(Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482
Potsdam, Germany)
Carbon monoxide in the solar atmosphere II. Radiative cooling by CO lines
The role of carbon monoxide as a cooling agent for the thermal structure of
the mid-photospheric to low-chromospheric layers of the solar atmosphere in
internetwork regions is investigated. The treatment of radiative cooling via
spectral lines of carbon monoxide (CO) has been added to the radiation
chemo-hydrodynamics code CO5BOLD. The radiation transport has now been solved
in a continuum band with Rosseland mean opacity and an additional band with
CO opacity. The latter is calculated as a Planck mean over the CO band
between 4.4 and 6.2 microns. The time-dependent CO number density is derived
from the solution of a chemical reaction network. The CO opacity indeed causes
additional cooling at the fronts of propagating shock waves in the
chromosphere. There, the time-dependent approach results in a higher CO
number density compared to the equilibrium case and hence in a larger net
radiative cooling rate. The average gas temperature stratification of the
model atmosphere, however, is only reduced by roughly 100 K. Also the
temperature fluctuations and the CO number density are only affected to
small extent. A numerical experiment without dynamics shows that the CO
cooling process works in principle and drives the atmosphere to a cool
radiative equilibrium state. At chromospheric heights, the radiative
relaxation of the atmosphere to a cool state takes several 1000 s. The CO
cooling process thus would seem to be too slow compared to atmospheric
dynamics to be responsible for the very cool temperature regions observed
in the solar atmosphere. The hydrodynamical timescales in our solar
atmosphere model are much too short to allow for the radiative relaxation
to a cool state, thus suppressing the potential thermal instability due to
carbon monoxide as a cooling agent. Apparently, the thermal structure and
dynamics of the outer model atmosphere are instead determined primarily by
shock waves.
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Vol. 462 No. 3 (February II 2007)
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