Thermospheric General Circulation Model Research Articles

The Effects of the Upper Atmosphere and Corona on the Solar Wind Interaction With Venus

AbstractSince Venus has no substantial planetary magnetic field, the fast‐flowing solar wind plasma interacts directly with its ionosphere, upper atmosphere, and corona. The thermal atmosphere and hot oxygen corona of Venus are expected to play essential roles in the interaction process. To quantify their effects, we combine three major state‐of‐the‐art three‐dimensional numerical models, including (a) The Venus thermosphere general circulation model (VTGCM), (b) The adaptive mesh particle simulator (AMPS), and (c) A multi‐species Magnetohydrodynamics (MHD) model of Venus based on Block Adaptive Tree Solar‐wind Roe Upwind Scheme. The global multi‐species MHD model is used to simulate the planet's interaction with the ambient solar wind, where the background neutral densities are provided by VTGCM and AMPS. Using the above‐mentioned numerical models, the effect of the upper atmosphere and corona on solar wind driven ion escape rates are examined for three typical solar cycle conditions, including solar maximum, solar median, and solar minimum conditions. The model results suggest that even though the hot oxygen corona only has a minor effect on the ion density profiles in the ionosphere, it can enhance ion loss rate up to ∼25% for high solar activity.

A 3D Physics‐Based Particle Model of the Venus Oxygen Corona: Variations With Solar Activity

AbstractDue to Venus not having a substantial planetary magnetic field the fast‐flowing solar wind plasma can propagate to regions close to the planet. Therefore, thermal atomic oxygen in the thermosphere, hot oxygen in the corona, and the resulting pickup oxygen ions are essential for determining the overall interaction of the planet with plasma of the ambient solar wind. To investigate this complex system, we have initiated a project where a combination of Venus Thermosphere General Circulation Model (VTGCM) and Adaptive Mesh Particle Simulator (AMPS) codes are used to determine the variability of the ”hot” O corona depending on the solar conditions. Here we present the results of modeling Venus' oxygen corona using the VTGCM ionosphere/thermosphere and AMPS kinetic particle models. VTGCM produces a self‐consistent calculation of the thermosphere/ionosphere, providing the spatial distributions of the dominant species. That is further used in AMPS’ modeling of Venus' exosphere (a) to specify the source of the newly created hot O atoms produced by dissociative recombination of ions and (b) to account for thermalization of these energetic oxygen atoms as they propagate in the upper thermosphere. The altitude distribution of hot O calculated for the solar maximum conditions agree well with Pioneer Venus Orbiter observations of the oxygen corona. The modeling that we have performed for the solar minimum conditions indicates a decrease of the oxygen density in the corona by almost a factor of six compared to that at solar maximum. That is consistent with the non‐detection of the oxygen corona from Venus Express. As expected, the solar moderate case is between the solar maximum and minimum cases.

Modeling of observations of the OH nightglow in the venusian mesosphere

Venus airglow emissions have been unambiguously detected in the wavelength ranges of 1.40–1.49 and 2.6–3.14 μm in limb observations by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) onboard the Venus Express (VEx) spacecraft and are attributed to the OH(2–0) and OH(1–0) Meinel band transitions. The integrated (limb slant path) emission rates for these bands were measured by Piccioni et al. (2008).Photochemical (Caltech/JPL KINETICS) and global circulation (Venus Thermospheric General Circulation Model - VTGCM) model calculations suggest the observed OH emission is produced primarily via the Bates-Nicolet mechanism, as on the Earth, although Venus' background atmosphere is different than that of the Earth, but the modeled contribution of the HO2 + O → OH(v) + O2 reaction increases in the lower portion of the OH airglow layer. An overall difference of ~2 km in the peak heights of the OH(1–0) and OH(2–0) layers is seen in both the KINETICS and VTGCM simulations as a result of this change in the relative importance of H + O3 → OH(v) + O2 versus HO2 + O → OH(v) + O2 reactions with altitude.First time 3-D simulations of the OH Δv = 1 nightglow limb slant emission calculate a peak intensity of ~0.6 ± 0.3 MegaRayleighs at ~102 km altitude, an intensity that is consistent with Venus Express VIRTIS observations (Gérard et al. 2010; Soret et al. 2010, 2012) and KINETICS results. Soret et al. (2010) reported the intensity of the peak OH airglow increased from 0.30 to 0.40 MR from dusk to dawn but noted the observations used are not uniformly distributed and the observed emission is extremely variable, so a more detailed assessment of the observations was not possible. Our simulations show a decrease in the average OH(1–0) emission is symmetric about the midnight meridian, but the simulations find an asymmetric decrease from the equator to the poles. Consideration of transport and chemical lifetimes suggests modeling of OH above ~96 km requires explicit description of transport and vibrational-state-dependent chemistry.

