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Sökning: L773:0148 0227 > Forskningsöversikt

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1.
  • Chaufray, J. Y., et al. (författare)
  • Mars solar wind interaction : Formation of the Martian corona and atmospheric loss to space
  • 2007
  • Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 112:E9, s. E09009-
  • Forskningsöversikt (refereegranskat)abstract
    • A three- dimensional ( 3- D) atomic oxygen corona of Mars is computed for periods of low and high solar activities. The thermal atomic oxygen corona is derived from a collisionless Chamberlain approach, whereas the nonthermal atomic oxygen corona is derived from Monte Carlo simulations. The two main sources of hot exospheric oxygen atoms at Mars are the dissociative recombination of O-2(+) between 120 and 300 km and the sputtering of the Martian atmosphere by incident O+ pickup ions. The reimpacting and escaping fluxes of pickup ions are derived from a 3- D hybrid model describing the interaction of the solar wind with our computed Martian oxygen exosphere. In this work it is shown that the role of the sputtering crucially depends on an accurate description of the Martian corona as well as of its interaction with the solar wind. The sputtering contribution to the total oxygen escape is smaller by one order of magnitude than the contribution due to the dissociative recombination. The neutral escape is dominant at both solar activities ( 1 x 10(25) s(-1) for low solar activity and 4 x 10(25) s(-1) for high solar activity), and the ion escape flux is estimated to be equal to 2 x 10(23) s(-1) at low solar activity and to 3.4 x 10(24) s(-1) at high solar activity. This work illustrates one more time the strong dependency of these loss rates on solar conditions. It underlines the difficulty of extrapolating the present measured loss rates to the past solar conditions without a better theoretical and observational knowledge of this dependency.
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2.
  • Tomasi, C., et al. (författare)
  • Aerosols in polar regions : A historical overview based on optical depth and in situ observations
  • 2007
  • Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 112:D16, s. D16205-
  • Forskningsöversikt (refereegranskat)abstract
    • Large sets of filtered actinometer, filtered pyrheliometer and Sun photometer measurements have been carried out over the past 30 years by various groups at different Arctic and Antarctic sites and for different time periods. They were examined to estimate ensemble average, long-term trends of the summer background aerosol optical depth AOD(500 nm) in the polar regions ( omitting the data influenced by Arctic haze and volcanic eruptions). The trend for the Arctic was estimated to be between -1.6% and -2.0% per year over 30 years, depending on location. No significant trend was observed for Antarctica. The time patterns of AOD( 500 nm) and angstrom ngstrom's parameters a and beta measured with Sun photometers during the last 20 years at various Arctic and Antarctic sites are also presented. They give a measure of the large variations of these parameters due to El Chichon, Pinatubo, and Cerro Hudson volcanic particles, Arctic haze episodes most frequent in winter and spring, and the transport of Asian dust and boreal smokes to the Arctic region. Evidence is also shown of marked differences between the aerosol optical parameters measured at coastal and high-altitude sites in Antarctica. In situ optical and chemical composition parameters of aerosol particles measured at Arctic and Antarctic sites are also examined to achieve more complete information on the multimodal size distribution shape parameters and their radiative properties. A characterization of aerosol radiative parameters is also defined by plotting the daily mean values of a as a function of AOD( 500 nm), separately for the two polar regions, allowing the identification of different clusters related to fifteen aerosol classes, for which the spectral values of complex refractive index and single scattering albedo were evaluated.
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