LYMAN-a OBSERVATIONS OF SUNGRAZING COMETS WITH THE SOHO/UVCS INSTRUMENT - Advances in Geosciences - page 171

LYMAN- OBSERVATIONS OF SUNGRAZING COMETS

WITH THE SOHO/UVCS INSTRUMENT

α

A. BEMPORAD ∗ , G. POLETTO † ,J.RAYMOND ‡ and S. GIORDANO §
∗ Astronomy Department Florence University
L.go E. Fermi 2, 50125 Florence, Italy
† INAF-Arcetri Astroph. Obs., L.go E. Fermi 5, 50125 Firenze, Italy
‡ Harvard-Smithsonian CFA, 60 Garden St., Cambridge, MA 02138, USA
§ INAF-Torino Astronom. Obs., Strada Osservatorio 20
10025 P. Torinese, Italy
∗ [e-mail]

The Large angle and spectrometric coronagraphs aboard the Solar and helio-
spheric observatory (SOHO) spacecraft observed a large (more than 1000) num-
ber of sungrazers. This led to many studies which tried to explain their origin
and the peculiar shape of the observed cometary lightcurves. However, in the
last years a few sungrazers have been observed also in the hydrogen Lyman- α
spectral line by the UV coronagraph spectrometer on SOHO. This instrument
allowed to perform UV spectroscopical observations of sungrazing comets on
their final stage of life at projected heliocentric distances between 1.4 and
10 solar radii. Ultra violet coronagraph spectrometer (UVCS) detected in the
sungrazer UV spectra mainly the Lyman- α spectral line. Typically, emission
in this line originates in the hydrogen cloud produced by the water photodis-
sociation, but, at these low heights, it is necessary to take into account also
strong interaction processes (e.g., mass-loading and charge exchange) between
the solar wind and the outgassed materials. From these observations, it has
been possible to estimate cometary parameters such as the outgassing rates
and the nucleus sizes, as well as parameters of the coronal plasma encountered
by the comet. In this work we review the main results derived from the UVCS
observations of sungrazing comets: the detection of a “hidden” mass below
∼

6R , the indirect and direct evidences for the occurrence of fragmentation
processes and a tentative estimate for the pyroxene dust grain number density.
Moreover, we present here preliminary results on the UVCS data interpretation
of a sungrazer observed in 2002.

1. Introduction: State of Art

Sungrazer comets have a perihelion distance of a few solar radii and in
some cases collide with the Sun. The first reliable discovery of a sungrazer
occurred in 1680 (the Great Comet); then, after the Great Comet of 1843,
four individual sungrazing comets have been observed in the 1880s, while
the next one did not appear until 1945. Two further sungrazers were dis-
covered in the 1960s, Comet Pereyra in 1963 and Comet Ikeya-Seki in 1965.

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