Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997icar..128..368k&link_type=abstract
Icarus, Volume 128, Issue 2, pp. 368-385.
Astronomy and Astrophysics
Astronomy
48
Scientific paper
EUVE and ROSAT observations of Comet Hyakutake revealed emission of 7 x 10^24 photons per second in the soft X-ray. We discuss and develop methods to calculate production of soft X-ray photons in cometary dust and gas by the following processes: (1) scattering and (2) fluorescence of solar X-rays; (3) K- and L-shell ionization by solar-wind protons and (4) electrons, and (5) by high-energy cometary ions; (6) bremsstrahlung of solar-wind electrons; (7) cometary magnetospheric substorms; (8) collisions between cometary and interplanetary dust particles; (9) scattering, fluorescence, and bremsstrahlung by very small particles with mass on the order of 10^-19 g; and (10) charge transfer of the solar-wind heavy ions with cometary molecules suggested by T. E. Cravens (1997, Geophys. Res. Lett. 24, 105-108). Very small attogram particles were detected in Comet Halley by N. G. Utterback and J. Kissel (1990, Astron. J. 100, 1315-1322) using the PUMA and PIA dust analyzers from the Vega and Giotto spacecraft. Of all these processes, only scattering by very small particles and charge transfer of the solar-wind heavy ions are capable of producing the measured soft X-ray emission. In the case of very small particles, the mean particle mass of 4.6 x 10^-19 g suggested by Utterback and Kissel implies that the total production of these particles, which exceeds that of gas by a factor 3.6 (in mass), is inconsistent with the polarization and color of Comet Halley. Both polarization and color require a reduction of the mean mass to (1-2) x 10^-19 g. This reduction strongly affects the visible brightness of the particles, which is proportional to m^2, with a relatively weak effect on the soft X-ray emission. We calculate the charge transfer process using the solar-wind ion densities and velocities in comet from both the model of T. I. Gombosi et al. (1994, J. Geophys. Res. 99, 21,525-21,539) and the Giotto measurements of alpha-particles and He^+ ions in Comet Halley. The calculated emission constitutes 20-35% of the measured value. X-ray spectroscopy of the observed emission, spectroscopy of the He^+ 304 A and He 522 A lines, and UV spectroscopy of cometary dust down to 1800 A may help in determining the contributions of the above processes.
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