On the Nature of Soft X-Ray Radiation in Comets
Abstract
EUVE and ROSAT observations of Comet Hyakutake revealed emission of 7 × 1024photons 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 10-19g; 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 × 10-19g 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) × 10-19g. This reduction strongly affects the visible brightness of the particles, which is proportional tom2, 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. Gombosiet al.(1994,J. Geophys. Res.99,21,525-21,539) and the Giotto measurements of α-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 Å and He 522 Å lines, and UV spectroscopy of cometary dust down to 1800 Å may help in determining the contributions of the above processes.- Publication:
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Icarus
- Pub Date:
- August 1997
- DOI:
- 10.1006/icar.1997.5722
- Bibcode:
- 1997Icar..128..368K