S. Engel

The University of Arizona, Tucson, Arizona, United States

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Publications (11)19.47 Total impact

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    ABSTRACT: Martian electron density profiles provided by the Mars Global Surveyor (MGS) Radio Science (RS) experiment over the 95-200 km altitude range indicate what 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 - 77.6 N) making use of four of these data sets. A mean ionospheric peak height of 133.5 - 135 km is obtained near SZA = 78 - 82 deg.; a corresponding mean peak density of 7.3 - 8.5 x l0(exp 4)/ qu cm 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. 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 non-migrating tidal mode with semidiurnal period (sigma = 2) and zonal wave number 1 (s = -1) characteristics. The height of this photochemically determined ionospheric peak should be monitored regularly.
    Journal of Geophysical Research Atmospheres 04/2004; · 3.44 Impact Factor
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    ABSTRACT: [1] The calculation of the dication CO22+ density in the atmosphere of Mars is performed for the first time. The metastable lifetime of these species reaches 4 seconds. The density of a layer centered around 155–160 km altitude can reach from 3 to and 5 106 m−3. The ions are produced by CO2 photoionization and photoelectron impact on CO2. They are lost by dissociative recombination with the thermal electrons and chemical reaction with CO2. This work is based upon relevant chemical reaction coefficient rates that have been measured in laboratory very recently. We suggest that this ion layer can be detectable by a mass spectrometer onboard the orbiter of the CNES PREMIER 07 mission. This work opens a series of promising studies on double ionization processes in the Mars ionosphere, such as their implications for the production of energetic electrons or fast fragmentation ion products which could participate in atmospheric escape.
    Geophysical Research Letters 03/2003; 30(7):12-1. · 3.98 Impact Factor
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    ABSTRACT: The Mars Global Surveyor (MGS) Radio Science (RS) experiment employs an ultrastable oscillator aboard the spacecraft. The signal from the oscillator to Earth is refracted by the Martian ionosphere, allowing retrieval of electron density profiles versus radius and geopotential. The present analysis is carried out on five sets of occultation measurements: (1) four obtained near northern summer solstice (Ls = 74-116, near aphelion) at high northern latitudes (64.7-77.6N), and (2) one set of profiles approaching equinox conditions (Ls = 135- 146) at high southern latitudes (64.7-69.1S). Electron density profiles (95 to 200 km) are examined over a narrow range of solar zenith angles (76.5-86.9 degrees) for local true solar times of (1) 3-4 hours and (2) 12.1 hours. Variations spanning 1-Martian year are specifically examined in the Northern hemisphere.
    02/2003;
  • S. Bougher, J. Murphy, S. Engel
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    ABSTRACT: Recent Mars Global Surveyor (MGS) (1997-1999) and 2001 Mars Odyssey (ODY) (2001-2002) aerobraking exercises confirm that the Mars lower thermosphere (roughly 100-160 km) is a highly variable region on time scales of a day or less. Orbitto-orbit 2-sigma variability of MGS densities at a constant height was observed to be at least 70% [Keating et al., 1998], in accord with Mariner and Viking values [Stewart, 1987]. Longitude fixed thermospheric variations were also observed throughout MGS aerobraking at low to mid-latitudes that seem to be related to the large scale features of the topography at similar latitudes [e.g. Keating et al. 1998; Forbes et al., 2001; Bougher et al., 2001; Wilson, 2002; Forbes et al., 2002]. Diurnal Kelvin waves have been identified as likely responsible for these oscillations. ODY accelerometer data at high Northern latitudes failed to identify longitude fixed features, but rather observed what may be traveling (baroclinic) waves during local Northern winter. These aerobraking experiences (near Mars perihelion and aphelion seasons) suggest that the strong coupling of the Mars lower and upper atmospheres is composed of : (1) inflation/contraction of the entire atmosphere due to infrared and aerosol heating, and (2) upward propagating waves (tides, planetary waves, gravity waves) which reach the thermosphere, and are modulated by the largely unexplored middle atmosphere (50-100 km) [Forbes et al., 2002]. Mars three-dimensional general circulation models of the lower (NASA Ames GCM) and upper (NCAR/Michigan MTGCM) atmospheres were recently coupled in an effort to capture both upward propagating tidal/planetary waves (below 100 km) and in-situ forced waves (above 100 km) resulting from local solar EUV/UV heating. Coupled simulations will be presented that illustrate longitude fixed oscillations for aphelion and perihelion seasons. Comparisons will be made with corresponding MGS and ODY thermospheric datasets, respectively . Both thermal and tidal diagnostics will be presented to confirm the tidal modes and heat balances responsible for the simulated features. The capabilities and shortcomings of this coupled model approach will be summarized.
    01/2002;
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    ABSTRACT: 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.
    Advances in Space Research 01/2002; · 1.18 Impact Factor
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    ABSTRACT: The Mars Global Surveyor (MGS) Radio Sci-ence (RS) experiment permits retrieval of electron density profiles 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.
    Geophysical Research Letters 01/2001; 28(15):3091-3094. · 3.98 Impact Factor
