Jahresbericht 08 - PMOD/WRC

28
Scientific Research Activities
Calculations of the Spectral Solar Irradiance with Molecular Lines
Alexander Shapiro and Micha Schöll
We introduce the calculation of molecular lines to the
radiative transfer code COSI (Code for Solar Irradiance).
The molecular lines significantly contribute to
the total opacity in the solar atmosphere and therefore
play an important role in the understanding of spectral
solar irradiance.
Robust knowledge of the opacity in the solar atmosphere
is crucial for spectral irradiance calculations. The COSI
code explicitly calculates only a small number of selected
transitions (about 100) in non local thermodynamic equilibrium,
while all other line transitions are taken into account
via the iteratively calculated opacity distribution function
(Haberreiter et al., 2008).
Molecular lines play an important role in forming the solar
spectrum and dominate in specific spectral regions. We introduce
the calculation of the chemical equilibrium and the
formation of the most prominent molecular systems (i.e. G
band, CN violet system, Herzberg band) to the COSI code
which enables us to include the contribution from molecules
to the total opacity distribution function.
As a significant fraction of atoms can be associated with
molecules the chemical equilibrium calculation also affects
the atomic lines. In Fig. 1 we present the deviations in carbon
and oxygen concentrations due to the association with
molecules together with CN and CO concentrations for
three atmospheric models by Fontenla et al. (1999). Both
molecular concentrations and deviations in atomic concentration
show strong temperature sensitivity which can
be important for the solar variability study.
In Fig. 2 we illustrate the comparison between synthetic
COSI spectra calculated with and without molecular lines
in the CH G-band region and compare it to SORCE observations.
Both spectra were calculated with the same atmospheric
structure (ASUN) and slightly modified to give
better continuum level agreement abundances (K91) by
Kurucz (2005). The inclusion of the CH G-band is necessary
to reproduce the spectral irradiance in the 410–450
nm region, however, because of the non local thermodynamic
equilibrium effect, it can also affect UV irradiance,
which is important for the terrestrial climate.
References: Haberreiter M., Schmutz W., Hubeny I.,
2008, A&A, 492, 833.
Fontenla J., White O.R., Fox P.A., Avrett E.H.,
Kurucz R.L., 1999, ApJ, 518, 480.
Kurucz R.L., 2005, Mem. Soc. Astron. Ital. Supp., 8, 189.
R
N [cm -3 ]
1.1
1.0
0.9
0.8
0.7
0 200 400 600 800 1000
CN
10 10
10 8
10 6
10 4
10 2
CARBON
0 200 400 600 800 1000
HEIGHT [km]
0.7
0 200 400 600 800 1000
CO
10 12
10 10
0 200 400 600 800 1000
HEIGHT [km]
Figure 1. The ratio R of unattached carbon and oxygen molecules to their
total amount (upper panels) and the CN and CO concentrations (lower
panels) for three solar atmospheric models: the relatively cold supergranular
cell center model FALA (blue curves), the averaged “quiet” Sun
model FALC (black curves) and the relatively worm bright network model
FALP (red curves).
Irradiance (W m -2 nm -1 )
2.5
2.0
1.5
Figure 2. Two synthetic spectra calculated with and without molecular
lines (magenta and cyan curves accordingly) in comparison to SORCE
observations (black curve).
1.1
1.0
0.9
0.8
10 8
10 6
10 4
OXYGEN
1.0
410 420 430
Wavelength [nm]
440 450