Modeling of Cometary X-rays
Caused by Solar Wind Minor Ions

Roman M. Haberli, Tamas I. Gombosi, Darren L. De Zeeuw, Michael R. Combi
 Space Physics Research Laboratory, University of Michigan, Ann Arbor
 Kenneth G. Powell
 Department of Aerospace Engineering, University of Michigan, Ann Arbor

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ABSTRACT:

X-ray emission was discovered in comet Hyakutake (C/1996 B2) by the Rontgen satellite in 1996, and these emissions were attributed to the excitation of high charge state solar wind ions due to electron capture from cometary molecules or atoms. Using the plasma flow in the coma of Hyakutake calculated by a three-dimensional adaptive magneohydrodynamic model, the density distribution of solar wind ions in the coma and the resulting x-ray emission were computed. The calculated High Resolution Imager count rate of 4.4 per second and the spatial distribution of the x-ray emission agree with the observations. A detailed energy spectrum of cometary x-rays is predicted in the 80 electronvolt (eV) to 2 kiloelectronvolt (keV) energy range. Cometary x-rays present a new and sensitive tool to monitor cometary activity and solar wind ion composition.

FIGURES:


Figure 1. Plasma flow field in the coma of Hyakutake on 27 March 1996, as computed by the 3D MHD model. Solar wind streamlines originating from the Sun, which is to the left in the figure, are shown as white and purple lines. The yellow line shows the location of the bowshock. Color code gives logarithm of plasma mass density, increasing from blue to red. The bulb in the center of the figure (also on the inset) shows a surface of constant plasma density. Scales are given in 106 km.


Figure 2. Image (A) of the x-ray emission from Hyakutake, measured by the ROSAT HRI on 27 March 1996. Computed image (B) of sum of all emission lines in the HRI passband caused by the charge exchange of solar wind ions with cometary gas. In both images, the x-ray emission shows a crescent shape and the maximum of the emission is displaced Sunward from the nucleus. The scale of both images is the same.


Figure 3. Measured (diamonds) and computed profile (solid line) of x-ray intensity for the WFC. The computed results have been scaled to the same integrated brightness as the measurements from 27.8 March 1996. The dashed lines show the computed profiles when the position of the nucleus is shifted sunward by 15,000 km (approximate resolution of WFC). We assumed that the measurement error is 10% of the peak value which approximately corresponds to the noise level in the observations. (Top) Shows the profile perpendicular to the sun-comet line and (Bottom) shows the profile along the sun-comet line. The direction of the sun in noted by the arrow.


Figure 4. Calculated x-ray energy spectrum in the 80 eV-2 keV range. We note that the spectrum is fairly universal and it is expected to change only moderately from comet to comet. The location of the spectral lines and their intensity is primarily controlled by the solar wind ion composition.