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

Spectra of comet Hyakutake at 80-700 Angstrom observed with a resolving pow er of 10 reveal for the first AZ time the emission lines of multiple-charge d ions that are brought to the comet by the solar wind and excited in charg e exchange with cometary neutral species. The most prominent lines are O4lambda 215, C4+ lambda 249, and He+ lambda 304. Some other lines, which are of comparable strength, are blended. The results convincingly prove that t he 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 collisionall y thick parts of comae, and the photon luminosity of charge exchange at ene rgy below 100 eV exceeds that above 100 eV by a factor of 2. However, the e nergetic luminosity below 100 eV is smaller than that above 100 eV by a fac tor 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 t he Jupiter-Neptune region of the solar nebula.