University of Kansas
X-Ray Emission in the Solar System
(DRAFT)
|
|
University of KansasX-Ray Emission in the Solar System |
(DRAFT)
|
Image: Jovian soft X-rays from ROSAT; courtesy of J. H. Waite.
X-rays have been observed from places in the solar system other than the Sun, including the lunar surface, the Earth's and Jupiter's upper atmosphere, and comets. Table 1 summarizes these observations and some of the mechanisms that have been suggested to explain them. Note that the references listed are just a sample and are incomplete. None of these non-solar X-ray sources are hot coronal-type plasma environments but are instead all rather cold neutral environments. For example, the upper atmospheres of Earth and Jupiter have temperatures only between 300 K and 1500 K or so. The moon's surface temperature is only about 300 K, and the temperature of the cometary neutral gas is typically less than about 100 K. In all cases, the X-rays must "derive," or "borrow," their energy from an external environment. For example, the solar corona acts as this external source by producing solar X-rays or the solar wind plasma which can then interact with the colder planetary and cometary environments.
Lunar X-rays are thought to be produced by the scattering or fluorescence of solar X-rays from the Moon's surface (Schmitt et al., 1991). Terrestrial X-rays are thought to be produced by bremsstrahlung collisions of precipitating electrons (with energies of several keV or greater) with atoms and molecules in the high latitude auroral, upper atmosphere (Anderson et al., 1998). These auroral electrons originate in the plasmasheet region of the Earth's magnetosphere and derive their energy in a complex manner from the solar wind interaction with the magnetosphere. Similarly, the emission of X-rays from Jupiter's auroral region is associated with collisions of either energetic ions or energetic electrons that precipitate into the upper atmosphere from the magnetosphere where the particles were energized (Barbosa, 1990, 1992; Waite et al., 1988, 1994). And again, although the cometary environment is known to be quite cold, the X-rays are thought to be produced by the interaction of solar wind ions (or perhaps solar wind electrons) with the neutrals. Solar wind ions originate in the solar corona. Scattering of solar X-rays from cometary dust has also been suggested as a possible mechanism.
Next: Jovian X-Ray Emission
 |
Tizby Hunt-Ward tizby@ku.edu |