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University of KansasX-Ray Emission in the Solar System |
(DRAFT)Temporal and Spatial Variations of Heliospheric X-Ray Emissions Associated with Charge Transfer of the Solar Wind with Interstellar Neutrals I. P. Robertson, T. E. Cravens, and S. Snowden |
Image: Jovian soft X-rays from ROSAT; courtesy of J. H. Waite.
In 1996 X-ray emission from the comet Hyakutake was discovered (Lisse et al. 1996). Cravens (1997) proposed that this emission could be explained by charge exchange collisions between heavy solar wind ions and cometary neutrals. The product ion is left in an excited state and eventually emits a photon in the X-ray or EUV region of the spectrum (Cravens 2002). Cox (1998) suggested that this same process could be applied to interaction between solar wind ions and interstellar neutrals, or neutrals in the Earth's geocorona. and might be able to explain some of the temporal variation in the observed soft X-ray background (SXRB). Dennerl et al. (1997) even suggested that this charge exchange mechanism might be able to explain the Long Term Enhancement (LTE) part of the SXRB. Cravens (2000) consequently created a simple model of the heliospheric x-ray emissions and concluded that about half of the SXRB could be explained by this mechanism. Robertson (2001) and Cravens (2001) slowly increased the complexity of the model and found a significant correlation between solar wind proton fluxes and LTE X-ray intensities.
Originally, the interstellar neutral density was approximated with a simple mathematical formula. The model has been improved upon by using Fahr's (1971) hot model for interstellar neutrals, as described in the current paper. Time independent maps were created and time dependent behavior was also studied.
The following equation was used by Cravens (1997, 2000, 2001) to calculate the X-ray and EUV power density:
Px-ray = alpha*nnnsw usw (eV cm-3 s-1)
where alpha contains all the atomic cross sections, the transition information and solar wind heavy ion composition. Alpha is different for interstellar helium and hydrogen and also varies with solar wind speed. For the time being we have used the same slow solar wind value of alpha for both helium and hydrogen, although the helium value should probably be somewhat less than the hydrogen value. The value used is: alpha approx.= 6 x 10-16 eV cm 2. The solar wind density is denoted as nsw and the solar wind speed as usw. The solar wind density is presently considered to be spherically symmetric and its dependence on radial distance (r) is as follows: nsw = nsw0 (ro/r)2, where ro is 1 AU and nsw0 is the solar wind density at 1 AU.
The interstellar neutral density (a combination of interstellar helium and hydrogen) is denoted as nn. Originally the helium and hydrogen densities were approximated by a simple mathematical formula. Fahr's hot model has been adopted to better calculate these densities. In this model gravitational focusing, ionization losses and radiation pressure are taken into account. Due to these factors the neutral hydrogen density is lower in the downwind direction from the sun than in the upwind direction. For helium, however, gravitational focusing produces a cone of enhanced helium abundance downwind from the sun near 1 AU. The unperturbed interstellar hydrogen density is taken to be 0.1 cm-3 and that of helium to be 0.02 cm-3. Fahr (1971) noted that even though the helium density is initially much smaller than that of interstellar hydrogen, its density reaches the same order of magnitude as the hydrogen density 1 AU on the upwind side. On the downwind side the density can be more than an order of magnitude larger than the helium density.
The production rate, integrated over a path length s, starting at Earth and going out to 200 AU, yields the X-ray intensity. Solar wind ions can also charge transfer with neutral hydrogen in the Earth's geocorona outside the magnetopause and produce X-rays. For the time being a simple mathematical formula is used to estimate this geocoronal intensity:
where Rmp approx.= 15 RE is the magnetopause distance from the Earth in the flanks and nH0 = 25 cm-3 is a reference value of the exospheric hydrogen density at 10 RE. The results are in agreement with an estimate of Cox (1998).
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Tizby Hunt-Ward tizby@ku.edu |