Spectroscopy of comet Hyakutake at 80-700 Å:: First detection of solar wind charge transfer emissions

Spectra of comet Hyakutake at 80-700 Angstrom observed with a resolving power of 10 reveal for the first AZ 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+ lambda 215, C4+ lambda 249, and He+ lambda 304. 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 Angstrom is emitted in a similar process by the solar wind AZ alpha -particles. The quantum yield of charge exchange is approximate to4 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 Angstrom 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.