Stephen W. Bougher

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 Earths 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).

Mars Global Surveyor radio science electron density profiles : Neutral atmosphere implications

The Mars Global Surveyor (MGS) Radio Sci- ence (RS) experiment permits retrieval of electron density prof iles versus height (∼90-200 km) from occultation mea- surements. An initial set of electron profiles is examined spanning high northern latitudes, early morning solar local times and high solar zenith angles (78 to 81 ◦ ) near aphelion. Sampling for these 32-profiles is well distributed over longi- tude. The height of the photochemically driven ionospheric peak is observed to respond to the background neutral den- sity structure, with a mean height during this season at this location of ∼134.4 km. Strong wave-3 oscillations about this mean are clearly observed as a function of longitude, and correspond to neutral density variations measured by the MGS Accelerometer (ACC) experiment. The wave-3 tidal pattern implicated by both the RS and ACC datasets is consistent with a semi-diurnal wave frequency. Clearly, the height of the martian dayside ionospheric peak is a sensitive indicator of the state of the underlying Mars atmosphere. This ionospheric peak height can be used as a proxy of the longitude specific non-migrating tidal variations present in the Mars lower thermosphere.

Models of Venus neutral upper atmosphere - Structure and composition

Abstract Models of the Venus neutral upper atmosphere, based on both in-situ and remote sensing measurements, are provided for the height interval from 100 to 3,500 km. The general approach in model formulation was to divide the atmosphere into three regions: 100 to 150 km, 150 to 250 km, and 250 to 3,500 km. Boundary conditions at 150 km are consistent with both drag and mass spectrometer measurements. A paramount consideration was to keep the models simple enough to be used conveniently. Available observations are reviewed. Tables are provided for density, temperature, composition (CO 2 , O, CO, He, N, N 2 , and H), derived quantities, and day-to-day variability as a function of solar zenith angle on the day- and nightsides. Estimates are made of other species, including O 2 and D. Other tables provide corrections for solar activity effects on temperature, composition, and density. For the exosphere, information is provided on the vertical distribution of normal thermal components (H, O, C, and He) as well as the hot components (H, N, C, O) on the day- and nightsides.

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

density of 7.3–8.5 � 10 4 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 (s = 2) and zonal wave number 1 (s = � 1) characteristics. The height of this photochemically determined ionospheric peak should be monitored regularly. INDEX TERMS: 5435 Planetology: Solid Surface Planets: Ionospheres (2459); 5409 Planetology: Solid Surface Planets: Atmospheres—structure and dynamics; 6225 Planetology: Solar System Objects: Mars; KEYWORDS: ionosphere, Mars, thermosphere

The spatial distribution of planetary ion fluxes near Mars observed by MAVEN

We present the results of an initial effort to statistically map the fluxes of planetary ions on a closed surface around Mars. Choosing a spherical shell ~1000 km above the planet, we map both outgoing and incoming ion fluxes (with energies >25 eV) over a 4 month period. The results show net escape of planetary ions behind Mars and strong fluxes of escaping ions from the northern hemisphere with respect to the solar wind convection electric field. Planetary ions also travel toward the planet, and return fluxes are particularly strong in the southern electric field hemisphere. We obtain a lower bound estimate for planetary ion escape of ~3 × 1024 s−1, accounting for the ~10% of ions that return toward the planet and assuming that the ~70% of the surface covered so far is representative of the regions not yet visited by Mars Atmosphere and Volatile EvolutioN (MAVEN).

Structure, Luminosity, and Dynamics of the Venus Thermosphere

We review here observations and models related to the chemical and thermal structures, airglow and auroral emissions and dynamics of the Venus thermosphere, and compare empirical models of the neutral densities based in large part on in situ measurements obtained by the Pioneer Venus spacecraft. Observations of the intensities of emissions are important as a diagnostic tool for understanding the chemical and physical processes taking place in the Venus thermosphere. Measurements, ground-based and from rockets, satellites, and spacecraft, and model predictions of atomic, molecular and ionic emissions, are presented and the most important sources are elucidated. Coronas of hot hydrogen and hot oxygen have been observed to surround the terrestrial planets. We discuss the observations of and production mechanisms for the extended exospheres and models for the escape of lighter species from the atmosphere. Over the last decade and a half, models have attempted to explain the unexpectedly cold temperatures in the Venus thermosphere; recently considerable progress has been made, although some controversies remain. We review the history of these models and discuss the heating and cooling mechanisms that are presently considered to be the most important in determining the thermal structure. Finally, we discuss major aspects of the circulation and dynamics of the thermosphere: the sub-solar to anti-solar circulation, superrotation, and turbulent processes.

The Martian Thermosphere-Ionosphere at High and Low Solar Activities

Abstract 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 A 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.

