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the observed constant background of the Ly
α
counts, they finally find that
the pyroxene dust grain number density needed to reproduce the obser-
vations is
10
−
10
cm
−
3
. This has been the firt order-of-magnitude
determination of the pyroxene dust grain number density in sungrazing
comets.
Before concluding this section, we briefly discuss the formation of the
cometary bow shock for sungrazers. Close to the Sun the magnetic field, the
solar wind velocity
v
w
and the sungrazer velocity
v
com
are nearly radial;
ashockshouldformwithashockvelocity
v
sh
=
v
w
+
v
com
and a frac-
tion of the energy dissipated in the shock should go into plasma heating
(detectable as an increase in the FWHM of the Ly
α
line profile). However,
sungrazers C/2000 C6 and C/2001 C2 were immersed in a slow wind region
(
v
w
∼
6
.
2
×
150-200 km/s) and no significant plasma heating was observed: as we
already mentioned, the Ly
α
cometary and coronal profiles had the same
FWHM. On the contrary, sungrazer C/1996 Y1 crossed a fast wind region:
its Ly
α
profile was much broader (kinetic temperature
T
k
∼
10
6
K) than
the background coronal profile and Doppler shifted because of the comet
motion along the line of sight (see Fig. 2). From the observed proton kinetic
temperature, the authors
3
9
×
620 km/s. Obvi-
ously the formation of the bow shock for sungrazers is strongly dependent
on the physical parameters of the coronal region crossed by the comet.
inferred a wind speed of
v
w
5. Ultraviolet Coronagraph Spectrometer Observation of
C/2002 S2: First Results
On September 18-19, 2002 UVCS observed the sungrazer C/2002 S2
18
at
four different heliocentric distances. In this section we briefly describe the
first results obtained from an analysis of these data. Figure 4 (top left
panel) shows the reconstructed Ly
α
appearance of this comet at the helio-
centric distances of 4.64 and 6.84 R
. These panels clearly show the presence
of 2 Ly
α
tails at 6.84 R
and possibly a similar multiplicity also at lower
heliocentric distance. The 6.84 R
image seems to be similar to the 4.98 R
image of the C/2001 C2 sungrazer (see Fig. 1), but there is a strong differ-
ence between the two. As revealed by a Gaussian fit of the C/2002 S2 Ly
α
line profiles, while the emission from the main tail is red-shifted by more
than
60 km/s, the Ly
α
profiles of the secondary tail are blue-shifted by
more than
∼
120 km/s. The resulting Doppler shift image of the comet is
puzzling and the two observed Ly
α
tails cannot be easily interpreted as the
signature of two fragments.
∼
