X-ray velocimetry of solar wind ion impact on comets

Laboratory measurements of the interaction of low-energy, bare, and hydrogen-like ions with neutral gases are presented. The measurements demonstrate that charge-exchange-induced cometary K-shell X-ray spectra represent rich spectral diagnostics for determining the speed of the solar wind and the collision dynamics within the coma. We show that the K-shell spectrum observed from low-energy ion-neutral collisions (less than or equal to 50 km s(-1)) has a distinct high-energy component that is suppressed in high-energy collisions (greater than or equal to 800 km s(-1)). As a result, the hardness ratio of the K-shell spectrum increases by as much as a factor of 4 as the ions decelerate in the coma. The change in spectral shape can be observed even with low-resolution energy dispersive solid-state detectors, opening the possibility of spatial imaging of the solar wind heavy-ion velocity profile in the coma. Our results clearly show that energy-dependent data are needed to fully describe charge-exchange-induced Xray production in the heliosphere.