University of Kansas
X-Ray Emission in the Solar System
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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.
The soft x-ray intensities in the Wisconsin survey (or in the ROSAT maps) vary by over a factor of 2 across the sky, with higher intensities typically being found at higher galactic latitudes. Higher interstellar column densities of H in the galactic plane, and the resulting higher x-ray absorption, have been invoked to explain this variation. The spatial morphology of the heliospheric x-ray emission will differ from the interstellar or extra-galactic emission and should depend on the spatial morphology of the interstellar hydrogen and helium within the heliosphere, as well as on the spatial morphology of the solar wind. The volume emission rate for the hydrogen x-ray source should have the same spatial morphology (but different magnitude) as the volume emission rate of Lyman alpha resonantly scattered from interstellar hydrogen, at least for a spherically symmetric solar wind density that varies only as the inverse square of r. The Lyman alpha emission rate maximizes at a heliospheric distance of about 2.5 AU in the upwind direction (Bertaux et al. 1996). Our x-ray volume emission rate, using the simple function presented above, also maximizes at r = l / 2 = 2.5 AU, so that the H part of the x-ray intensity (also see Cox (1998)) should have a "dipole" pattern like the observed Lyman alpha intensity pattern. The Ulysses mission (Phillips et al. 1995; Marsden 1996) has shown that the solar wind speed is about a factor of 2 higher on the average at high heliospheric latitudes than in the ecliptic plane. The solar wind composition, and hence the x-ray emissivity (Neugebauer et al. 1999; Schwadron and Cravens 1999) also varies with heliospheric latitude. A significant effort will be required to combine all the relevant spatial variations and construct a map of predicted heliospheric x-ray intensity. This task is beyond the scope of this initial study.
The heliospheric x-ray emission should be temporally variable due to the temporal variability of the solar wind. X-ray emission from comet Hyakutake and other comets has been observed to vary by factors of 2 to 4 over two-day time periods (Lisse et al. 1996, 1999). This variability has been explained by observed variations of the solar wind proton flux (Neugebauer et al. 1999) and by changes in the solar wind composition (Neugebauer et al. 1999; Schwadron and Cravens 1999). The heliospheric x-ray intensity observed at Earth should not be as variable as the cometary emission because the former originates from a much greater volume of space than does the latter, which will partially "average out" variations.
A rough estimate of how the heliospheric x-ray intensity might vary with time can be obtained from equation (3). Consider a factor of F solar wind flux enhancement over "background" values (e.g., perhaps a corotating interaction region with F = 3) which propagates outward from the Sun at the solar wind speed (usw) and has a radial extent of D r = usw D t. D t is the temporal extent of the enhancement. The term in brackets in equation (3) multiplied by F provides an estimate of the associated relative enhancement of the observed x-ray intensity if we make the identification, rmax = usw t, where t is time, and rmin = rmax - D r. Consider H and He separately. For atomic hydrogen (l = 5), for D r = 1 AU (or 3 days), and for F = 3, the maximum enhancement of I is about 32%, which occurs about 6 days after the enhancement leaves the Sun. The enhancement declines to about 15% after about 2 weeks. For helium (l = 1 AU) we estimate that an F = 3 and D r = 1 AU (3 days) enhancement yields a 70% increase in the He part of I (about 30% change overall) over a time interval of about 1.5 days.
An even more rapid time response of x-ray emission to solar wind variations results from the SWCX mechanism applied to neutral H in the hydrogen geocorona (suggested by Cox (1998) and Freyberg (1998)) beyond the magnetopause (located at a geocentric distance of Rmp = 10 RE where 1 RE is an Earth radius). Our estimate for this contribution is less than Cox's. Only escaping neutrals make it out past Rmp, in which case the H density versus radial distance, R, can be expressed as nH = (RE / R)2 (fH / un), where un is the average neutral speed and fH is the escape flux at the top of the atmosphere. We take un to be the escape speed uescape = 10 km/s and fH = 1.5 x 108 cm-2 s-1 (Shizgal and Arkos 1996). The geocoronal contribution of the SWCX mechanism is 4 p I = b RE nsw (fH / un) = 2 x 10-5 fH = 4 keV cm-2 s-1 or I = 0.3 keV cm-2 s-1 sr-1, which is only about 3% of the heliospheric contribution to the SXRB.
The heliospheric x-ray emission should vary significantly over time periods of a few days to a week, which should distinguish it from the part of the SXRB originating outside the heliosphere (e.g., from the interstellar medium), which is expected to be essentially independent of time. Snowden et al. (1994, 1998) identified several sources of "noncosmic background contamination" in the ROSAT/PSPC data, including an energetic particle-induced background, a solar x-ray scattering background, short-term enhancements (STE) background associated mainly with auroral x-rays, and long-term enhancements (LTE) background which were unexplained. The typical LTE time scale is several days and the lowest two energies (i.e., energies less than about 0.5 keV) are most strongly affected (Snowden et al. 1994). LTE enhancements of about 30% are evident and with a typical time scale of about a week, although shorter and longer time scales are also present (see Figure 2). The previous discussion of the SWCX heliospheric mechanism suggests that an enhancement should have a faster rise time than decay time, and although some of the enhancements in Figure 2 might fall into this category, many others do not. Overall, however, the identification of the ROSAT/PSPC LTEs with heliospheric x-ray emission seems plausible. If the heliospheric x-ray emission can be distinguished from the remaining (e.g., interstellar) part of the soft x-ray background, perhaps by means of LTE-type temporal variations, then measurements of the x-ray background (for different look directions) can be used to remotely monitor the solar wind across the sky.
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Tizby Hunt-Ward tizby@ku.edu |