X-ray emissions from cometary atmospheres were modeled from first principles using the charge-exchange interaction with solar wind ions as well as coherent scattering of solar X-rays from dust and ice grains .
Scattering cross-sections were interpolated over the 1 nm–1 cm grain radius range using approximations based on the optically thin or thick nature of grains with different sizes .
The theoretical emission model was compared to Chandra observations of Comets ISON and Ikeya–Zhang due to their high signal-to-noise ratios and clearly defined spectral features .
Comparing the observed intensities to the model showed that the charge-exchange mechanism accurately reproduced the emission spectra below 1 keV , while dust and ice scattering was negligible .
Examining the 1–2 keV range found dust and ice scattering emissions to agree well with observations , while charge-exchange contributions were insignificant .
Spectral features between the scattering model and observations also trended similarly over the 1–2 keV range .
The dust and ice density within the cometary atmosphere n was varied with respect to grain size a as the function n ( a ) \propto a ^ { - \alpha } , with Ikeya–Zhang requiring \alpha = 2.5 and ISON requiring \alpha = 2.2 to best fit the observed spectral intensities .
These grain size dependencies agreed with independent observations and simulations of such systems .
The overall findings demonstrate evidence of significant scattering emissions present above 1 keV in the analyzed cometary emission spectra and that the dust/ice density dependence on grain radius a may vary significantly between comets .