Mars Thermospheric General Circulation Model Research Articles

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.

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.

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.

Water loss and evolution of the upper atmosphere and exosphere over martian history

The loss of water from Mars can be evaluated by studying the evolution of the escape rate of atomic oxygen over time. Throughout martian history, the evolution of solar radiation has led to significant variations in the macroscopic parameters of the thermosphere/ionosphere, which in turn govern the hot species population of the exosphere and especially the atmospheric loss rates. In this study, the combination of our Direct Simulation Monte Carlo (DSMC) kinetic model and the 3D Mars Thermosphere General Circulation Model (MTGCM) [Valeille, A., Combi, M.R., Tenishev, V., Bougher, S.W., Nagy, A., 2009. Icarus. doi:10.1016/j.icarus.2008.08.018] is used to describe self-consistently ancient upper atmospheres of Mars for different solar inputs. 3D descriptions from the MTGCM of the ancient thermosphere/ionosphere are presented and discussed for the first time, including density profiles and temperature maps of the background neutrals and ions for the three epochs considered, which can be related to a solar EUV (Extreme Ultraviolet) flux enhancement of 1, 3 and 6 times the present values. Furthermore, solar cycle effects are quantified and discussed for both present and past conditions. Along with maps of ion production by photoionization (PI), charge exchange (CE) and electron impact (EI), the DSMC model provides density and temperature profiles, return fluxes and atmospheric loss rates of suprathermal exospheric oxygen as functions of the Solar Zenith Angle (SZA). This approach allows us to study the effects of dynamics on the ancient Mars upper atmosphere structure. Thermospheric variations are found to be not as large as previous 1D models predicted. The study of the evolution of the heat balance suggests that the ancient Mars thermosphere, of about 3.5 billion years (Gyr) ago, was relatively similar to the present Venus thermosphere. While O 2 + dissociative recombination (DR) is by far the main source of atmospheric escape at present, its relative contribution is shown to be reduced in the past compared to secondary escape processes. Nevertheless, O 2 + DR is found to remain the main driver of atmospheric escape over this period. The long-term decay of atmospheric loss throughout the martian history (until about 3.5 Gyr ago) may be comparable to the short-term periodic variations at present. A heuristic scaling law for evolution of water over time is proposed and a conservative estimate of about 10 m of water is found to have escaped globally to space over the last ∼3.5 Gyr.

Solar cycle variability of Mars dayside exospheric temperatures: Model evaluation of underlying thermal balances

The response of the Mars dayside exospheric temperatures to short and long term solar flux changes was recently established. Characterization of the relative importance of various thermospheric heating and cooling mechanisms for maintaining these Mars exospheric temperatures requires the systematic application of modern global dynamical models that capture both lower and upper atmosphere thermal and dynamical processes. Coupled Mars General Circulation Model (MGCM) plus Mars Thermospheric General Circulation Model (MTGCM) simulations are utilized for this study, closely matching conditions during Mars Global Surveyor drag measurements. Simulations confirm the major balance of EUV heating and thermal heat conduction at dayside exospheric altitudes. However, the influence of variable Martian global winds is significant and must be carefully considered when investigating the global regulation of Mars exospheric temperatures over the solar cycle and Martian seasons. Finally, the present MGCM‐MTGCM heating and cooling processes suggests that an EUV‐UV heating efficiency of 19% yields net heating in accord with MGS exospheric temperatures.

A study of suprathermal oxygen atoms in Mars upper thermosphere and exosphere over the range of limiting conditions