Planetary‐Scale Wave Impacts on the Venusian Upper Mesosphere and Lower Thermosphere

AbstractThis work examines the planetary wave‐induced variability within the upper mesosphere/lower thermosphere of Venus by utilizing the Venus Thermospheric General Circulation Model (VTGCM). Rossby and Kelvin wave perturbations are driven by variations in the geopotential height of the VTGCM lower boundary (∼70 km). A suite of simulations was conducted to examine the impact of the individual and combined waves propagating from two different lower boundary conditions (uniform and varying). The Kelvin wave is the more dominant wave which produces the most variability. The combination of the Kelvin and Rossby waves provides a maximum temperature amplitude of 13 K at 92 km and maximum zonal wind amplitude of 23 m/s at 102 km. The combined waves overall are able to propagate up to 125 km. Most of the variation within the temperature, winds, and composition occurs between 70 and 110 km. The varying lower boundary increases the magnitude of the wave deposition and atmospheric responses, but weakly changes the propagation altitude. The thermal variation due to the planetary waves does not reproduce most observed variations. The simulated O2 IR nightglow emission is sensitive to the waves with respect to intensity and local time, but lacks latitudinal variation. The integrated intensity ranges from 1.2 MR to 1.65 MR and the local time ranges from 0.33 local time to 23.6 local time. Overall, planetary waves do affect the atmospheric structure, but there are still large observed variations that planetary waves alone cannot explain (i.e., thermal structure).

Coupled Earth’s Thermosohere-Ionosphere Global Dynamics Model

A coupled Earth’s thermosohere-ionosphere global dynamics model (for altitudes of 90–500 km) is presented. This model is based on a three-dimensional thermospheric general circulation model and a dynamical model of the ionospheric F-layer, which takes into account plasma-chemical processes, ambipolar diffusion, and advective ion transport due to neutral wind. General upper atmosphere characteristics have adequately been reproduced and the thermosphere–ionosphere interaction has quantitatively been estimated based on this coupled model. The sensitivity of thermospheric characteristics to ionospheric parameters and the sensitivity of the electron-concentration field distribution in the ionospheric F-layer to thermospheric parameters have been studied within a specified diurnal cycle.

Global response of the upper thermospheric winds to large ion drifts in the Jovian ovals

AbstractWe use our fully coupled 3‐D Jupiter Thermosphere General Circulation Model (JTGCM) to quantify processes which are responsible for generating neutral winds in Jupiter's oval thermosphere from 20 µbar to 10−4 nbar self‐consistently with the thermal structure and composition. The heat sources in the JTGCM that drive the global circulation of neutral flow are substantial Joule heating produced in the Jovian ovals by imposing high‐speed anticorotational ion drifts (~3.5 km s−1) and charged particle heating from auroral processes responsible for bright oval emissions. We find that the zonal flow of neutral winds in the auroral ovals of both hemispheres is primarily driven by competition between accelerations resulting from Coriolis forcing and ion drag processes near the ionospheric peak. However, above the ionospheric peak (<0.01 µbar), the acceleration of neutral flow due to pressure gradients is found to be the most effective parameter impacting zonal winds, competing mainly with acceleration due to advection with minor contributions from curvature and Coriolis forces in the southern oval, while in the northern oval it competes alone with considerable Coriolis forcing. The meridional flow of neutral winds in both ovals in the JTGCM is determined by competition between meridional accelerations due to Coriolis forcing and pressure gradients. We find that meridional flow in the lower thermosphere, near the peak of the auroral ionosphere, is poleward, with peak wind speeds of ~0.6 km s−1 and ~0.1 km s−1 in the southern and northern oval, respectively. The corresponding subsiding flow of neutral motion is ~5 m s−1 in the southern oval, while this flow is rising in the northern oval with reduced speed of ~2 m s−1. We also find that the strength of meridional flow in both auroral ovals is gradually weakened and turned equatorward near 0.08 µbar with wind speeds up to ~250 m s−1 (southern oval) and ~75 m s−1 (northern oval). The corresponding neutral motion in this region is upward, with wind speeds up to 4 m s−1 in both ovals.

