The ionospheric peak height changes greatly with the passage of a Mars global dust storm event. In addition, Martian dayside exobase temperatures are generally warmer during dusty periods, in accord with Mariner 9 UVS data (Stewart et al., 1972). During the Planet-B mission, the NMS team intends to use the MTGCM as a facility tool whose simulated output can be utilized to aid various investigations.

P.5.003 Pilot study of D2 dopamine receptor, ALDH2, ADH2 and ADH3 gene allelic association in polynesian alcohol and cannabis users

The upper mesosphere and the lower thermosphere of Venus (from 90 to 150 km altitude) seems to play a transition region in photochemistry, dynamics and radiation, but is still very poorly constrained observationally. Since 2006 VIRTIS on board Venus Express has been obtaining limb observations of CO fluorescent infrared emissions in a systematic manner. This study represents the scientific exploitation of this dataset and reports new information on the composition and temperature at those altitudes. This work is focused on the 4.7 μm emission of CO as observed by VIRTIS, which contains two emission bands, the fundamental and the first hot of the main CO isotope. A specific scheme for a simultaneous retrieval of CO and temperature is proposed, based on results of a comprehensive non-LTE model of these molecular emissions. A forward model containing such non-LTE model is used at the core of an inversion scheme that consists of two steps: (i) a minimization procedure of model-data differences and (ii) a linear inversion around the solution of the first step. A thorough error analysis is presented, which shows that the retrievals of CO and temperature are very noisy but can be improved by suitable averaging of data. These averages need to be consistent with the non-LTE nature of the emissions. Unfortunately, the data binning process reduced the geographical coverage of the results. The obtained retrieval results indicate a global distribution of the CO in the Venus dayside with a maximum around the sub-solar point, and a decrease of a factor 2 towards high latitudes. Also a gradient from noon to the morning and evening sides is evident in the equator, this being smaller at high latitudes. No morning–afternoon differences in the CO concentration are observed, or are comparable to our retrieval errors. All this argues for a CO distribution controlled by dynamics in the lower thermosphere, with a dominant sub-solar to anti-solar gradient. Similar variations are found with the Venus Thermospheric General Circulation Model (VTGCM), but the VIRTIS CO is systematically larger than in the model. The thermal structure obtained by VIRTIS presents a hint of local maximum around 115 km near the terminator at equatorial latitudes, but not at noon, in clear contrast to VTGCM predictions and to an upper mesosphere in pure radiative balance. A few tentative ideas to explain these model-data discrepancies are discussed.

A meridional wind climatology from a fast model for the derivation of meridional winds from the height of the ionospheric F2 region

Results of a semi-empirical model of neutral wind speeds in the upper thermosphere are presented. The derived wind speeds will be useful in studies of ionospheric behavior and for validating thermospheric general circulation models. The Meridional Wind Model is based on a refinement of the technique of Miller et al. (1986, 1993) who used the height of the peak electron density in the ionosphere (hmF2) and a comprehensive ionospheric model to deduce thermospheric winds. Computation requirements were drastically reduced by parameterizing the essential aspects of the ionospheric model while preserving the accuracy of the original method. The model can accept hmF2 directly or calculate it from fundamental scaled ionosonde parameters to produce winds locally. It also includes the capability to produce winds on a global scale by using the International Reference Ionosphere (IRI) model. Since there is no provision for electric fields, the model is accurate only at mid-latitudes. The Meridional Wind Model is used to examine the long-term averages of meridional winds at different levels of solar activity. The winds are derived from F2-layer heights from an ionosonde data base that extends from 1958 to 1990. Results show smaller night-time winds when the solar flux is greatest. Daytime wind speeds increase with latitude and vary by season, but are not as strongly dependent on solar flux. Winds from the ionosonde data base and derived using the IRI are in general agreement with the winds from the Horizontal Wind Model (HWM90). There are, however, significant differences, especially at higher latitudes.

