Catching some Sun:: Probing the solar wind with cometary X-ray and far-ultraviolet emission

Strong X-ray and far-ultraviolet emission from comets is the direct result of charge exchange reactions of solar wind ions with the neutral coma of comets. Here we report experimental state-selective cross sections of electron capture and use these to predict cometary line emission. Our results show that helium line ratios are a direct diagnostic of the solar wind velocity, while their absolute intensities are linked to the local density of the solar wind. Comparison with observations of Hale-Bopp yields a velocity of 220 km s(-1) and a helium density of 0.6 cm(-3), and observations of Hyakutake show that it interacted with a faster wind of 375 km s(-1) and a helium density of 10(-3) cm(-3). The potential of line emission by heavy minor ions is also demonstrated. Spectral lines with energies greater than 650 eV contain a wealth of information on the original composition of the wind, while lines at lower energies are used best to study the interaction processes themselves.