The contribution of charge exchange to extreme ultra-violet and X-ray astronomy

Charge exchange (also referred to also charge transfer or electron capture) has been studied by the atomic collisions community for many years. Apart from the fundamental interest in the subject, much of the research has been driven by the needs of the fusion community where it plays an important role in radiative heat loss and diagnostics in tokamak plasmas. It is therefore surprising that it has generally not been applied as extensively in astrophysical plasmas. Atomic data for astrophysics includes comprehensive data for electron process but little for charge exchange, due to the fact that the data does not exist or is not considered important. However, it has been shown in the last few years that charge exchange is the dominant mechanism for X-ray production for a number of newly discovered X-ray sources in the solar system. These results have demonstrated the need to understand this mechanism for the particular species of interest. Recent experimental work in our laboratory, complimented by other groups, has shown that the relative intensities of these X-ray and extreme ultra-violet lines can be highly dependent on the nature of the neutral target and the collision velocity.