The heliosphere is a bubble of hot gas in interstellar space stretching from the Sun to greater than 100 Sun-Earth distances. It contains some fraction of inflowing neutral interstellar hydrogen and helium atoms. Highly charged positive ions emanate from the Sun in the solar wind and impact the neutral material. The ions capture electrons from the atoms into high excited states that radiate primarily in the soft X-ray spectral region. The spectra provide information about the solar wind composition and velocity and the distribution of atoms in the heliosphere. The X-rays generated by the solar wind contribute to the diffuse soft X-ray background. Similar processes occur in the interaction of the solar wind ions with comets and with the extended exospheres of the planets. We are engaged in theoretical efforts to predict the spectra resulting from the collisions of the solar wind ions (recent references).
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V. Kharchenko and A. Dalgarno