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.
Approximately 0.1% of the solar wind consists of species more massive than helium and these species are found in high charge states (e.g., O+7, O+6, O+5, Fe+13, Fe+12) (cf. Bame et al. 1972). For example, the charge exchange reaction of O+q (where charge states q = 6, 7, 8 are most likely) with a neutral, designated M, is represented by:
The neutral M would be H2O, OH, O, or H in comets, but H or He in the heliosphere. The resulting O+(q-1) ion is almost always highly excited (cf. Wegmann et al. 1998) and, hence, emits an x-ray photon if q is large enough. The relevant cross sections are very large (in excess of 10-15 cm2).
Cravens (1997) devised the following expression for the x-ray power produced per unit volume assuming only one charge transfer collision per solar wind ion:
where nn is the neutral density, nsw is the solar wind density, and usw is the solar wind speed. Cravens (1997) originally estimated that a = 6.3 x 10-17 eV cm2 (or a usw = 2.5 x 10-9 for usw = 400 km s-1) for photon energies greater than 100 eV, and for a 10% excitation efficiency. It has since been pointed out that the excitation efficiency is more like 100%, and recent work indicates that a = 6 x 10-16 - 6 x 10-15 eV cm3 s-1 (Wegmann et al. 1998; Kharchenko and Dalgarno 1999; Schwadron and Cravens 1999).
The solar wind and interstellar neutral densities as functions of r are needed in equation (2). The former is given by the expression nsw = nsw0(r0 / r)2, where r0 = 1.5 x 1013 cm is 1 AU and nsw0 is the solar wind density at 1 AU. An interstellar atomic hydrogen density (far from the Sun) of nH0 = 0.15 cm-3 is adopted (Quemerais et al. 1993; Bertaux et al. 1996). Hydrogen is depleted in the inner solar system due to photoionization by solar radiation, solar radiation pressure, and charge transfer of the H atoms with solar wind protons. This depletion, or attenuation, is much more severe on the downwind side of the heliospheric cavity than on the upwind side (i.e., pointing into the interstellar wind). The following very approximate, but mathematically simple, expression is used for the H density as a function of heliocentric distance, r: nn = nH0exp(-l/r). The attenuation length, l, is related to the neutral flow speed and the lifetime of the H atoms against ionization, charge transfer, etc., but in this paper it is merely used as a parameter chosen (l = 5 AU upwind and 20 AU downwind) to reasonably match Lyman alpha data (e.g., Quemerais et al. 1993). The factor of 2 or so density enhancement associated with the "hydrogen wall" near the heliopause (Fahr 1996) is not included. The density of helium in the local interstellar medium is approximately nHe0 = 0.015 cm-3 (Gloeckler 1996; Mobius et al. 1995). The ionization frequency and proton charge transfer cross section are much less for He than for H, and thus the attenuation length is much less for He than for H. He is actually enhanced in the downwind region due to gravitational focusing (Mobius et al. 1995). An attenuation length for He of l = 1 AU is adopted.
The total heliospheric x-ray luminosity can be estimated by integrating equation (2) over the volume of the heliosphere. A rough luminosity estimate of 1016 W is obtained by assuming spherical symmetry and adopting the values l = 5 AU and nsw0 = 7 cm-3. This is 107 times greater than the x-ray luminosity of comet Hyakutake (Lisse et al. 1996). The part of the heliosphere outside the termination shock could make a comparable contribution to the luminosity inside due to the higher solar wind density in the shocked flow and due to the higher H density in the "wall." The solar wind heavy ion flux should begin to be significantly attenuated in the outer heliosphere (100 - 150 AU) due to the charge transfer reactions.
Next: X-Ray Intensity at the Earth
 |
Tizby Hunt-Ward tizby@ku.edu |