University of Kansas

X-Ray Emission in the Solar System

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


Heliospheric X-Ray Emission Associated with Charge Transfer of the Solar Wind with Interstellar Neutrals

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 Astrophysical Journal Letters, 532, L153, 2000.)

ABSTRACT. X-rays should be generated throughout the heliosphere as a consequence of charge transfer collisions between heavy (Z > 2) solar wind ions and interstellar neutrals. The high charge state solar wind ions resulting from these collisions are left in highly excited states and emit extreme ultraviolet (EUV) or soft x-ray photons. This solar wind charge exchange mechanism applied to cometary neutrals has been used to explain the soft x-ray emission observed from comets. A simple model demonstrates that heliospheric x-ray emission can account for about 25 - 50% of the observed soft x-ray background intensities. The spatial and temporal variations of heliospheric x-ray emission should reflect variations in the solar wind flux and composition as well as variations in the distribution of interstellar neutrals within the heliosphere. The heliospheric x-ray "background" can perhaps be identified with the "long term enhancements" in the soft x-ray background measured by ROSAT. Heliospheric x-rays can potentially be used to remotely monitor the solar wind flux and composition.

Acknowledgments. Support from NASA Planetary Atmospheres grant NAG5-4358 and NSF grant ATM-9815574 is gratefully acknowledged.

Figure 1. X-ray intensity spectrum calculated for the solar wind charge exchange mechanism applied to heliospheric neutrals (diamonds) and all-sky averaged data (triangles) from the Wisconsin survey of the x-ray background (McCammon & Sanders 1990).
Figure 2. X-ray intensity for a particular 20 degree scan from the ROSAT/PSPC instrument for the 1/4 keV channel versus orbit number. One orbit is 100 minutes and the plot extends over 50 days. Reproduced from Snowden et al. (1994).

Last modified January 9, 2004
Tizby Hunt-Ward