Winds and composition changes in the thermosphere using the transfer function model

Abstract Different theoretical formulations are used to describe the thermosphere, ranging from fully analytical, linear models based, in the case of gravity waves, on the work of C.O. Hines, to fully numerical, thermospheric general circulation models (TGCMs), which account for non-linear processes and coupling with the ionospheric plasma. The semi-analytical Transfer Function Model (TFM) describes perturbations in the wind field, temperature and composition, accounting in self-consistent form for the momentum transfer between species. The perturbations are driven by external energy and momentum sources associated with absorbed solar radiation and magnetospheric processes. Compared with TGCMs, the TFM is simplified mainly by its linear approximation. But the model is not limited in its spatial and temporal resolutions, and it describes acoustic gravity waves that are generated in the thermosphere and partially propagate through the lower atmosphere. Moreover, the model is semi-analytical, which helps in delineating and understanding the dynamical characteristics. Using expansions in terms of vector spherical harmonics and Fourier transformation, the transfer function of the atmosphere is obtained through numerical height integration, which is time consuming computationally but needs to be done only once. Once the transfer function is constructed, the thermospheric response is obtained virtually instantaneously, thus providing an efficient tool for numerical simulation and experimentation. After a review of the TFM, results are discussed, which describe a number of thermospheric phenomena covering a wide range of spatial and temporal scales. These include magnetic storm effects as well as gravity waves propagating in the thermosphere, which are partially reflected from the Earth's surface and ducted through the lower atmosphere.

Dust storm impacts on the Mars upper atmosphere

Coupling between the Mars lower (≤80 km) and upper atmospheres is a fundamental issue affecting dynamics, tracer distributions, and energy balance. Also, this coupling has significant bearing on future mission planning to the planet ( e.g. Mars Global Surveyor aerobraking). Two general circulation models are currently being run in tandem to investigate tides and their generation, propagation, and impact upon the Mars lower thermosphere. The NASA Ames Mars General Circulation Model (MGCM) is first run for Mars near-perihelion (Ls ∼ 270) conditions for which a 20-SOL dust-storm is initiated and allowed to self-consistently evolve. Subsequently, selected MGCM upper boundary fields are used to specify the height of the lower boundary and to drive semi-diurnal tides in the NCAR Mars Thermospheric General Circulation Model (MTGCM) for identical Mars parameters. MTGCM key fields are presented to illustrate the profound impact of Mars lower atmosphere heating and tides upon the Mars thermospheric structure and dynamics.

The impact of gravity waves on the Venus thermosphere and O2 IR nightglow

The terrestrial Fritts and Lu [1993] gravity wave parameterization, based on the theory of wave saturation, has been adopted within the Venus thermospheric general circulation model (VTGCM) to examine the impact of upward propagating gravity waves on the Venus thermospheric subsolar‐to‐antisolar (SS‐AS) circulation and the resultant O2 IR nightglow. It is shown that the drag force generated by the breakdown of gravity waves can effectively slow down the winds, which results in reproducing the observed day/night contrasts in temperatures and densities. With stronger drag force on the dawn terminator than on the dusk terminator, the apparently superrotating zonal wind can be forced by the dissipation of gravity waves. The equivalent superrotating zonal wind varies both with local time and altitude. The simulated distribution of the O2 IR nightglow is strongly sensitive to small changes of the characteristic intrinsic phase speeds at the model lower boundary. This sensitivity implies that minor variations in gravity wave momentum fluxes to the thermosphere can give rise to large O2 IR emission variations with time and space, in agreement with ground‐based observations.

The Martian thermosphere/ionosphere at high and low solar activities

We compare here models of the thermosphere/ionosphere of Mars at low and high solar activities, and we present heating rates and efficiencies due to the absorption of solar radiation in the 18 to 2000 Å range. Using neutral model densities from the NCAR Mars Thermospheric General Circulation Model (MTGCM) of Bougher and co-workers, and solar fluxes from W. K. Tobiska, we have modeled the density profiles of 14 ions and 5 minor neutral species. We predict the variations in the ion densities with solar activity, and describe the sources and sinks of the ions. The major sources and sinks differ in some respects from those for the Venus ionosphere, and these differences are discussed as well. We find that the predicted total electron density profile computed using solar fluxes from Tobiska is somewhat different from that obtained using the fluxes of Hinteregger. One possible conclusion is that, at the time of Mariners 6 and 7, the soft x-ray fluxes were midway between those of the Tobiska and Hinteregger spectra.