As a part of a global effort, the dynamics of energetic particles flowing through the martian upper atmosphere is studied. Most of the production of hot atomic oxygen occurs deep in the day-side thermosphere of Mars, where dissociative recombination (DR) of O 2 + ion is the dominant source. The study of an upper atmosphere is complicated by the change in the flow regime from a thermospheric collisional to an exospheric collisionless domain. To understand the martian exosphere, it is then highly desirable to employ a global kinetic model that includes a self-consistent description of both thermospheric and exospheric regions. In this work, a combination of our Direct Simulation Monte Carlo (DSMC) model [Tenishev, V., Combi, M.R., 2005. Monte-Carlo model for dust/gas interaction in rarefied flows. AIAA, 2005–4832] and the 3D Mars Thermosphere General Circulation Model (MTGCM) [Bougher, S.W., Bell, J.M., Murphy, J.R., Lopez-Valverde, M.A., Withers, P.G., 2006. Geophys. Res. Lett. 32, doi:10.1029/2005GL024059. L02203] is used to describe self-consistently the exosphere and the upper thermosphere. Along with the effect of ionization, the model provides profiles of density and temperature, atmospheric loss rates and return fluxes as functions of the Solar Zenith Angle (SZA) for all cases considered. To present a complete description of the effects of a 3D thermosphere onto the exosphere, several of the most limiting cases spanning spatial and temporal domains are examined. Along with solar activity variability, the influences of position on the planet and of different seasons are investigated and their relative contribution to the atmospheric loss is shown to be of the same order.

Vertical dust mixing and the interannual variations in the Mars thermosphere

Mars winter polar warming is a phenomenon of the lower thermosphere temperatures and densities that is well documented by in situ accelerometer data taken during spacecraft aerobraking maneuvers. Previous work by Bougher et al. (2006) simulates two specific time periods, corresponding to existing aerobraking data sets, and confirms the existence of winter polar warming structures in the Martian upper atmosphere, using the coupled Mars General Circulation Model–Mars Thermosphere General Circulation Model (MGCM‐MTGCM). The present work investigates the underlying mechanisms that drive winter polar warming in three major studies: (1) a systematic analysis of vertical dust mixing in the lower atmosphere and its impact upon the dynamics of the lower thermosphere (100–130 km), (2) an interannual investigation utilizing 3 years of lower atmosphere infrared (IR) dust optical depth data acquired by the Thermal Emission Spectrometer (TES) instrument on board Mars Global Surveyor (MGS), and finally (3) a brief study of the MTGCM's response to variations in upward propagating waves and tides from the lower atmosphere. From the first study, we find that the vertical dust mixing greatly modifies the simulated winter polar warming features in the MTGCM's thermosphere at both solstice seasons. Furthermore, this sensitivity study confirms that an interhemispheric Hadley circulation and the concomitant adiabatic heating rates are the primary causes for these winter polar warming structures. From the second study, we find that as revealed in the lower atmosphere by Liu et al. (2003), the polar lower thermosphere exhibits a high degree of variability at LS = 270, with reduced yet significant variability at LS = 090. Finally, the third numerical experiment indicates, as was found in the lower atmosphere by Wilson (1997), that upward propagating waves and tides allow meridional flows to access the thermosphere's winter polar latitudes. This last investigation also indicates that the Hadley circulation responsible for thermospheric winter polar warming originates in the lower atmosphere and extends high into the upper atmosphere. In summary, these new efforts establish a baseline numerical study upon which more comprehensive model‐to‐data comparisons may be conducted for the Martian thermosphere.

Zonal Wind Calculations from Mars Global Surveyor Accelerometer and Rate Data

TheMars Global Surveyor spacecraft was initially placed into a high-eccentricity, nearly polar orbit about Mars with a 45-h period. To accomplish the science objectives of the mission, a 2-h circular orbit was required. Using a method known as aerobraking, numerous passes through the upper atmosphere slowed the spacecraft, thereby reducing the orbital period and eccentricity. To successfully perform aerobraking, the spacecraft was designed to be longitudinally, aerodynamically stable in pitch and yaw. Because the orbit was nearly polar, the yaw orientation of the spacecraft was sensitive to disturbances caused by the zonal components of wind (east to west or west to east) acting on the spacecraft at aerobraking altitudes. Zonal wind velocities were computed by equating the aerodynamic and inertia-related torques acting on the spacecraft. Comparisons of calculated zonal winds with those computed from the Mars thermospheric general-circulation model are discussed.

