EUVE Science Highlights -
October 9, 2000
EUVE Science Highlights -
October 9, 2000
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This science highlight was submitted by Vladimir Krasnopolski of the Physics Department, Catholic University of America.
Based on papers: 1) EUVE Search for X-Rays From Comets Encke, Mueller (C/1993 A1), Borrelly, and post-perihelion Hale-Bopp, V. A. Krasnopolsky, M. J. Mumma, and M. J. Abbott, Icarus, accepted January 2000. 2) Spectroscopy of Comet Hyakutake at 80-700 A: Evidence for Solar Wind Charge Transfer, V. A. Krasnopolsky and M. J. Mumma, Science, submitted February 2000.
Soft X-ray emission from comets was discovered in the EUVE and ROSAT observations of Comet Hyakutake in 1996. The Comet was a more efficient emitter of soft X-rays than the Moon by a factor of 80,000. Results of nine EUVE observations of comets have been published, and X-rays were detected in six of them. It is clear that cometary X-rays is a general phenomenon.
However, the nature of this phenomenon was puzzling, and a few excitation processes were suggested for its explanation. Recent analysis of the EUVE spectra of comets revealed for the first time - before the coordinated EUVE/Chandra observation of comet LINEAR S4 - emissions of multiple charged ions that are brought to comets by the solar wind (see Figure). Those ions get electrons from neutral molecules and atoms in comets and emit photons in the X-ray and extreme ultraviolet (EUV) regions of the spectrum.
Therefore, the main puzzle has been solved, first of all due to the unprecedented quality of the EUVE spectrum (see Figure) and similar spectra in the ranges of 80-180 and 300-700 Å. While most sources of X-rays and EUV emission are point-like, comets are extended sources. The basic factor which determines the quality of X-ray and EUV spectra of extended sources is the instrument dispersion. For extended sources, the EUVE instrumental dispersion is better than on all other X-ray and EUV observatories, including Chandra (by a factor of 9).