Dayside temperatures in the Venus upper atmosphere from Venus Express/VIRTIS nadir measurements at 4.3μm

In this work, we analysed nadir observations of atmospheric infrared emissions carried out by VIRTIS, a high-resolution spectrometer on board the European spacecraft Venus Express. We focused on the ro-vibrational band of CO 2 at 4.3 µm on the dayside, whose fluorescence originates in the Venus upper mesosphere and above. This is the first time that a systematic sounding of these non-local thermodynamic equilibrium (NLTE) emissions has been carried out in Venus using this geometry. As many as 143,218 spectra have been analysed on the dayside during the period 14/05/2006 to 14/09/2009. We designed an inversion method to obtain the atmospheric temperature from these non-thermal observations, including a NLTE line-by-line forward model and a pre-computed set of spectra for a set of thermal structures and illumination conditions. Our measurements sound a broad region of the upper mesosphere and lower thermosphere of Venus ranging from 10 −2 –10 −5 mb (which in the Venus International Reference Atmosphere, VIRA, is approximately 100–150 km during the daytime) and show a maximum around 195 ± 10 K in the subsolar region, decreasing with latitude and local time towards the terminator. This is in qualitative agreement with predictions by a Venus Thermospheric General Circulation Model (VTGCM) after a proper averaging of altitudes for meaningful comparisons, although our temperatures are colder than the model by about 25 K throughout. We estimate a thermal gradient of about 35 K between the subsolar and antisolar points when comparing our data with nightside temperatures measured at similar altitudes by SPICAV, another instrument on Venus Express (VEx). Our data show a stable temperature structure through five years of measurements, but we also found episodes of strong heating/cooling to occur in the subsolar region of less than two days.

Doppler winds mapped around the lower thermospheric terminator of Venus: 2012 solar transit observations from the James Clerk Maxwell Telescope

Doppler shifts of sub-millimeter 12CO (346GHz) and 13CO (330GHz) and millimeter 12CO (230GHz) line absorptions were mapped around the circum-disk terminator of Venus before, during, and after the June 5, 2012 solar transit, employing the James Clerk Maxwell Telescope (JCMT). Radiative transfer analysis of the solar transit 12CO thermal line absorptions yields cross-terminator winds in the Venus lower thermosphere (100–120km) over the local time (LT) and latitude extent of the atmospheric limb presented by the inferior conjunction, nightside apparent disk of Venus. The unique solar transit geometry provides enhanced spatial resolution of the terminator (0.2h in local time, LT) associated with solar illumination of this atmospheric limb region, and so provides the first characterization of the instantaneous distribution of cross terminator flow in the Venus lower thermosphere versus LT and latitude. Furthermore, by mapping Doppler winds over the nightside disk preceding and following the solar transit, we place the highly variable zonal and subsolar-to-antisolar (SSAS) circulation components of the nightside lower thermosphere (Clancy, R.T., Sandor, B.J., Moriarty-Schieven, G.H. [2012a]. Icarus 217, 794–812) in the context of the day-to-night cross terminator flow that drives this chaotic nightside dynamical regime. The solar transit observations indicate substantially supersonic (200–300m/s) day-to-night cross terminator winds that are significantly (by 50–150m/s) stronger over the evening versus the morning terminator. They also exhibit surprisingly large (50%) variations over a 1–2h timescale that challenge explanation. These behaviors likely contribute to both the variability and the apparent retrograde zonal component of circulation in the Venus nightside upper atmosphere. Hence, these observations support dynamical arguments for preferential deceleration of the morning sector SSAS circulation (e.g., Alexander, M.J. [1992]. Geophys. Res. Lett. 19, 2207–2210), as recently simulated in the Venus thermospheric general circulation model of Hoshino et al. (Hoshinom, N. et al. [2013]. J. Geophys. Res. 118, 2004–2015).

