University of Kansas
X-Ray Emission in the Solar System
X-Ray Emission from the Terrestrial Magnetosheath
by Robertson and Cravens
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University of KansasX-Ray Emission in the Solar System |
Draft X-Ray Emission from the Terrestrial Magnetosheath by Robertson and Cravens |
Image: Jovian soft X-rays from ROSAT; courtesy of J. H. Waite.
We now consider time variations of the geocoronal X-ray emission due to solar wind variations. The geocoronal X-ray intensities from the SWCX mechanism should scale with solar wind flux (see equation (1)) as predicted by Cravens et al. [2001] and Robertson et al. [2001], if the magnetopause stays fixed. However, an increasing solar wind dynamic pressure moves the magnetopause closer to the Earth, allowing the solar wind to enter regions with higher hydrogen densities. This effect was not included by Cravens et al. [2001] and Robertson et al. [2001]. According to the Chapman-Ferraro theory [cf. Chapman and Ferraro, 1931] the relationship between the location of the magnetopause and the solar wind dynamic pressure (rsw u2 sw, where the mass density is rsw ~=nsw mp), is:
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(4) |
BE is the equatorial magnetic field at the Earth's surface, mo is the permeability of free space, Rmp is the subsolar magnetopause distance, and RE is an Earth radius.
We numerically determined X-ray intensities for a variety of solar wind densities ranging from 0.3 =< rsw/r sw0 =< 10, and for a view direction through the magnetospheric flank (corresponding with the most probable ROSAT look direction). We divided each X-ray intensity by the upstream proton flux used and normalized the result with respect to the reference flux rsw0 usw0. The ratio of X-ray intensity corrected for magnetopause distance and the "linear intensity" was found to vary as (rsw u2sw/ rsw0u2 sw0)1/3, which is just the functional dependence one expects from equation (4), and a geocoronal H density which varies as R-3.
Figure 3 is a more accurate recalculation of the heliospheric X-ray intensities with interstellar helium and hydrogen contributions, using an updated heliospheric neutral model, a smaller a for He (50% of H), and the X-ray emission for geocoronal hudrogen (as discussed in the current paper). Note that Figure 3 includes the non-linear magnetopause effect. The interstellar H contribution exhibits little variability due to its large (many AU) emitting volume [see Cravens et al., 2001].
Figure 4 is an expanded view of a portion of Figure 3. Most of the time there is only a slight difference between the intensities with and without the non-linear contribution. The greatest difference is in the "higher" peaks, when the solar wind density goes up to as much as 53 cm-3 or when the solar wind speed increases to about 780 km/s. The X-ray intensities can then be double what the "linear model" would predict, due to a drastically reduced magnetopause distance.
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Figure 3. Heliospheric X-ray intensities similar to those in Cravens et al. [2001], but using an improved interstellar neutral model and a different a. The look direction is north of the ecliptic plane. A look direction in the plane would increase the linear intensities by a factor of 1.4 [Hodges, 1994]. Day numbers start at Jan. 1, 1996. |
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Figure 4. Closeup of the spiked region of the geocoronal X-ray intensities in Figure 3. The total intensity differs dramatically from the linear intensity due to a much large solar wind flux which compresses the magnetosphere. |
Next: Discussion
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Tizby Hunt-Ward tizby@ku.edu |