E Region neutral winds in the postmidnight diffuse aurora during the atmospheric response in Aurora 1 Rocket Campaign

Measured E region neutral winds from the Atmospheric Response in Aurora (ARIA 1) rocket campaign are compared with winds predicted by a high‐resolution nonhydrostatic dynamical thermosphere model. The ARIA 1 rockets were launched into the postmidnight diffuse aurora during the recovery phase of a substorm. Simulations have shown that electrodynamical coupling between the auroral ionosphere and the thermosphere was expected to be strong during active diffuse auroral conditions (Walterscheid and Lyons, 1989). This is the first time that simulations using the time history of detailed specifications of the magnitude and latitudinal variation of the auroral forcing based on measurements have been compared to simultaneous wind measurements. Model inputs included electron densities derived from ground‐based airglow measurements, precipitating electron fluxes measured by the rocket, electron densities measured on the rocket, electric fields derived from magnetometer and satellite ion drift measurements, and large‐scale background winds from a thermospheric general circulation model. Our model predicted a strong jet of eastward winds at E region heights. A comparison between model predicted and observed winds showed modest agreement. Above 135 km the model predicted zonal winds with the correct sense, the correct profile shape, and the correct altitude of the peak wind. However, it overpredicted the magnitude of the eastward winds by more than a factor or 2. For the meridional winds the model predicted the general sense of the winds but was unable to predict the structure or strength of the winds seen in the observations. Uncertainties in the magnitude and latitudinal structure of the electric field and in the magnitude of the background winds are the most likely sources of error contributing to the differences between model and observed winds. Between 110 and 135 km the agreement between the model and observations was poor because of a large unmodeled jetlike feature in the observed winds (140 m s−1). Agreement between the present simulation and the earlier simulations of Walterscheid and Lyons (1989) is favorable, although the winds in the present simulation are considerably weaker for particle precipitation of similar characteristic energy and flux. The reasons for the difference were the smaller latitudinal extent of the model diffuse aurora and the weaker electric fields in our simulation. We have shown that the enhanced electron densities and electric fields associated with the postmidnight diffuse aurora provide the potential for a rapid acceleration of the zonal winds as shown by Walterscheid and Lyons (1989). However, the modeled response to the large‐scale electric field is too great. This suggests that the assimilated mapping of ionospheric electrodynamics (AMIE) electric field is also too large. The actual electric field is most likely reduced locally in regions of enhanced ionization and conductivity within the diffuse aurora. In addition, we have shown that the “exotic” jetlike wind feature between 110 and 135 km is not aurorally forced. However, it may be the result of an enhancement of the Hall drag relative to the Coriolis force that modifies the geostrophic balance with the large‐scale pressure gradient.

Atmospheric Response in Aurora experiment: Observations of E and F region neutral winds in a region of postmidnight diffuse aurora

The goal of the Atmospheric Response in Aurora (ARIA) experiment carried out at Poker Flat, Alaska, on March 3, 1992, was to determine the response of the neutral atmosphere to the long‐lived, large‐scale forcing that is characteristic of the diffuse aurora in the postmidnight sector. A combination of chemical release rocket wind measurements, instrumented rocket composition measurements, and ground‐based optical measurements were used to characterize the response of the neutral atmosphere. The rocket measurements were made at the end of a 90‐min period of strong Joule heating. We focus on the neutral wind measurements made with the rocket. The forcing was determined by running the assimilated mapping of ionospheric electrodynamics (AMIE) analysis procedure developed at the National Center for Atmospheric Research. The winds expected at the latitude and longitude of the experiment were calculated using the spectral thermospheric general circulation model developed at the Danish Meteorological Institute. Comparisons of the observations and the model suggest that the neutral winds responded strongly in two height ranges. An eastward wind perturbation of ∼100 m s−1 developed between 140 and 200 km altitude with a peak near 160 km. A southwestward wind with peak magnitude of ∼150 m s−1 developed near 115 km altitude. The large amplitude winds at the lower altitude are particularly surprising. They appear to be associated with the upward propagating semidiurnal tide. However, the amplitude is much larger than predicted by any of the tidal models, and the shear found just below the peak in the winds was nominally unstable with a Richardson number of ∼0.08.