Upper atmosphere temperature structure at the Venusian terminators: A comparison of SOIR and VTGCM results

Venus Express SOIR terminator profiles of CO2 densities and corresponding temperatures have been determined for 132 selected orbits obtained between 2006 and 2013. These recently recalibrated measurements provide temperature profiles at the Venusian terminator over approximately 70–160km, revealing a striking permanent temperature minimum (at about 125km) and a weaker temperature maximum (over 100–110km). In addition, topside temperatures (above 140km) reveal a warming trend consistent with a typical thermospheric structure. These features are reflected in the corresponding CO2 density profiles, and provide detailed constraints for global circulation models of the upper atmosphere.New Venus Thermospheric General Circulation Model (VTGCM) simulations are presented for conditions appropriate to these SOIR measurements. In particular, solar minimum to moderate fluxes are specified and mean values of eddy diffusion and wave drag parameters are utilized. Recent upgrades to the VTGCM code now include more realistic lower boundary conditions at ~70km near cloud tops. Model temperature profiles are extracted from the terminators that correspond to five latitude bins presently used in the SOIR data analysis. Averaging of VTGCM temperature profiles in each of these bins (at each terminator) is conducted to match SOIR sampling. Comparisons of these SOIR and VTGCM temperature profiles are shown. Most notably, the observed temperature minimum near 125km and the weaker temperature maximum over 100–110km are generally reproduced by the VTGCM at the correct pressure/altitude levels. However, magnitudes of simulated and measured temperatures are somewhat different as a function of latitude. In addition, VTGCM evening terminator (ET) temperatures are simulated to be modestly warmer than corresponding morning terminator (MT) values, a result of stronger ET than MT zonal winds at/above about 130km. The SOIR terminator temperatures thus far do not reveal this consistent trend, suggesting the VTGCM climate based winds may not precisely represent the averaged conditions during SOIR sampling. Overall, these data-model comparisons reveal that both radiative and dynamical processes are responsible for maintaining averaged temperatures and driving significant variations in terminator temperature profiles.

Carbon monoxide and temperature in the upper atmosphere of Venus from VIRTIS/Venus Express non-LTE limb measurements

The upper mesosphere and the lower thermosphere of Venus (from 90 to 150km 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 4.7μm 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 115km 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.

Hot carbon corona in Mars’ upper thermosphere and exosphere: 1. Mechanisms and structure of the hot corona for low solar activity at equinox

Two important source reactions for hot atomic carbon on Mars are photodissociation of CO and dissociative recombination of CO+; both reactions are highly sensitive to solar activity and occur mostly deep in the dayside thermosphere. The production of energetic particles results in the formation of hot coronae that are made up of neutral atoms including hot carbon. Some of these atoms are on ballistic trajectories and return to the thermosphere, and others escape. Understanding the physics in this region requires modeling that captures the complicated dynamics of hot atoms in 3-D. This study evaluates the carbon atom inventory by investigating the production and distribution of energetic carbon atoms using the full 3-D atmospheric input. The methodology and details of the hot atomic carbon model calculation are given, and the calculated total global escape of hot carbon from the assumed dominant photochemical processes at a fixed condition, equinox (Ls = 180°), and low solar activity (F10.7 = 70 at Earth) are presented. To investigate the dynamics of these energetic neutral atoms, we have coupled a self-consistent 3-D global kinetic model, the Adaptive Mesh Particle Simulator, with a 3-D thermosphere/ionosphere model, the Mars Thermosphere General Circulation Model to provide a self-consistent global description of the hot carbon corona in the upper thermosphere and exosphere. The spatial distributions of density and temperature and atmospheric loss are simulated for the case considered.

Solar wind interaction with Mars upper atmosphere: Results from the one‐way coupling between the multifluid MHD model and the MTGCM model

The 3‐D multifluid Block Adaptive Tree Solar‐wind Roe Upwind Scheme (BATS‐R‐US) MHD code (MF‐MHD) is coupled with the 3‐D Mars Thermospheric general circulation model (MTGCM). The ion escape rate from the Martian upper atmosphere is investigated by using a one‐way coupling approach, i.e., the MF‐MHD model incorporates the effects of 3‐D neutral atmosphere profiles from the MTGCM model. The calculations are carried out for two cases with different solar cycle conditions. The calculated total ion escape flux (the sum of three major ionospheric species, O+, , and ) for solar cycle maximum conditions (6.6×1024 s−1) is about 2.6 times larger than that of solar cycle minimum conditions (2.5×1024 s−1). Our simulation results show good agreement with recent observations of 2–3×1024 s−1 (O+, , and ) measured near solar cycle minimum conditions by Mars Express. An extremely high solar wind condition is also simulated which may mimic the condition of coronal mass ejections or corotating interaction regions passing Mars. Simulation results show that it can lead to a significant value of the escape flux as large as 4.3×1025s−1.

