Geoscience Reference
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Fig. 1. Left: composite image of the C/2000 C6 Ly
α
tail at 4.56 R
after the subtrac-
tion of the coronal emission (adapted from Ref. 4). Right: the same for the C/2001 C2
sungrazing comet (adapted from Ref. 5). In both images the pixel size along the
y
-axis
correponds to the spatial resolution along the UVCS slit, while the pixel size along the
x
-axis corresponds to the comet's radial velocity multiplied by the exposure time.
the interaction of the cometary ejecta with the solar wind. We notice here
that the correction for the comet motion along the slit revealed the Ly
α
tails shown in Fig. 1 to be both oriented with the cometary orbital path.
Theangle(projectedontheplaneofthesky)betweentheradialfromthe
Sun and the orbital path is on the order of 14 and 13
◦
, respectively, for the
C/2000 C6 (Fig. 1, left) and the C/2001 C2 (Fig. 1, right) Ly
α
images. We
discuss this properties of the sungrazers Ly
α
tails in Sec. 3. As revealed
by a comparison between the two panels of Fig. 1, while the C/2000 C6
sungrazer shows a single Ly
α
tail, the C/2001 C2 image shows two tails.
5
We discuss this issue in Sec. 4: here we anticipate only that the presence
of two tails has been interpreted as a signature of a subfragment traveling
with the main nucleus.
In Sec. 3 we briefly discuss the origin for the enhanced (with respect to
the coronal plasma density) number of H atoms responsible for the observed
cometary emission.
3. The Origin of the Observed Ly
Sungrazer Emission
α
10
6
K) and
In typical coronal conditions of high-electron temperature (
T
e
∼
10
6
-10
7
cm
−
3
) the observed spectral line emis-
sion is due to the collisional excitation with thermal electrons and the reso-
nant scattering of the chromospheric radiation (only for spectral lines which
connect to the lowest levels). Hence, the first question we have to solve is
whether the observed cometary Ly
α
emission arises from collisional and/or
low-electron density (
n
e
∼
