Geoscience Reference
In-Depth Information
Fig. 3. Left: the C/2001 C2 Ly
α
counts (summed over the slit length) versus time
curve as observed at 3.6 R
(solid line) and the curve fit (dotted line). Right: the iso-
χ
2
curves for the determination of the (
τ
ion
,
t
st
)valuesatwhichthe
χ
2
value is minimum
(see text).
second produced by outgassing) and to the unknown ionization and charge
exchange rates (
τ
−
1
ion
cx
, respectively). These are proportional
11
,
12
to
the local electron density
n
e
encountered by the comet. The
N
coma
atoms
undergo a charge transfer with the ambient protons creating a number
N
tail
of neutrals (responsible for the observed Ly
α
emission)atarate
τ
−
1
and
τ
−
1
cx
:this
process locally increases the number density of H atoms with respect to
the coronal density. As a consequence, the number of H atoms
N
tail
left
by the comet along its path exponentially decays as exp(
t/τ
ion
)untilthe
cometary Ly
α
signal disappears (Fig. 3, left panel). By fitting with an expo-
nential curve (depending on
N
,
τ
ion
,thetime
t
st
at which the comet first
entered the slit and other known parameters) the observed Ly
α
intensity
decay after the comet transit, it is possible to derive an estimate for the
cometary outgassing rate
N
and the local electron density
n
e
(Fig.3,right
panel). Assuming a balance between the energy supplied by the solar radia-
tion impinging on a spherical nucleus and the energy required to sublimate
the quantity of ice derived from the
N
value, we may estimate (for isotropic
outgassing) the nucleus equivalent radius.
Table 1 gives the results obtained at different heliocentric distances
h
(R
) of the comet for the sungrazers C/1996 Y1, C/2000 C6, and C/2001
C2, in particular the ice mass outgassed per second
Q
H
2
O
(kg/s) and the
estimated nucleus radius
r
(m). An interesting result from Table 1 is the
UVCS detection of a significant surviving mass even after the comets disap-
peared from the LASCO/C2 FOV (hence below 6 R
), implying the pres-
ence at very low heliocentric distances of a hidden mass “inconsistent with
the nominal models based on the coronagraphic observations alone”.
13
This
−