Incorporation of a gravity wave momentum deposition parameterization into the Venus Thermosphere General Circulation Model (VTGCM)

The gravity wave‐drag parameterization of Alexander and Dunkerton (1999) was implemented into a Venus Thermosphere General Circulation Model (VTGCM) to investigate breaking gravity waves as a source of momentum deposition in Venus' thermosphere. Previously, deceleration of zonal jets on the morning and evening terminators in models was accomplished via Rayleigh friction, a linear drag law that is not directly linked to any physical mechanism. The Alexander and Dunkerton (1999) parameterization deposits all of the momentum of a breaking wave at the breaking altitude and features a spectrum of wave phase speeds whose amplitudes are distributed as a Gaussian about a center phase speed. We did not find a combination of wave parameters (namely, center phase speed, amplitude at center phase speed, and distribution width) to produce sufficient drag in the jet cores that would bring VTGCM density and nightglow emissions into agreement with Venus Express observations. The zonal wind shear from 100 to 120 km altitude is very strong. Gravity waves launched below 100 km either break in the strong shear zones below 115 km or are reflected and do not propagate into the jet core regions where drag is needed. The results we present demonstrate that parameterizations developed for the middle atmosphere do not work in the thermosphere and that appropriate damping mechanisms other than nonlinear breaking/saturation dominate and should be accounted for at these heights.

Dayside thermal structure of Venus' upper atmosphere characterized by a global model

Observations of Venus' dayside thermal structure are being conducted through ground based observatories. These temperature measurements, along with those from several instruments onboard the current Venus Express mission, are augmenting the previous thermal structure data from past missions (e.g., Veneras', Pioneer Venus Orbiter, Pioneer Venus Probes). These recent ground‐based and VEx observations reveal the Venus dayside lower thermosphere to be considerably warmer and dynamically important than previously understood. In this study, a three dimensional general circulation model, the Venus Thermospheric General Circulation Model (VTGCM), is used to provide dayside temperature predictions for comparison to these recent ground based observations. Such a comparison serves to identify and quantify the underlying thermal processes responsible for the observed dayside temperature structure. The VTGCM reproduces the dayside temperatures observed near 110 km at noon from 40°S to 40°N very well. In addition, the global winds generated by these warm dayside temperatures are shown to give rise to dayside upwelling (divergence) and nightside subsidence (convergence) resulting in nightside warming near the anti‐solar point at ∼104 km. Corresponding nightside temperatures reach ∼198 K, in accord with averaged measurements. This agreement implies (1) it is important for GCMs to include the updated radiative heating and cooling rates presented in Roldán et al. (2000) and (2) the current VTS3 and VIRA empirical models are in‐sufficient in representing the warm regions observed in the thermal structure of the dayside lower thermosphere (∼100 to 130 km) and need to be updated.

Understanding the variability of nightside temperatures, NO UV and O2IR nightglow emissions in the Venus upper atmosphere

[1] Venus Express (VEX) has been monitoring key nightglow emissions and thermal features (O2 IR nightglow, NO UV nightglow, and nightside temperatures) which contribute to a comprehensive understanding of the global dynamics and circulation patterns above ∼90 km. The nightglow emissions serve as effective tracers of Venus' middle and upper atmosphere global wind system due to their variable peak brightness and horizontal distributions. A statistical map has been created utilizing O2 IR nightglow VEX observations, and a statistical map for NO UV is being developed. A nightside warm layer near 100 km has been observed by VEX and ground-based observations. The National Center for Atmospheric Research (NCAR) Venus Thermospheric General Circulation Model (VTGCM) has been updated and revised in order to address these key VEX observations and to provide diagnostic interpretation. The VTGCM is first used to capture the statistically averaged mean state of these three key observations. This correspondence implies a weak retrograde superrotating zonal flow (RSZ) from ∼80 km to 110 km and above 110 km the emergence of modest RSZ winds approaching 60 m s−1 above ∼130 km. Subsequently, VTGCM sensitivity tests are performed using two tuneable parameters (the nightside eddy diffusion coefficient and the wave drag term) to examine corresponding variability within the VTGCM. These tests identified a possible mechanism for the observed noncorrelation of the O2 and NO emissions. The dynamical explanation requires the nightglow layers to be at least ∼15 km apart and the retrograde zonal wind to increase dramatically over 110 to 130 km.

