University of Kansas X-Ray Emission in the Solar System

Draft Temporal Variations of Geocoronal and Heliospheric X-Ray Emission Associated with the Solar Wind Interaction with Neutrals by Cravens et al.

Image: Jovian soft X-rays from ROSAT; courtesy of J. H. Waite.

4. Geocoronal X-Ray Production by the Solar Wind Charge Exchange Mechanism

Solar wind ions can charge transfer with neutral hydrogen atoms in the portion of the Earth's geocorona existing outside the magnetopause. This produces X-rays in the manner described earlier, and the intensity will be a function of both time (due to solar wind variations) and look direction of the X-ray observations, primarily due to the spatial variations of the solar wind flow around the magnetopause surface and secondarily due to spatial variations of the geocoronal neutral densities. Some solar wind plasma might also enter the magnetosphere, particularly in the cusp region [Reiff et al., 1977], but we neglect this contribution to the geocoronal X-ray production. We include the geocoronal contribution in our calculations of the time variation of the X-ray emission in a manner similar to that described by Cravens [2000] but using correct hydrogen densities from the Hodges [1994] model and using a time-varying solar wind. Our calculations are appropriate for observational pointing directions toward the flank magnetopause, which is at a distance from Earth of about R_mp ~= 15 R_E where 1 R_E is 1 Earth radius. This look direction is also consistent with our assumed observation geometry for the heliospheric contribution. An examination of the density profiles presented by Hodges for radial distances from Earth of about r ~= 10 R_E suggests the following simple approximation for the hydrogen density: n_H ~= n_H0 (10 R_E/r)³ with n_H0 = 25 cm^-3.

The X-ray production rate can be determined using this density expression and (2). The solar wind flux in the flank magnetosheath is roughly comparable to the upstream flux. The next step in sophistication would require realistic spatial variations of the solar wind parameters in the magnetosheath as well as a time-dependent magnetopause structure. The integration of the production rate for radial distances greater than the magnetopause distance yields the following X-ray intensity expression for the geocoronal contribution:

where cgs units are assumed. Equation (5) gives an estimate of the geocoronal X-ray intensity which is about a factor of 8 greater than the estimate of Cravens [2000] but which is in agreement with the estimate of Cox [1998] (i.e., ~30 keV cm^-2 s^-1 rather than 4 keV cm^-2 s^-1). The "new" geocoronal contribution is roughly 20-25% of the total heliospheric contribution. However, the geocoronal contribution is more time variable than the total heliospheric contribution to the SXRB, particularly the hydrogen part. Delay or dispersion times between the solar wind observed by IMP-8 near Earth and the X-ray intensity from geocoronal hydrogen are of the order of R_mp/u_sw ~= 300 s, which is a very short time, which we thus neglect in our model.

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Last modified January 8, 2004 Tizby Hunt-Ward tizby@ku.edu