Geoscience Reference
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A second alternative explanation for the two tails could be that the
comet is crossing an inhomogeneous coronal region. The secondary H
atoms travel with the solar wind, hence the sungrazers Ly
α
image strongly
depends on the local magnitude and direction of the wind velocity vector. If
the comet encounters, for instance, a transition region between slow and fast
wind regime, we could expect to observe an “inhomogeneous Ly
α
image” of
the comet: in this sense, UVCS observations of sungrazers could be used as
“tracers” of the solar wind inhomogeneities. In order to solve these prob-
lems it is necessary to better understand the relationships between the
shape of the observed Ly
α
tails and the physical parameters of the coronal
plasma crossed by the comet. To this end, Giordano
et al.
18
are work-
ing on a comet simulation code based on the Monte Carlo technique. The
aim of this code is to reproduce the observed sungrazer Ly
α
image as a
function of both coronal (e.g. kinetic temperature, electron density, elec-
tron temperature, wind speed, etc.
...
) and cometary (e.g. outgassing rate,
velocity distribution and kinetic temperature of the outgassed, etc.
...
)free
parameters. Figure 4 (right panels) shows the simulated dependence of the
H atoms distribution from the wind speed
v
w
for
v
w
= 0 km/s (panel a)
and
v
w
= 200 km/s (panel b). These results are preliminary: a full paper is
in preparation.
6. Conclusions
Sungrazing comets have been detected by UVCS in the Ly
α
spectral line.
Data analysis provided many new results about these comets, first of all
the presence of a hidden mass, unobserved by LASCO coronagraphs and
ascribed to the presence of slowly eroding subfragments traveling with the
main nucleus. The fragmentation processes play a fundamental role in the
formation and evolution of sungrazers; from UVCS data the presence of
subfragments has been inferred from the observed sudden increases in the
Ly
α
intensity. A subfragment has been directly observed for the first time
(thanks to the high UVCS spatial resolution), as derived from the obser-
vation of a secondary Ly
α
tail. Recent UVCS observation of a sungrazer
revealed again a secondary Ly
α
tail: however, because of the puzzling Ly
α
Doppler shift image, this cannot explained as a signature of two fragments.
In order to find an alternative explanation, the creation of a Ly
α
tail sim-
ulation code is in progress: this code will help us to better understand the
dependence of the observed Ly
α
image on the physical parameters of the
coronal plasma encountered by the comet.
