Kometen und die Sonne | Comets and sun.

As much as 300 sun-grazing comets were observed between January 1996 and June
2001 by the sensitive Solar Heliospheric Observatory (SOHO) instrumentation capa-
ble of detecting faint objects. The average number of sun-grazers is 60 per year. A
comet was discovered that did not reappear after the time of closest approach to the
Sun. Much later, extremely broad diffuse brightening of the corona over one solar
hemisphere was observed. This brightening was interpreted as having been caused by
the cometary debris back-scattered into the ambient solar wind.
One June 11, 1998, two sun-grazing comets following similar but not identical or-
bits were viewed. Shortly after they disappeared behind the occulting disks of the
coronographs, a coronal mass ejection (CME) with an enormous erupting prominence
appeared on the limb of the Sun. Observations like these raise the question of whether
these have been pure coincidences or whether, if only in some cases, there exists a
causal link between cometary impacts on the Sun and the release of CMEs. Here we
present the first results of an attempt to infer whether CMEs can in principle be caused
by cometary impacts on the Sun.
The typical mass a CME lifted against the solar gravity is of the same order of mag-
nitude the mass of a 1 km icy sphere. The energy released in the impact of such a
comet may be sufficient to produce a “directly driven” CME, when the sudden re-
lease of thermal energy in the corona (“thermal blast”) creates a highly enhanced gas
pressure that cannot be contained by the solar magnetic field. In this case the corona
is literally blown open, intense shock waves are generated, propagating out to large
distances from the Sun. Historic evidence shows that among sun-grazers there were
several large comets. Our previous investigations have shown that large impactors may
deeply penetrate into the solar photosphere (e.g. down to an altitude of about 1500 km
for a comet 30 km in radius). Energy of these impactors is sufficient for direct igni-
tion of a CME. But impacts of large bodies are rather infrequent, while there are 270
to about 1500 CMEs per year. It is obvious that the main part of the relatively faint
sun-grazers is rather small and their mass and energies are much less than those of a
typical CME.
Currently the best accepted model of generation of CMEs refers to “stored and re-
leased” coronal magnetic energy. This model proposes that CMEs are triggered by
micro-instabilities causing anomalous resistivity, current dissipation, and reconnec-
tion. The energy fuelling the CME is stored in the corona. The energy of the impactors
which may initiate the evolution of micro-instabilities, reconnection of magnetic field
lines and ultimately trigger a CME may be substantially less than the final energy of
CME. So we should investigate the peculiar properties of impacts of small comets,
which previously have not been studied thoroughly.
Simple estimates show that near the Sun the mass of an icy comet evaporated dur-
ing its approach to the Sun under the action of solar radiation is equivalent to the
mass of an icy layer about 100 m thick. Therefore, small sun-grazers may be fully
evaporated. These estimates should, however, be corrected as the surface of the comet
reaches temperatures much higher than the temperature of volatile evaporation. This
claim is supported by the observation of the light curves which show that the normal-
ized brightness depends only on the heliocentric distance of the comet. That can be
understood if we assume that the maximum temperature is given by the equilibrium
blackbody temperature TB . At temperatures of 1600 to 2700 K the refractory material
becomes intensely evaporated, while volatiles may evaporate at smaller heliocentric
distances.
The determination of the ablation is made more difficult by the complicated radiation
transfer processes in the vapour layer. Detailed spectral absorption coefficients in a
wide range of temperatures, densities and spectral intervals (in the IR, visible and UV
wavelength ranges) were calculated for typical volatile-rich cometary substances and
for chondritic components, which may represent a comet after the evaporation of ices.
The analysis of these coefficients shows that the evaporating surface is at least partially
screened by the vapour layer.
Because of the multitude of molecule absorption bands and lines involved, one needs
to introduce a very large number of spectr

https://www.meetings.copernicus.org/www.cosis.net/abstracts/EGU05/04384/EGU05-J-04384.pdf