Spectroscopy of Comet Hyakutake at 80–700 A: First Detection of Solar Wind Charge Transfer Emissions

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Abstract
Spectra of comet Hyakutake at 80-700 Å observed with a resolving power of 10 reveal for the first time the emission lines of multiple-charged ions that are brought to the comet by the solar wind and excited in charge exchange with cometary neutral species. The most prominent lines are O4+ λ215, C4+ λ249, and He+ λ304. Some other lines, which are of comparable strength, are blended. The results convincingly prove that the charge exchange mechanism is the dominant process in excitation of X-ray and EUV emissions from comets. The He+ line at 304 Å is emitted in a similar process by the solar wind α-particles. The quantum yield of charge exchange is ≈4 photons per heavy ion in collisionally thick parts of comae, and the photon luminosity of charge exchange at energy below 100 eV exceeds that above 100 eV by a factor of 2. However, the energetic luminosity below 100 eV is smaller than that above 100 eV by a factor of 2. The O+ lines at 538/539, 617, and 430/442 Å are formed by photoionization of atomic oxygen similarly to those in Earth's dayglow. The observed depletion of neon relative to the solar abundance by more than a factor of 2600 confirms the current view that Oort cloud comets formed in the Jupiter-Neptune region of the solar nebula.