18 April 2014 - Mission Day: 6713 - DOY: 108

First UVCS Observations of a Comet's Tail

    UVCS Image of SOHO Comet 104
    The four image segments on the left are the UVCS observations. Each section of the image is a composite of several 200 second exposures. The instrument stays at one of the four heights as the nucleus and the tail pass through the observed region. The position and size of the Sun is indicated by an EIT 304 Å image.
    Also: High-resolution TIFF image.

Caption: Comet SOHO-104 (officially designated C/2000 C6), a member of the Kreutz family of Sun-grazing comets, was first discovered by the Large Angle and Spectrometric Coronagraph (LASCO) on 9 February 2000. The group of images shown here illustrate the four heights (6.37, 5.71, 4.57, and 3.27 Solar radii) where the comet was observed by the Ultraviolet Coronagraph Spectrometer (UVCS) on the following day.

The light detected from the comet has a wavelength of 1216 Angstroms, referred to as Lyman-alpha. Hydrogen atoms produced by the dissociation of water vapor outgassed from the nucleus scatter the Lyman-alpha from the Sun towards the UVCS instrument. As its orbit takes it closer to the sun, the comet brightens as ice evaporation increases, and then fades as less material is available for vaporization.

Since the neutral Hydrogen atoms are "destroyed" (ionized) by collisions with the surrounding corona, the comet can be used as a probe to determine the density of of the region of the corona it is passing through. This is accomplished by measuring the decrease in the Lyman-alpha flux from the tail to determine the time it takes for the intensity to drop by half. When the result is combined with the ionization rate coefficient for Hydrogen (taken from Scholz & Walters, ApJ 380, 302), the coronal density can be determined.

For more images of comets discovered by SOHO, see the Hot Shot for SOHO Comet 100.

Instrument: UVCS (Ultraviolet Coronagraph Spectrometer); EIT (Extreme ultraviolet Imaging Telescope);
Taken: 10 February 2000
Credits: Early reporting and position estimates provided by Doug Biesecker. Orbit prediction provided by Gareth Williams. Observation, image, and caption provided the UVCS operations and science teams. Image montage by Steele Hill.


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