Chandra Observations of Comet 2P/Encke 2003: First Detection of a Collisionally Thin, Fast Solar Wind Charge Exchange System

We report the results of 15 hr of Chandra observations of comet 2P/Encke 2003 on November 24. X-ray emission from comet Encke was resolved on scales of 500-40,000 km, with unusual morphology due to the presence of a low-density, collisionally thin (to charge exchange) coma. A light curve with peak-to-peak amplitude of 20% consistent with a nucleus rotational period of 11.1 hr was found, further evidence for a collisionally thin coma. We confirm emission lines due to oxygen and neon in the 800-1000 eV range but find very unusual oxygen and carbon line ratios in the 200-700 eV range, evidence for low-density, high effective temperature solar wind composition. We compare the X-ray spectral observation results to contemporaneous measurements of the coma and solar wind made by other means and find good evidence for the dominance of a postshock bubble of expanding solar wind plasma, moving at 600 km s(-1) with charge state composition between that of the "fast'' and "slow'' solar winds.

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