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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.
Figure 1 shows the heliospheric X-ray intensities in the equatorial plane for a solar wind speed of 400 km/s, a solar wind density with no = 7/cm3, and neutral densities from Fahr's hot model. The location of the Earth is at the vernal equinox, as indicated on the small insert in the figure. The direction of the interstellar wind is also indicated with the large arrow. The coordinates used are Earth-centered solar ecliptic; consequently, zero degrees points towards the sun (an area blocked out in the graph), -90 deg. points into the interstellar wind and +90 deg. points towards the tail. There is little variation in the X-ray intensity due to charge exchange with interstellar hydrogen. The contribution due to helium varies much more, with a significant enhancement near 30 deg. when the look direction intersects with the "helium cone" due to gravitational focusing. Note, though, that a more careful determination of alpha for helium could reduce these intensities by as much as a factor of 2.
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Figure 1. Time independent variation with ecliptic longitude of heliospheric x-ray intensities for look directions in the ecliptic plane. See text for details. |
Time dependent X-ray intensities were calculated for fixed look directions. We used solar wind proton fluxes measured by the IMP-8 spacecraft for the time period 1996-1998. The day number is the number of days after January 1, 1996. Once again the solar wind proton flux is assumed to be spherically symmetric and the flux decreases as 1/r2. Again Fahr's hot model is used to model the interstellar neutral density. Figures 2 and 3 show the X-ray intensity due to helium, hydrogen and the Earth's geocorona. The total intensity is also plotted. Cravens et al. (2001) show a plot for the same time period, but it was produced without the use of Fahr's model. Two cases were considered: the Earth located in the upwind direction (summer) and the Earth located in the downwind direction (winter). In both cases depicted, the look direction is northward from the ecliptic plane. In Figure 2, the Earth is in the downwind direction (winter solstice). Because the Earth is immersed in the helium cone, the dominant contribution, both in variation as well as intensity, is from helium. The variation in hydrogen is not noticeable at this scale. Even though the geocoronal contribution is minimal, it does contribute to the variation in total intensity. In Figure 3, the Earth is in the upwind direction (summer solstice). The hydrogen contribution is dominant but has very little variation. More variation can be seen in the helium contribution and even more in the contribution from the geocorona. It should be noted that these plots were generated using identical alphas for the calculations of the helium as well as the hydrogen contributions. As noted earlier, that is probably not the case. It can be noticed, however, that even if the alpha for helium would be smaller by a factor of 2, the helium continues to be dominant in the X-ray variability.
 | Figure 2. X-ray intensity at winter solstice versus time. Earth is located in the downwind direction. |
 | Figure 3. X-ray intensity at summer solstice versus time. Earth is located in the upwind direction. |
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Tizby Hunt-Ward tizby@ku.edu |