The first Mars thermospheric general circulation model: The Martian atmosphere from the ground to 240 km

The Martian general circulation model (GCM) at the Laboratoire de Météorologie Dynamique (LMD) has been extended up to approximately 240 km and it is now the first GCM to fully simulate the Martian atmosphere from the ground up to the thermosphere. In this paper we summarize the processes that have been added and give a first simple study of the effects of parameterized orographic gravity waves on the zonal mean fields. This first study clearly emphasizes the coupling between the different atmospheric regions and notes the importance of having fully coupled models to simulate the atmosphere of Mars.

Global MGS TES data and Mars-GRAM validation

Mars Global Reference Atmospheric Model (Mars-GRAM 2001) is an engineering-level Mars atmosphere model widely used for many Mars mission applications. From 0 to 80 km, it is based on NASA Ames Mars General Circulation Model (MGCM), while above 80 km it is based on University of Michigan Mars Thermospheric General Circulation Model. Mars-GRAM 2001 and MGCM use surface topography from Mars Global Surveyor Mars Orbiter Laser Altimeter (MOLA). Validation studies are described comparing Mars-GRAM with a global summary data set of Mars Global Surveyor Thermal Emission Spectrometer (TES) data. TES averages and standard deviations were assembled from binned TES data which covered surface to ∼40 km, over more than a full Mars year (February 1999–June 2001, just before start of a Mars global dust storm). TES data were binned in 10° × 10° latitude–longitude bins (36 longitude bins, centered at 5°–355°, by 18 latitude bins, centered at −85° to +85°), and 12 seasonal bins (based on 30° increments of L s angle). Bin averages and standard deviations were assembled at 23 data levels (temperature at 21 pressure levels, plus surface temperature and surface pressure). Two time-of-day bins were used: local time near 2 or 14 h. Two dust optical depth bins were used: infrared optical depth, either less than or greater than 0.25 (which corresponds to visible optical depth less than or greater than about 0.5). For interests in aerocapture and precision entry and landing, comparisons focused on atmospheric density. TES densities versus height were computed from TES temperature versus pressure, using assumptions of perfect gas law and hydrostatics. Mars-GRAM validation studies used density ratio (TES/Mars-GRAM) evaluated at data bin center points in space and time. Observed average TES/Mars-GRAM density ratios were generally 1 ± 0.05, except at high altitudes (15–30 km, depending on season) and high latitudes (>45°N), or at most altitudes in the southern hemisphere at L s ∼ 90° and 180°. Compared to TES averages for a given latitude and season, TES data had average density standard deviation about the mean of ∼2.5% for all data, or ∼1–4%, depending on time of day and dust optical depth. Average standard deviation of TES/Mars-GRAM density ratio was 8.9% for local time 2 h and 7.1% for local time 14 h. Thus standard deviation of observed TES/Mars-GRAM density ratio, evaluated at matching positions and times, is about three times the standard deviation of TES data about the TES mean value at a given position and season.

MGS Radio Science electron density profiles: Interannual variability and implications for the Martian neutral atmosphere

Martian electron density profiles provided by the Mars Global Surveyor (MGS) Radio Science (RS) experiment over the 95–200 km altitude range indicate wthat the height of the electron peak and the longitudinal structure of the peak height are sensitive indicators of the physical state of the Mars lower and upper atmospheres. The present analysis is carried out on five sets of occultation profiles, all at high solar zenith angles (SZA). Variations spanning 2 Martian years are investigated near aphelion conditions at high northern latitudes (64.7–77.6N) making use of four of these data sets. A mean ionospheric peak height of 133.5–135 km is obtained near SZA = 78–82°; a corresponding mean peak density of 7.3–8.5 × 104 cm−3 is also measured during solar moderate conditions at Mars. Strong wave number 2–3 oscillations in peak heights are consistently observed as a function of longitude over the 2 Martian years. These observed ionospheric features are remarkably similar during aphelion conditions 1 Martian year apart. This year‐to‐year repeatability in the thermosphere‐ionosphere structure is consistent with that observed in multiyear aphelion temperature data of the Mars lower atmosphere [Clancy et al., 2000; Smith, 2004]. Coupled Mars general circulation model (MGCM) and Mars thermospheric general circulation model (MTGCM) codes are run for Mars aphelion conditions, yielding mean and longitude variable ionospheric peak heights that reasonably match RS observations. A tidal decomposition of MTGCM thermospheric densities shows that observed ionospheric wave number 3 features are linked to a nonmigrating tidal mode with semidiurnal period (σ = 2) and zonal wave number 1 (s = −1) characteristics. The height of this photochemically determined ionospheric peak should be monitored regularly.

