University of Kansas
X-Ray Emission in the Solar System
(DRAFT)
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University of KansasX-Ray Emission in the Solar System |
(DRAFT)
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Image: Jovian soft X-rays from ROSAT; courtesy of J. H. Waite.
This paper has so far discussed X-ray emission observed from comets, the Earth, Jupiter, and the Moon. What about X-ray emission from the other planets or objects in the solar system? Consider just the X-rays that can be generated by the solar wind charge transfer mechanism. This mechanism should operate wherever the solar wind interacts directly with neutral gases.
Planets with only small intrinsic magnetic fields, such as Venus and Mars, are obvious candidates for this mechanism because the solar wind interacts directly with the upper atmospheres and ionospheres of such planets (cf. Luhmann and Cravens, 1991). The boundary between the solar wind and the ionosphere is called the ionopause, and the X-ray luminosity can be estimated by integrating the X-ray production rate given by equation (4) over altitude, starting at the ionopause. The ionopause is located at about 500 km on average for Venus, but this varies with solar wind dynamic pressure (Brace and Kliore, 1991). The main neutral species relevant here is the atomic oxygen in the hot oxygen corona (Nagy and Cravens, 1988). Using an exobase hot O density of about 105 cm-3, a shocked solar wind density of about 30 cm-3, and integrating equation (4) over the dayside hemisphere, a total X-ray luminosity of 105 W is obtained. For Mars, a similar estimate yields about 104 W. The X-rays from this mechanism will be emitted from a narrow crescent surrounding the dayside hemisphere of each planet. These estimates were for average solar wind conditions, but when a coronal mass ejection (CME) encounters Venus or Mars, greatly enhanced X-ray emission is expected because the solar wind flux is greatly enhanced and the ionopause is located at much lower altitudes (below 300 km for Venus). A lower ionopause is relevant because the solar wind gains access to altitudes where the neutral density is much higher. The X-ray luminosity in the event of a CME could be temporarily enhanced by at least a factor of 10, giving an X-ray power for Venus of more than 1 MW (private communication -- Waite, Elsner, Gladstone, and Cravens). Another potential source of X-rays from Venus and Mars, which should be evaluated, is the fluorescence associated with K-shell absorption by carbon atoms in the atmospheric CO2 molecules. This type of mechanism was mentioned earlier for Jupiter.
Interstellar neutral H and He atoms are streaming into the heliosphere where they can interact with the solar wind (see review by Suess, 1990). The interstellar H, detected via resonantly scattered solar Lyman alpha, has a density of about 0.1 cm-3 but is attenuated within a few astronomical units (AU) from the Sun due to photoionization by solar radiation. We can again apply equation (4) for the charge transfer mechanism by using a solar wind density falling off as the square of the heliospheric distance and adopting an interstellar H density of 0.1 cm-3, plus a suitable cutoff at small heliocentric distances. Integrating the resulting volume emission rate over the entire heliosphere, one obtains a total soft X-ray luminosity of about 1016 W. Of course, the X-rays are emitted from the entire sky and become part of the soft X-ray background which has been observed by the Einstein Observatory as well as ROSAT (e.g., McCammon and Sanders, 1990). The portion of the observed X-ray background which is attributable to this heliospheric charge transfer source needs to be determined (Cox, 1997).
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Tizby Hunt-Ward tizby@ku.edu |