Statistics
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sh21a05k&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #SH21A-05 INVITED
Statistics
2164 Solar Wind Plasma, 2780 Solar Wind Interactions With Unmagnetized Bodies, 6025 Interactions With Solar Wind Plasma And Fields, 6210 Comets
Scientific paper
We analyzed 8 observations of comets with the Extreme Ultraviolet Explorer (EUVE). A soft X-ray camera in the range of 97-165 eV FWHM with a peak effective area of 28 cm2 and three spectrometers at 80-180, 170-360, and 300-720 Å with peak effective areas of 2.1, 0.5, and 0.8 cm2, respectively, were used for those observations. The detection limit of the X-ray camera corresponds to the X-ray luminosity of 1.9x 1014 Δ 2 erg s-1 for photon energy ɛ > 100 eV. (Δ is the geocentric distance in AU.) Five comets were detected with the X-ray camera: Hyakutake, Borrelly, d'Arrest, pre- and postperihelion Hale-Bopp. Their images reveal a crescent-like structure with peak brightness offsets from the nuclei between the sunward and comet orbital velocity directions. X-ray luminosities and their spatial distributions were determined from the observations. The measured luminosities are in excellent correlation with gas production rates in comets, resulting in the efficiency of (6.4 +/- 0.9)x 10-5 AU3/2 in the range of 97-165 eV. Correlation with dust production rates is poor, and this favor a gas-related excitation process. The peak brightnesses scaled to r2 are constant and equal to 26+/- 9 millirayleighs. This means that comae are optically or collisionally thick near the brightness centers. Of a few suggested excitation mechanisms, only charge exchange between solar wind heavy ions and cometary neutrals agrees with both these facts. The EUVE spectra of comets Hale-Bopp and Hyakutake have been analyzed. Due to the close flyby of Hyakutake at 0.1 AU, its spectra are of exceptionally high quality and exceed the currently published spectra of comets by a factor of 3 in resolving power and by two orders of magnitude in photon statistics. The spectra reveal for the first time the emission lines of multiple charged ions which are brought to the comet by the solar wind and excited in charge exchange with cometary neutral species. The most prominent lines are O4+ 215 Å, C4+ 249 Å, and He+ 304 Å. 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 alpha-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 similar 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. Recent observations of comet McNaught-Hartley using the Chandra X-Ray Observatory will be also discussed. Detailed analyses of all papers that prove electron impact and bremsstrahlung as the main processes of X-ray excitation in comets reveal significant numerical and conceptual errors. Processes of excitation by electrons are proportional to square of gas production rate in comets, and this also disagrees with our observations. My calculations result in Mars' X-ray luminosity of 4x 1022 ph s-1. This work was supported by NSF grant AST-9732895.
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