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radiative excitation. In order to distinguish between these two processes,
we may resort to the ratio Ly
β
/Ly
α
between the observed intensities of
these spectral lines. At typical coronal temperatures, the ratio between the
collisional components of these line is expected to be
0.13-0.14,
3
while
the ratio between the radiative components has a much lower value on
the order of
∼
0.001-0.002.
3
Because in the UVCS observations of sungraz-
ers C/1996 Y1, C/2000 C6, and C/2001 C2 the Ly
β
line was very faint
or absent, it can be concluded that the observed cometary Ly
α
emission
arises almost entirely from radiative excitation (as is usually the case in the
corona
7
).
Another question concerning the observed Ly
α
tails is the origin of the
neutral Hydrogen atoms responsible for this emission. In principle these
neutrals could be a “first generation” of H atoms formed from the pho-
todissociation of the H
2
O molecules outgassed from the nucleus, and/or
a “second generation” of H atoms formed
via
the charge exchange process
between these atoms and the coronal protons
p
+
of the solar wind. First, we
have to take into account that the velocity imparted to the H atoms from
outgassing
8
and photodissociation processes
9
is less than about 40 km/s,
much smaller than the typical sungrazer speed close to the Sun
3
-
5
(
∼
250-
300 km/s). This implies that the H atoms from the photodissociation of H
2
O
molecules move, in first approximation, with the comet, hence (opposite to
what Fig. 1 shows) the Ly
α
emission from these atoms would drop as soon
as the comet travels beyond the spectrograph slit. Second, because of the
high radial component of the cometary speed, the atomic absorption profile
of these H atoms is Doppler shifted with respect to the chromospheric emis-
sion profile, and the radiatively excited Ly
α
emission is strongly reduced (by
a factor 0.2-0.1 for radial speeds between 250-300 km/s
10
) by the Swings
effect. The emission from outgassed H atoms would result in a narrow line
profile, while the FWHM of the cometary and coronal profiles turns out
to be equal (Fig. 2). Moreover, because of the comet velocity component
along the line of sight, we should expect a Doppler shift of the Ly
α
line
profile from outgassed H atoms with respect to the coronal profile, while no
significant shifts are observed
c
(see Fig. 2). We conclude that the H atoms
responsible for the Ly
α
sungrazer emission belong to the “second genera-
tion” of neutrals, i.e. to the H atoms from the photodissociation of H
2
O
which, before being collisionally or radiatively ionized, undergo a charge
∼
c
Partial filling of the UVCS aperture by the sungrazer at the time the comet entered the
UVCS FOV results in a small shift in the Ly
α
line profile observed only in that exposure.
