Geoscience Reference
In-Depth Information
distances. From these observations we have an unique opportunity to obtain
information on the constituents of the cometary dust. However, despite the
very large amount of sungrazing comets discovered by LASCO, as of today
UVCS observed only 13 comets; 10 of these belong to the Kreutz group.
So far, detailed study of sungrazers C/1996 Y1,
3
C/2000 C6,
4
and C/2001
C2
5
have appeared: in the following sections we describe the main results
from UVCS observations of these comets.
2. The UVCS Sungrazer Emission: Main Properties
Once the cometary orbital parameters have been computed from LASCO
observations, the UVCS slit is set along the trajectory to observe the comet
transit at 4-5 different heliocentric distances. At each position, the instru-
ment starts to observe at a fixed altitude
15-20 min before the comet
enters the UVCS FOV and stops about 30-40 min later, depending on
the cometary orbital parameters. When the comet, traveling along its
orbit, intersects the slit, cometary emission is seen typically over a sec-
tion
∼
100-200
long. The spectral intervals for sungrazer observations typ-
ically include, beside neutral Hydrogen Ly
αλ
1215.7 ALy
βλ
1025.7 Aand
the O
∼
λλ
1031.9-1037.6 A lines, spectral lines from species that might be
present in cometary spectra such as He
vi
i
i
iii
i
ii
ii
i
ii
,N
,Al
,Si
,Si
,P
,O
,C
,
i
and Ar
. However, in the selected spectral ranges, UVCS usually detected
a cometary emission only in the H Ly
α
line
b
; hence, in the following we
discuss the origin and main characteristics of this line in sungrazer spec-
tra. The H Ly
α
line is observed also in coronal spectra: hence, in order
to identify the cometary signal, we need to calculate its average coronal
intensity in spectra taken before the comet entered the UVCS FOV and to
subtract this background signal from the following exposures. This proce-
dure removes also the possible Ly
α
interplanetary emission and the detector
dark counts. From the cometary Ly
α
intensity observed along the spectro-
graph slit in different exposures it is possible to reconstruct the cometary
Ly
α
image and then the shape of the Hydrogen cloud. Figure 1 shows the
reconstructed Ly
α
images for the sungrazers C/2000 C6 (left panel) and
C/2001 C2 (right panel). In both cases it is clear that the Hydrogen clouds
appear to be strongly “stretched” along the comet-Sun direction leading
to the formation of a “Ly
α
tail”: as we discuss later, this is due mainly to
b
O
have been detected in UVCS spectra of non-sungrazing comets,
6
partly
because these comets are much brighter.
i
,C
ii
,andC
iii