Mars-GRAM validation with Mars global surveyor data

Mars Global Reference Atmospheric Model (Mars-GRAM 2001) is an engineering-level Mars atmosphere model widely used for many Mars mission applications. From 0–80 km, it is based on NASA Ames Mars General Circulation Model (MGCM), while above 80 km it is based on University of Arizona Mars Thermospheric General Circulation Model. Mars-GRAM 2001 and MGCM use surface topography from Mars Global Surveyor Mars Orbiter Laser Altimeter (MOLA). Validation studies are described comparing Mars-GRAM with Mars Global Surveyor Radio Science (RS) and Thermal Emission Spectrometer (TES) data. RS data from 2480 profiles were used, covering latitudes 75°S–72°N, surface to ∼40 km, for seasons ranging from areocentric longitude of Sun ( L s) = 70–160° and 265–310°. RS data spanned a range of local times, mostly 0–9 and 18–24 h. For interests in aerocapture and precision landing, comparisons concentrated on atmospheric density. At a fixed height of 20 km, measured RS density varied by about a factor of 2.5 over the range of latitudes and L s values observed. Evaluated at matching positions and times, average RS/Mars-GRAM density ratios were generally 1 ± 0.05, except at heights above ∼25 km and latitudes above ∼50°N. Average standard deviation (SD) of RS/Mars-GRAM density ratio was 6%. TES data were used covering surface to ∼40 km, over more than a full Mars year (February, 1999 through June, 2001, just before start of a Mars global dust storm). Depending on season, TES data covered latitudes 85°S–85°N. Most TES data were concentrated near local times 2 and 14 h. Observed average TES/Mars-GRAM density ratios were generally 1 ± 0.05, except at high altitudes (15–30 km, depending on season) and high latitudes (>45°N), or at most altitudes in the southern hemisphere at L s ∼ 90° and 180°. Compared to TES averages for a given latitude and season, TES data had average density SD about the mean of ∼2.5% for all data, or ∼1–4%, depending on time of day and dust optical depth. Average SD of TES/Mars-GRAM density ratio was 8.9% for local time 2 h and 7.1% for local time 14 h. Thus SD of observed TES/Mars-GRAM density ratio, evaluated at matching positions and times, is about three times the SD of TES data about the TES mean value at a given position and season.

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.

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.

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 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.

Atomic oxygen in the Martian thermosphere

Modern models of thermospheric composition and temperature and of excitation and radiative transfer processes are used to simulate the O I 130‐nm emission from Mars measured by the Mariner 9 ultraviolet spectrometer. We use the Mars thermospheric general circulation model calculations (MTGCM) of Bougher et al. (1988a) and the Monte Carlo partial frequency redistribution multiple scattering code of Meier and Lee (1982). We find that the decline in atomic oxygen through the daylight hours predicted by the MTGCM cannot be reconciled with the excess afternoon brightness seen in the data. Oxygen concentrations inferred from the data show a positive gradient through the day, in agreement with the original analysis by Strickland et al. (1973), although the absolute amounts are somewhat less because we use a larger photoelectron impact excitation and a somewhat larger solar flux in the 130‐nm triplet. In addition, the data suggest that the oxygen abundance increases toward high southerly latitudes, in contrast with the MTGCM prediction of high values in the northern (winter) hemisphere. It appears that solar forcing alone cannot account for the observed characteristics of the Martian thermosphere and that wave and tidal effects may profoundly affect the structure, winds, and composition.