Venus mesosphere and thermosphere: III. Three-dimensional general circulation with coupled dynamics and composition

Abstract The National Center for Atmospheric Research (NCAR) thermospheric general circulation model (TGCM) for the Earths thermosphere has been modified to examine the three-dimensional (3D) structure and circulation of the upper mesosphere and thermosphere of Venus (VTGCM). This model used the parameterizations from our earlier two-dimensional (2D) Venus model, including eddy diffusion and wave-drag parameterizations, and the 15-μm cooling scheme from Bougher et al . (S W. Bougher, R. E. Dickinson, E. C. Ridley, R. G. Roble, A. F. Nagy, and T.E. Cravens 1986, Icarus 68, 284–312). Processes unique to the Earths thermosphere (ion drag, magnetospheric convection, etc.) are removed, but the TGCM computational framework is retained. A symmetric version of the VTGCM is used first to simulate the mean subsolar-to-antisolar variation of observed composition and temperatures, as determined from Pioneer Venus data and subsequent emperical models. The VTGCM equatorial fields are shown to be largely consistent with previous symmetric 2D model fields of Bougher et. al . (S. W. Bougher, R. E. Dickinson, E. C. Ridley, R. G. Roble, A. F. Nagy, and T. E. Cravens 1986, Icarus 68, 284–312). The VTGCM is then used to examine the 3D character of the Venus asymmetric circulation and structure. A prescribed Venus retrograde zonal wind profile has been superimposed on the mean subsolar-to-antisolar circulation, and results show that the major (heavy) species and temperatures are not greatly affected by this superrotation. A shift in the exospheric temperature minimum to LT = 2 AM occurs, which is consistent with the convergence of the horizontal winds after midnight. Many of the observed features of the Venus thermosphere can be reproduced by the 3D VTGCM. Calculated terminator winds reach 230 m sec − , with exospheric temperatures ranging from 309 (day) to 136°K (night). Model atomic oxygen concentrations show little diurnal variation along constant pressure surfaces. Prescribed wave drag is primarily responsible for this weak global circulation, which is consistent with the observed day-night contrast in calculated densities and temperatures. Furthermore, the incorporation of a retrograde zonal momentum source helps to simulate observed asymmetries in Venus thermospheric fields. The present VTGCM thus serves as a useful benchmark upon which to incorporate additional minor constituents and test new self-consistent parameterizations for wave drag and superrotation.

Three‐dimensional study of Mars upper thermosphere/ionosphere and hot oxygen corona: 1. General description and results at equinox for solar low conditions

important reaction, the dissociative recombination of O2 is responsible for most of the production of hot atomic oxygen deep in the dayside thermosphere/ionosphere. The investigation of the Martian upper atmosphere is therefore complicated by the change in the flow regime from a collisional to a collisionless domain. Past studies, which used simple extrapolations of 1-D thermospheric/ionospheric parameters, could not account for the full effects of realistic conditions, which are shown to be of significant influence on the exosphere both close to and far away from the exobase. In this work, the combination of the new 3-D Direct Simulation Monte Carlo kinetic model and the modern 3-D Mars Thermosphere General Circulation Model is employed to describe selfconsistently the Martian upper atmosphere at equinox for solar low conditions. For the first time, a 3-D analysis and shape of the Martian hot corona is provided, along with density and temperature profiles of cold and hot constituents as functions of position on the planet. Atmospheric loss and ion production (found to be more than an order of magnitude lower than the neutral escape), calculated locally all around the planet, provide valuable information for plasma models, refining the understanding of the ion loss, atmospheric sputtering, and interaction with the solar wind, in general.

Three-dimensional study of Mars upper thermosphere/ionosphere and hot oxygen corona: 2. Solar cycle, seasonal variations, and evolution over history

[1] The global dynamics of the flow of energetic particles through the Martian upper atmosphere is studied for different cases reflecting variations in solar cycle, seasons, and epochs over history. In this study, the combination of the new 3-D Direct Simulation Monte Carlo kinetic model and the modern 3-D Mars Thermosphere General Circulation Model is employed to describe self-consistently the Martian upper atmosphere (i.e., the thermosphere/ionosphere and the exosphere). The variations in the Martian upper atmosphere over long-term (seasons and solar cycle) and evolutionary (Martian history) time scales are presented and discussed using the equinox solar low case extensively described in the work of Valeille et al. (2009c) as reference throughout. These characteristic conditions lead to significant variations in the thermosphere/ionosphere temperatures, dynamical heating, winds, and ion/neutral density distributions, which, in turn, affect the exosphere general structure, the hot corona shape, and the escape rate and have important implications for the study of the ion loss, atmospheric sputtering, and interaction with the solar wind in general. Calculations for present conditions are performed for three characteristic seasons (aphelion, equinox, and perihelion), while solar activity is either fixed to low or high conditions. Calculations for past conditions are related to a solar EUV flux enhancement of 1, 3, and 6 times the present values. Spatial-, seasonal-, solar cycle–, and evolutionary-driven variations, although exhibiting very different time scales, are all shown to exert an influence of the same order. Models of Mars upper atmosphere should address them accordingly.