Model calculation of N2Vegard-Kaplan band emissions in Martian dayglow

[1] A model for N2 Vegard-Kaplan (VK) band (A3Σu+ − X1Σg+) emissions in Martian dayglow has been developed to explain the recent observations made by the Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars (SPICAM) ultraviolet spectrograph aboard Mars Express. Steady state photoelectron fluxes and volume excitation rates have been calculated using the analytical yield spectra technique. Since interstate cascading is important for triplet states of N2, the population of any given level of N2 triplet states is calculated under statistical equilibrium considering direct excitation, cascading, and quenching effects. Relative population of all vibrational levels of each triplet state is calculated in the model. Line of sight intensities and height-integrated overhead intensities have been calculated for VK, first positive (B3Πg − A3Σu+), second positive (C3Πu − B3Πg), and Wu-Benesch (W3Δu − B3Πg) bands of N2. A reduction in the N2 density by a factor of 3 in the Mars thermospheric general circulation model is required to obtain agreement between calculated limb profiles of VK (0–6) and SPICAM observation. Calculations are carried out to asses the impact of model parameters, namely, electron impact cross sections, solar EUV flux, and model atmosphere, on the emission intensities. Constraining the N2/CO2 ratio by SPICAM observations, we suggest the N2/CO2 ratios to be in the range 1.1–1.4% at 120 km, 1.8–3.2% at 140 km, and 4–7% at 170 km. During high solar activity the overhead intensity of N2 VK band emissions would be ∼2.5 times higher than that during low solar activity.

Characteristics of planetary-scale waves simulated by a new venusian mesosphere and thermosphere general circulation model

We have developed a new general circulation model (GCM) for the venusian mesosphere and thermosphere (80-about 180 km). Our GCM simulations show that winds in the subsolar-to-antisolar direction (SS–AS) are predominant above about 90 km. A weak return flow of the SS–AS is seen below about 90 km. We performed GCM simulations imposing the planetary-scale waves (thermal tides, Rossby wave, and Kelvin wave) at the lower boundary. Although the diurnal and semidiurnal tides are damped below 95 km, the Rossby wave propagates up to around 130 km. However, the amplitude of the Rossby wave is too small (<1 m/s) to affect the general circulation. On the other hand, the Kelvin wave propagates up to about 130 km with a maximum zonal wind fluctuation of approximately 5.9 m/s on average. The amplitude of the Kelvin wave sometimes exceeds 10 m/s around the terminator. The Kelvin wave causes a temporal variation in the wind velocity at the altitude of the O 2-1.27 μm nightglow emission (about 95 km). Using a newly developed 1-D nightglow model and the composition distribution calculated from our GCM, we investigated the impact of the Kelvin wave on the nightglow distribution. Our results suggest that the Kelvin wave would cause temporal variations in the nightglow emission in the 23:50–00:20 LT region with an intensity of 1.1–1.3 MR and a period of approximately 4 days.

Atomic oxygen distributions in the Venus thermosphere: Comparisons between Venus Express observations and global model simulations

Nightglow emissions provide insight into the global thermospheric circulation, specifically in the transition region (∼70–120 km). The O 2 IR nightglow statistical map created from Venus Express (VEx) Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) observations has been used to deduce a three-dimensional atomic oxygen density map. In this study, the National Center of Atmospheric Research (NCAR) Venus Thermospheric General Circulation Model (VTGCM) is utilized to provide a self-consistent global view of the atomic oxygen density distribution. More specifically, the VTGCM reproduces a 2D nightside atomic oxygen density map and vertical profiles across the nightside, which are compared to the VEx atomic oxygen density map. Both the simulated map and vertical profiles are in close agreement with VEx observations within a ∼30° contour of the anti-solar point. The quality of agreement decreases past ∼30°. This discrepancy implies the employment of Rayleigh friction within the VTGCM may be an over-simplification for representing wave drag effects on the local time variation of global winds. Nevertheless, the simulated atomic oxygen vertical profiles are comparable with the VEx profiles above 90 km, which is consistent with similar O 2 ( 1Δ) IR nightglow intensities. The VTGCM simulations demonstrate the importance of low altitude trace species as a loss for atomic oxygen below 95 km. The agreement between simulations and observations provides confidence in the validity of the simulated mean global thermospheric circulation pattern in the lower thermosphere.

