Planetary Magnetic Fields and Solar Forcing: Implications for Atmospheric Evolution

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• 作者：Rickard Lundin ; Helmut Lammer and Ignasi Ribas
• 关键词：Planetary magnetospheres ; Solar forcing ; Young Sun ; Ionospheric plasma escape ; Loss of planetary water
• 刊名：Space Science Reviews
• 出版年：2007
• 出版时间：March, 2007
• 年：2007
• 卷：129
• 期：1-3
• 页码：245-278
• 全文大小：1,084 KB
• 刊物类别：Physics and Astronomy
• 刊物主题：Physics
  Astronomy
  Extraterrestrial Physics and Space Sciences
  Astrophysics
  Planetology
  
• 出版者：Springer Netherlands
• ISSN：1572-9672

文摘

The solar wind and the solar XUV/EUV radiation constitute a permanent forcing of the upper atmosphere of the planets in our solar system, thereby affecting the habitability and chances for life to emerge on a planet. The forcing is essentially inversely proportional to the square of the distance to the Sun and, therefore, is most important for the innermost planets in our solar system—the Earth-like planets. The effect of these two forcing terms is to ionize, heat, chemically modify, and slowly erode the upper atmosphere throughout the lifetime of a planet. The closer to the Sun, the more efficient are these process. Atmospheric erosion is due to thermal and non-thermal escape. Gravity constitutes the major protection mechanism for thermal escape, while the non-thermal escape caused by the ionizing X-rays and EUV radiation and the solar wind require other means of protection. Ionospheric plasma energization and ion pickup represent two categories of non-thermal escape processes that may bring matter up to high velocities, well beyond escape velocity. These energization processes have now been studied by a number of plasma instruments orbiting Earth, Mars, and Venus for decades. Plasma measurement results therefore constitute the most useful empirical data basis for the subject under discussion. This does not imply that ionospheric plasma energization and ion pickup are the main processes for the atmospheric escape, but they remain processes that can be most easily tested against empirical data. Shielding the upper atmosphere of a planet against solar XUV, EUV, and solar wind forcing requires strong gravity and a strong intrinsic dipole magnetic field. For instance, the strong dipole magnetic field of the Earth provides a “magnetic umbrella”, fending of the solar wind at a distance of 10 Earth radii. Conversely, the lack of a strong intrinsic magnetic field at Mars and Venus means that the solar wind has more direct access to their topside atmosphere, the reason that Mars and Venus, planets lacking strong intrinsic magnetic fields, have so much less water than the Earth?