Thermosperic vertical winds above Mawson, Antarctica

High-resolution ( R ≅ 200,000) night-time Fabry-Perot spectra of the λ630 run OI thermospheric airglow emission obtained from Mawson, Antarctica, during the austral winter of 1992 have been used to provide estimates of thermospheric temperature and neutral wind. In this paper, we report on characteristics of the resulting vertical wind estimates. The final data set comprised 103 observing nights between 18 March and 20 September for a total of 2096 useable zenith spectra. Because the instrumental reference rest wavelength of the 7630 nm emission is not known, this experiment was only capable of measuring vertical wind variations relative to an arbitrarily-chosen reference velocity. For this work, we established our reference by requiring that the nightly mean vertical wind be zero. We have divided our data into two subsets, corresponding to days of quiet and moderately disturbed magnetic activity. In both cases, the mean daily variation in vertical wind velocity was dominated by its diurnal component with downward winds (relative to our chosen zero, and negative) appearing prior to local magnetic midnight and upward (positive) winds after. Averaged over all days of low magnetic activity, the diurnal range was −2.6 to 3.0 m s −1 whilst for days of moderate activity, it was −6.8 to 4.1 m s −1. Superimposed on this mean vertical wind are fluctuations at shorter periods than our sampling interval of 1 h. We estimate the distributions of vertical velocities prevailing during our observations to be characterized by one-sigma halfwidths of 14.3 and 18.7 m s −1 for low and moderate magnetic activity, respectively. This experiment has yielded generally smaller estimates of vertical wind speeds than those reported by previous authors from auroral latitudes for similar levels of magnetic activity. Comparison is also made with the UCL thermospheric general circulation model.

Thermospheric vertical winds above Mawson, Antarctica

High-resolution (R ≅ 200,000) night-time Fabry-Perot spectra of the λ630 run OI thermospheric airglow emission obtained from Mawson, Antarctica, during the austral winter of 1992 have been used to provide estimates of thermospheric temperature and neutral wind. In this paper, we report on characteristics of the resulting vertical wind estimates. The final data set comprised 103 observing nights between 18 March and 20 September for a total of 2096 useable zenith spectra. Because the instrumental reference rest wavelength of the 7630 nm emission is not known, this experiment was only capable of measuring vertical wind variations relative to an arbitrarily-chosen reference velocity. For this work, we established our reference by requiring that the nightly mean vertical wind be zero. We have divided our data into two subsets, corresponding to days of quiet and moderately disturbed magnetic activity. In both cases, the mean daily variation in vertical wind velocity was dominated by its diurnal component with downward winds (relative to our chosen zero, and negative) appearing prior to local magnetic midnight and upward (positive) winds after. Averaged over all days of low magnetic activity, the diurnal range was −2.6 to 3.0 m s−1 whilst for days of moderate activity, it was −6.8 to 4.1 m s−1. Superimposed on this mean vertical wind are fluctuations at shorter periods than our sampling interval of 1 h. We estimate the distributions of vertical velocities prevailing during our observations to be characterized by one-sigma halfwidths of 14.3 and 18.7 m s−1 for low and moderate magnetic activity, respectively. This experiment has yielded generally smaller estimates of vertical wind speeds than those reported by previous authors from auroral latitudes for similar levels of magnetic activity. Comparison is also made with the UCL thermospheric general circulation model.

Comparative thermospheres: Venus and Mars

The thermospheres of Venus and Mars are compared using Venera, Pioneer Venus, Mariner and Viking data. These measurements provide constraints for modeling efforts which probe thermospheric processes, balances, and responses to various forcings and input parameters. In particular, the National Center for Atmospheric Research (NCAR) non-LTE CO 2 radiative transfer model and the Thermospheric General Circulation Model (TGCM) have been adapted to the thermospheres of Venus and Mars for standard solar flux conditions. The better constraied Venus TGCM serves as a valuable precursor to the subsequent Mars TGCM, since both planets share many radiative and chemical processes peculiar to CO 2 atmospheres. Results indicate that planetary rotation significantly modifies thermospheric structure and winds; no nightside “cryosphere” of the type observed for Venus is predicted for Mars. Atomic O is greatly depleted on Mars compared to Venus. Resulting dayside heat balances suggest that CO 2 15-μm cooling strongly regulates Venus temperatures, while the Mars circulation is more effective as a thermostat. Fundamental planetary parameters are shown to be responsible for many of these differences in Venus and Mars thermospheric features.