Four Martian years of nightside upper thermospheric mass densities derived from electron reflectometry: Method extension and comparison with GCM simulations

The long‐term dynamics of the Martian upper thermosphere near the exobase (∼160–200 km) are still relatively poorly constrained by data. Electron reflectometry (ER) provides a way to derive, from electron loss cones, neutral mass densities at these altitudes in the night hemisphere. Because the Mars Global Surveyor Electron Reflectometer was not designed for this purpose, uncertainties in individual measurements are large and thus upper thermospheric variability can be characterized only on time scales of weeks or longer. Density measurements are presented at 2 A.M. local time and 185 km altitude, from April 1999 until November 2006, spanning ∼4 Martian years. We observe a weaker correlation with lower atmospheric dust activity than is seen in the lower thermosphere and a weaker correlation with solar EUV flux than is observed in the dayside exosphere. Seasonally repeating features are (1) overall expansion/contraction of the nighttime thermosphere with heliocentric distance, (2) much lower densities at the aphelion winter pole compared to the perihelion winter pole, and (3) a short‐lived local density maximum at aphelion in the southern hemisphere. Interannual differences are also observed; in particular, the interval of low densities in the southern winter occurs progressively later as solar EUV flux decreases from solar maximum to solar minimum. Results are compared with predictions from the Mars Thermosphere General Circulation Model and LMD Mars Global Circulation atmospheric model frameworks for Ls = 90°–180°, which generally underestimate and overestimate neutral densities, respectively. This disagreement reflects the difficulty in simulating nightside dynamical and cooling processes.

Simulating the density and thermal structure of the middle atmosphere (∼80–130 km) of Mars using the MGCM–MTGCM: A comparison with MEX/SPICAM observations

The objective of this work is to advance the understanding of the structure and dynamics of the middle altitude region of the martian atmosphere. While numerous advancements have been made in the level of scientific understanding of Mars’s upper and lower atmospheres over the past several decades, insight into the mechanisms of the middle atmosphere has come at a significantly slower pace due to the small number of datasets available for this region. Over the past decade this has begun to change, with renewed interest by NASA and ESA to send spacecraft to Mars. The result of these recent efforts is a growing database for Mars’s middle atmosphere, enabling long-awaited and necessary studies characterizing the middle altitude region. Various numerical models of the martian atmosphere can now be validated and constrained using this database. We utilize the Mars Express/Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (MEX/SPICAM) density and temperature datasets to characterize the middle atmosphere as well as validate and constrain the coupled multi-dimensional Mars General Circulation Model–Mars Thermosphere General Circulation Model (MGCM–MTGCM) at middle altitudes in order to explore the underlying physics controlling the structure and dynamics at these levels. The results of this study stress the importance of proper dust prescription within the MGCM–MTGCM for accurately reproducing the density and thermal structure of the middle and upper atmosphere regions on Mars. Simulations conducted with horizontal dust opacities that are consistent with the SPICAM observation period (i.e. Mars Odyssey/THEMIS opacities) yield modeled densities and temperatures that are closer to the observed values than simulations conducted with “typical” dust conditions (i.e. Mars Global Surveyor/TES opacities). We show that the MGCM–MTGCM closely reproduces the observed densities during low-dust and high-dust scenarios but displays difficulty during the pre-dust-season “ramp-up” period ( L s ∼ 120–200°) during MY27. In addition, we show that the MGCM–MTGCM accurately reproduces the temperature profiles below the mesopause, but, the mesopause altitude is too low and its temperature warmer (5–10 K) than observations. This may be related to nightside dynamical heating processes that require further refinement. In addition, CO 2 15-μm cooling rates may be too small, which would be consistent with underestimated atomic O abundances.