University of Kansas

X-Ray Emission in the Solar System

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

X-Ray Emission from the Outer Planets: Albedo for Scattering and Fluorescence of Solar X-Rays

T. E. Cravens, J. Clark, A. Bhardwaj, R. Elsner, J. H. Waite Jr., A. N. Maurellis, G. R. Gladstone and G. Branduardi-Raymont

(The final version of this paper was published in Journal of Geophysical Research, 111, A07308, doi:10.1029/2005JA011413, 2006.)

PDF of final submission version
Abstract with link to full article on the JGR website.

Abstract. Soft x-ray emission has been observed from the low-latitude "disk" of both Jupiter and Saturn as well as from the auroral regions of these planets. The disk emission as observed by ROSAT, the Chandra X-Ray Observatory, and XMM-Newton appears to be uniformly distributed across the disk and to be correlated with solar activity. These characteristics suggest that the disk x-rays are produced by: (1) the elastic scattering of solar x-rays by atmospheric neutrals and (2) the absorption of solar x-rays in the carbon K-shell followed by fluorescent emission. The carbon atoms are found in methane molecules located below the homopause. In this paper we present the results of calculations of the scattering albedo for soft x-rays. We also show the calculated x-ray intensity for a range of atmospheric abundances for Jupiter and Saturn and for a number of solar irradiance spectra. The model calculations are compared with recent x-ray observations of Jupiter and Saturn. We conclude that the emission of soft x-rays from the disks of Jupiter and Saturn can be largely explained by the scattering and fluorescence of solar soft x-rays. We suggest that measured x-ray intensities from the disk regions of Jupiter and Saturn can be used to constrain both the absolute intensity and the spectrum of solar x-rays.


Figure 1. Elastic scattering (dotted lines) and absorption cross sections as a function of wavelength for H, He, and C. From the NIST tabulations [Chantler, J. Phys. Chem. Ref. Data, 1995].
Figure 2a. Elastic scattering albedo for Jupiter and Saturn versus wavelength. The scattering angle is assumed to be 180 deg. (appropriate for the Earth and the planet being in opposition). Photon energy is also shown on the top scale.
Figure 2b. Albedo as a function of wavelength for carbon K-shell fluorescence from Jupiter. Photon energy is also shown on the top scale.
Figure 3. Elastic scattering albedo versus the fractional He to H2 abundance for Saturnian methane abundance (CH4/H2 = 0.0025) (the results for the Jovian methane abundance are almost the same). The albedo is shown for 3 wavelengths as noted.
Figure 4. Elastic scattering albedo versus the methane abundance for a Saturnian helium abundance (He/H2 = 0.06).
Figure 5. Solar irradiance spectra at 1 AU for low solar activity (denoted "low activity flux B" spectrum in the text.) Note: 1 angstrom = 0.1 nm.
Figure 6. Scattered Jovian and Saturnian x-ray intensities (normalized for 1 AU) versus photon energy at high resolution. The spectrum does not include the carbon K-shell line intensities from the fluorescence mechanism. The intensity points are "per bin." The inset is an expanded view of the low energy spectra.
Figure 7. Scattered Jovian x-ray intensity (normalized for 1 AU) versus photon energy at 50 eV-resolution for 2 different low solar activity solar fluxes, as well as the high solar activity case ("solar max"). Each bin is 50 eV wide for this figure. The spectra do not include the carbon K-shell line intensities from the fluorescence mechanism. The two "gaps" near 1 keV and 1.1 keV are due to the lack of solar intensity points for these energy intervals rather than due to any intrinsic structure in the spectrum.
Figure 8. Comparison of measured and modeled disk x-ray spectra for Jupiter. The modeled count rates are for the solar "low activity flux A" case and are generated by convolving our model with the CXO ACIS-S instrumental response. The model includes the carbon K-shell line intensities. The data shown are from CXO ACIS-S measurements of Jupiter's disk (i.e., auroral regions are excluded) during February 2003 (see the paper in preparation by Bhardwaj et al. for details of the observations; the auroral data from this same set of CXO observations are described by Elsner et al. [JGR, 2005]).
Figure 9. The CXO data is the same as in Figure 8 -- CXO ACIS spectrum of the Jovian disk (auroral regions excluded), but for this figure the comparison is with a MEKAL collisional plasma model intensity multiplied by the Jovian x-ray scattering albedo and convolved with the instrumental response function.

Last modified Oct. 18, 2006
Tizby Hunt-Ward