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 Comets and Planets

T. E. Cravens
University of Kansas, Department of Physics and Astronomy, Lawrence, KS 66045, U.S.A.

The final version of this paper appeared in Advances in Space Research, 26, 1443, 2000.
Abstract and link to complete article through ScienceDirect.

ABSTRACT. X-ray emission has been observed from many objects throughout the solar system including the Sun, the Moon, the Earth, Jupiter, and comets. A brief review of these observations and some of the emission mechanisms suggested to explain the observed X-rays are given in this paper. Cometary X-ray emission and Jovian X-ray emission will be emphasized. The initial discovery of X-rays from comet Hyakutake (Lisse et al., 1996) was surprising and a number of explanations were put forth, including bremsstrahlung associated with hot electron collisions with cometary neutrals or ions, scattering or fluorescence of solar X-rays from cometary neutrals or from dust particles, and charge transfer of heavy solar wind ions with neutrals. X-rays have also been observed both from Jupiter's auroral region as well as from low latitudes (Waite et al., 1997a). Again, a number of emission mechanisms have been proposed, including precipitation of energetic heavy ions from the magnetosphere. Predictions of X-ray emission from Venus, Mars, and from interstellar neutrals in the heliosphere are made.

Acknowledgments. Support from NASA Planetary Atmospheres grant NAG5-4358 and NSF grant ATM-98-15574 is gratefully acknowledged. The Kansas Center for Scientific Computing (NSF EPSCOR/KSTAR) and NASA EPSCOR/KTEC NCC5-168 are also acknowledged.

Figure 1. Temperature versus height in solar atmosphere. The solar corona is to the right of the dashed line. Adapted from Goldberg (1969).
Figure 2. ROSAT X-ray counting rate versus photon energy for Jupiter (Waite et al., 1994) and a spectrum from a theoretical model. From Cravens et al. (1995).
Figure 3. Photon emission rate per precipitating oxygen ion at Jupiter versus photon energy. From Kharchenko et al. (1998).
Figure 4. Data points for ROSAT X-ray intensity measured from comet Hyakutake versus radial distance along the sun-comet line and at right angles. The solid and dashed lines are model results. From Haberli et al. (1997).
Figure 5. X-ray emission rate versus photon energy for comet Hyakutake adapted from a theoretical calculation by Wegmann et al. (1998). Each point represents the emission rate for a specific line.

Last modified Sept. 11, 2006
Tizby Hunt-Ward