University of Kansas

X-Ray Emission in the Solar System

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


Temporal Variations of Geocoronal and Heliospheric X-Ray Emission Associated with the Solar Wind Interaction with Neutrals

T. E. Cravens and I. P. Robertson

Dept. of Physics and Astronomy, University of Kansas, Lawrence, KS

S. L. Snowden

NASA Goddard Space Flight Center, Greenbelt, MD

(The final version of this paper appeared in Journal of Geophysical Research, 106, 24883, 2001.
Abstract with link to full article on the AGU website.)

Abstract. X-ray emission due to charge transfer collisions between heavy solar wind ions and neutrals has been predicted to exist both in the heliosphere and in the geocorona. The heliospheric X-ray emission can account for roughly half of the observed soft X-ray background intensity. It was also suggested that temporal variations in the heliospheric and geocoronal soft X-ray intensities will result from solar wind variations. In this paper, a simple model of the charge exchange X-ray emission mechanism is combined with measured solar wind parameters as a function of time and used to generate predictions of the temporal variation of the X-ray intensity observed at Earth for the time periods 1990-1993 and 1996-1998. Measured solar wind proton fluxes are also directly compared with the "long-term enhancement" part of the soft X-ray background measured by the Rontgen Satellite (ROSAT). A significant positive correlation exists, which supports the existence of X-ray emission associated with the solar wind interaction with either interstellar neutrals and/or with geocoronal neutral hydrogen.

Acknowledgments: Support from NSF grant ATM-9815574 at the University of Kansas is gratefully acknowledged.

Figure 1. (top) Predicted X-ray intensity versus time at Earth for a 105 s long solar wind enhancement (factor of 10 enhancement over a steady solar wind) for interstellar hydrogen. The intensity scale is arbitrary. (bottom) For interstellar helium.
Figure 2. (top) Measured solar wind proton flux versus time for April/May 1998. Day numbering starts at the beginning of 1996. The solar wind timescale is adjusted so that time refers to time the solar wind left the Sun. (bottom) Predicted soft ( approx. 0.1-1.0 keV) X-ray intensity versus time; total intensity, the hydrogen contribution, the helium contribution, and the geocoronal contribution are shown separately.
Figure 3. Time history of measured solar wind proton flux (solid line) and the LTE part of the ROSAT 1/4 keV channel soft X-ray background count rate for September 28 to October 24, 1990 (dotted line). The solar wind timescale refers to the time of measurement at 1 AU, unlike Figure 2. Note that the LTEs already have the steady background removed. The LTE scale relative to the solar wind flux scale was adjusted to give the best comparison, although the individual scales are correct.
Figure 4. Model X-ray intensity versus time for the same time period as in Figure 3. Individual contributions from heliospheric H and He and from the geocorona are shown. The solar wind data used in the model included "interpolations" from other time periods (see text).
Figure 5. (top) Same time period as Figures 3 and 4. Model total X-ray intensity but with a straight line "background" removed and ROSAT LTE data for this time period are shown. (middle) Same as Figure 5 (top) but for just the heliospheric helium contribution. Note that the scaling factor for the LTE relative to the X-ray intensity is not the same as for the other panels but was adjusted to give a good comparison. (bottom) Same as the middle panel but for just the geocoronal contribution; no background removal was needed, and this contribution is really just a rescaled solar wind proton flux. Note that the scaling factor for the LTE relative to the X-ray intensity is not the same as for the other panels but was adjusted to give a good comparison.
Figure 6. Scatter plot of the daily averaged ROSAT soft X-ray background LTE count rate for the 1/4 keV channel and the hourly averaged measured solar wind proton flux for the time period 1990-1993. The best straight-line fit is shown, and the linear regression coefficient is R = 0.706. The dotted line marked "expected" is the linear relation expected from Figure 3.

Last modified Sept. 11, 2006
Tizby Hunt-Ward