Semidiurnal tide in the 80–150 km region: an assimilative data analysis

A set of tabulated functions called ‘Hough Mode Extensions’ (HMEs), which represent numerical extensions of classical Hough modes into the viscous regime of the thermosphere, are used to least-squares fit a climatological data base of tidal measurements. The data base consists of monthly average vertical profiles of semidiurnal amplitudes and phases at 17 radar sites accessing some part of the 80–150 km height region. The radars are distributed between 78 S and 70 N latitude, and each one provides measurements of one or more of the following: eastward wind, southward wind, perturbation temperature. As a result of the fitting process, a single complex normalizing coefficient is derived for each month and for each of the four HMEs, designated (2,2), (2,3), (2,4) and (2,5) after their classical Hough function designations. Once the complex coefficients are derived, reconstruction by weighted superposition of the HMEs results in globally continuous specifications of semidiurnal horizontal and vertical wind, temperature, pressure, and density throughout the 80–150 km height region. The tidal variations in density, in particular, provide greater accuracy for several aerospace applications. The methodology developed here can also be utilized to derive tidal lower boundary conditions for Thermospheric General Circulation Models (TGCMs), or as a basis for future empirical model development. Comparisons are also made with HME coefficients and global tidal fields from the Forbes and Vial [(1989) J. atmos. terr. Phys. 51, 649] numerical tidal model.

Venus O2 visible and IR nightglow: Implications for lower thermosphere dynamics and chemistry

The National Center for Atmospheric Research thermospheric general circulation model for the Venus thermosphere is modified to examine two observed night airglow features, both of which serve as sensitive tracers of the thermospheric circulation. New O2 nightglow data from the Pioneer Venus Orbiter (PVO) star tracker (O2 Herzberg II at 400–800 nm) and ground‐based telescopes (O2 IR at 1.27 μm) (Allen et al., 1992) yield additional model constraints for estimating Venus winds over 100–130 km. Atomic oxygen, produced by dayside CO2 photolysis peaking near 110 km, and transported to the nightside by the global wind system, is partially destroyed through three‐body recombination, yielding the O2 Herzberg II visible nightglow. This emission is very sensitive to horizontal winds at altitudes between 100 and 130 km. Other trace species catalytic reactions also contribute to the production of the very strong nightside infrared (1.27 μm) emission. This paper examines the dynamical and chemical implications of these new data using the Venus thermospheric general circulation model (VTGCM) as an analysis tool. Three‐dimensional calculations are presented for both solar maximum and solar medium conditions, corresponding to early PVO (1979–1981) and PVO entry (mid‐1992) time periods. Very distinct periods are identified in which zonal winds are alternately weak and strong in the Venus lower thermosphere. VTGCM sensitivity studies are conducted to assess the impacts of potential changes in thermospheric zonal and day‐to‐night winds, and eddy diffusion on the corresponding nightglow intensities. It appears that cyclostrophic balance extends above 80 km periodically, owing to a reversal of the upper mesosphere latitudinal temperature gradient, and thereby producing strong zonal winds and correspondingly modified O2 nightglow distributions that are observed.

Neutral composition measurements by the Pioneer Venus Neutral Mass Spectrometer during Orbiter re‐entry

Measurements by the Pioneer Venus Orbiter Neutral Mass Spectrometer (ONMS) instrument during Orbiter re‐entry have been made from 18–24 hours local solar time, above 170 km, of He, and from midnight to 4.5 hours below 200 km of He, N, O, CO, N2, and CO2. Preliminary results indicate that in the post‐midnight sector He is the dominant species above 170 km, O the dominant species from 140–170 km and CO2 the dominant species below 140 km. Estimated scale height temperatures for He, O and CO2 of about 105–120 K are similar to those observed in 1978–80 at higher solar activity. The densities at 1 am local solar time and at 150 km are within 35% of those measured earlier. The He bulge is also similar to that observed in 1978–80 confirming that thermosphere superrotation is still present. Comparison with the results of a Venus Thermosphere General Circulation Model suggests the nightside is not sensitive to changes in solar activity due to the isolation of the day and night thermospheres. Apparently, the relatively small changes in the dayside thermosphere with solar activity have little impact on the nightside thermosphere.