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Solar Cycle Variability of the Thermospheres of Mars and Earth

Solar Cycle Variability of the Thermospheres of Mars and Earth. Sean L. Bruinsma 1 , Jeffrey M. Forbes 2 , Jean-Charles Marty 1 , and Xiaoli Zhang 2 1 Department of Terrestrial and Planetary Geodesy Centre Nationale D'Etudes Spatiales,Toulouse, France

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Solar Cycle Variability of the Thermospheres of Mars and Earth

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  1. Solar Cycle Variability of the Thermospheres of Mars and Earth Sean L. Bruinsma1, Jeffrey M. Forbes2, Jean-Charles Marty1, and Xiaoli Zhang2 1Department of Terrestrial and Planetary Geodesy Centre Nationale D'Etudes Spatiales,Toulouse, France 2Department of Aerospace Engineering Sciences University of Colorado, Boulder, Colorado, US

  2. Previous Conclusions Concerning Exosphere Temperature Responses of the Terrestrial Planets to Long-Term Changes in Solar Flux [Forbes et al., GRL, 2008] • Venus is about 10% as responsive to solar flux changes as Earth, in terms of Texo change per unit change of flux received at the planet • Mars is about 36-50% as responsive as Earth • Plasma-neutral coupling in Earth’s atmosphere may be playing an important role in regulating its response in comparison to Mars. • Role of thermal expansion due to solar radiation absorption by dust on thermosphere variability (not related to solar flux variability) could not be determined with available data. Fall AGU 2007

  3. Probably dominates response at Venus compared to Earth May be playing an important role in relative response between Earth and Mars, at least on dayside Factors Affecting Planetary Response to Solar Flux Received at the Planet (e.g., Bougher et. al, 1999, 2000, 2006) • Heating efficiency • Infrared cooling, e.g., CO2 and O/CO2 ratio • Thermal conductivity (eddy & molecular) • Large-scale circulation (adiabatic heating & cooling) • and plasma-neutral coupling Fall AGU 2007

  4. Solar Cycle Variability of the Thermospheres of Mars and Earth (this study) • Infer thermosphere densities from precise determination of orbit changes (Mars MGS and MO satellites) and accelerometer measurements (Earth, CHAMP satellite) • How did Mars’ thermosphere/exosphere respond to the recent deep solar minimum? • Does dust loading affect Mars’ thermosphere/exosphere?

  5. MGS 81-day mean density @ 380 km 370 x 437 km ~ -40o to-60o lat ~14±1 LT 81-day mean F10.7 solar flux at Mars zonal mean dust optical depth ±30o latitude avg. Courtesy of Michael Smith MO 81-day mean density @ 380 km, 385 x 455 km ~ -55o to-90o lat, ~17±1 or 5±1 LT

  6. Empirical Fit to MO Data: Solar Cycle and Season MO density fit Mars Gram Model

  7. Empirical Fit to CHAMP Data (Earth): Solar Cycle

  8. Solar Flux Dependence of Density at Earth, and at Mars With Effect of Non-Circular Orbit Around the Sun and Seasonal Term Removed ρmax Mars Earth ρmin

  9. Solar Cycle Variability of the Thermospheres of Mars and Earth CONCLUSIONS • The long-term variability of Mars’ thermosphere/exosphere densities near 400 km altitude is mostly described by solar flux and a small seasonal term. • However, there are significant aspects of the observed variability that remain unexplained • The long-term solar flux-related variation in density at Earth is 2.5 times that at Mars • Heating associated with dust in the lower atmosphere does not affect the thermosphere/exosphere of Mars (ca. 400 km), at least at high southern latitudes. • Mars Gram Model seriously underestimates density and density variability at these altitudes. Fall AGU 2007

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