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    ABSTRACT: The Mars Global Surveyor (MGS) Radio Science (RS) experiment employs an ultrastable oscillator aboard the spacecraft. The signal from the oscillator to Earth is refracted by the Martian ionosphere, allowing retrieval of electron density profiles versus radius . The present analysis is carried out on four sets of occultation measurements : (1) three obtained near northern summer solstice (Ls = 74-116, near aphelion) at high northern latitudes (64.7-73.3N), and (2) one set approaching equinox conditions (Ls 135-146) at high southern latitudes (64.7-69.1S). Electron density profiles (95 to 200 km) are examined over a narrow range of solar zenith angles (SZA) (76.5-86.9 degrees) for local true solar times of (1) 3-4 hours and (2) 12.1 hours. In three of these datasets, sampling is well distributed over longitude. Specific attention is given to the height and magnitude of the primary F1-ionospheric peak observed in each of these profiles. The height of this photochemically driven peak is controlled by the neutral density structure. Variations are observed as a function of SZA (weak) and longitude (strong), with a mean height of 134-135 km for the aphelion profiles. The magnitude of this same photochemical peak (7-9 .0 E+4 cm-3 ) reflects solar moderate conditions. Seasonal inflation/contraction of the Mars atmosphere, dust storm expansion/abatement, and planetary wave processes are all thought to impact the integrated atmospheric column and the height of the dayside ionospheric peak. The Mars Thermospheric General Circulation Model (MTGCM) is exercised for Mars conditions appropriate to these RS observational periods in order to understand the underlying neutral atmosphere conditions giving rise to these ionospheric features (mean and variations). Solar moderate fluxes (F10.7 = 130), aphelion conditions (Ls = 90), and low dust opacities are specificed. The MTGCM simulations also incorporate wave features resulting from upward propagating migrating plus non-migrating tides as well as in-situ tidal forcing. Longitude variations in the height of the simulated ionospheric peak are contrasted with corresponding RS data. Tidal modes responsible for these longitude specific wave features are also identified. Clearly, the height of the dayside ionospheric peak is a sensitive indicator of the changing state of the Mars lower atmosphere. This research is funded by the NASA MGS Data Analysis Program.
    10/2000;
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    ABSTRACT: 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).
    Journal of Geophysical Research Atmospheres 01/2000; 105:17669-17692. · 3.44 Impact Factor
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    ABSTRACT: The present maturity of available planetary databases and modeling capabilities now permits us to extend the comparison of terrestrial planetary thermospheres beyond the limited capability of one-dimensional models to global multidimensional models [e.g., Bougher and Roble, 1997]. This effort focuses upon the comparison of the solar cycle responses of the thermospheres of Venus, Earth, and Mars using three-dimensional global models that couple the energetics, dynamics, and neutral-ion composition above ∼100 km for each planet. Standard solar EUV and UV fluxes are adopted for use in these simulations. The Venus, Earth, and Mars Thermosphere General Circulation Models (TGCMs) each share a common formulation scheme and development heritage making use of the computing facilities of the National Center for Atmospheric Research. The motivation of this research is not only to simulate the observed responses of these individual planets to solar EUV/UV flux variations but also to understand the relative importance of common processes that regulate this unique behavior. The role of O-CO2 enchanced 15-μm cooling is investigated in the context of global dynamics and its effect on atomic-O distributions. It is found that CO2 cooling is an effective thermostat for control of the Venus dayside temperatures, while Mars and Earth are only moderately affected. By contrast, the role of global dynamics in controlling temperature distributions is most pronounced for Mars and the Venus nightside but negligible for Earth. The net effect of these radiative and dynamical processes is to determine that Venus and Mars thermospheres respond rather quickly to solar flux variations (much less than an Earth day), while the Earth thermosphere is more sluggish in its behavior. This work confirms the relative importance of CO2 cooling in the Earth's lower thermosphere. Furthermore, the value of the CO2-O deactivation rate near 300 K is rather well constrained by these planetary comparisons.
    Journal of Geophysical Research Atmospheres 01/1999; 104:16591-16611. · 3.44 Impact Factor
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    S. Engel, S. W. Bougher
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    ABSTRACT: Recent aerobraking maneuvers in the Martian thermosphere demonstrated the need to understand the structure and dynamics of these upper atmospheres. 3-dimensional circulation models (MTGCM for Mars and VTGCM for Venus) are used to calculate relevant physical and chemical processes. In this study we focus on the thermal, compositional and dynamical responses of these upper atmospheres to solar EUV/UV flux variability. Standard EUV/UV solar flux data sets are adopted for minimum, moderate, and maximum solar conditions. The thermospheres of Mars and Venus are compared to each other, applying the same processes and parameters, however their responses to solar flux changes are quite different. We found that dynamics are mostly responsible for cooling the Mars thermosphere. An increase in temperature due to solar cycle variations (min to max) causes an increase in upwelling and downwelling winds, which regulate the overall thermospheric temperature increase. The temperature in the upper thermosphere changes by about 100 K over the solar cycle. The O/CO2 mixing ratio does change significantly, but the resultant CO2 cooling is secondary in regulating temperatures. For Venus, strong O-CO2 cooling serves as an effective thermostat that gives rise to small variations of thermospheric temperatures over the solar cycle. This is reflected in the increase of the noontime O/CO2 mixing ratio from 7-8 the upper thermosphere the change is about 80 K, which is small considering Venus' close proximity to the Sun. Fundamental planetary parameters are at the root of the differences observed in these Venus and Mars upper atmospheric responses to solar EUV/UV variability.
    08/1998; 30:1033.
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