Jahresbericht 08 - PMOD/WRC

Reconstructing the Long Term Spectral Solar Irradiance
Micha Schöll and Werner Schmutz
We present a spectral reconstruction of solar irradiance
back to 1610 utilizing sunspots for short-term
variations and the open magnetic flux for long-term
trend.
Solar irradiance variation is an important forcing component
in climate models as it is known that climate reacts to
changing solar irradiance. As Rozanov et al. (2002) have
shown, the Lyman-α line is of particular importance to
chemical reactions in the upper atmosphere.
To reconstruct past climate, spectra as a function of time
are required. Following the approach of Krivova et al. (2005)
the spectral reconstruction is based on a four component
model with filling factors for the area of quiet sun, sunspots,
faculae and network regions. Wenzler extracts the filling
factors using MDI magnetogram data and images of the
visible sun.
Utilizing the open magnetic flux to reconstruct the longterm
network evolution we obtain spectral irradiance back
to 1610 where the Maunder minimum is below the current
minimum.
The sunspot area and faculae are reconstructed using the
sunspot number. The long-term network is split into active
and passive networks. The active network corresponds to
the active areas on the sun and is hence reconstructed
using the sunspot number. The quiet network is assumed
to correspond to the open magnetic flux with the assumptions
that no open magnetic flux translates to a quiet sun
and that the current quiet network is 14 % in accordance
with Foukal et al. (1991). Using this approach, together with
the active area expansion (see Schöll and Schmutz 2007)
and a modeled spectrum using the COSI code, we obtain
the reconstructed spectrum, with the Lyman-α line (121.5
nm) shown in Figure 1 and 2. Figure 1 is an enlargement of
Figure 2 for the 1992 to 2001 period.
References: Haberreiter M., Krivova N.A., Schmutz W., Wenzler T.,
Reconstruction of the solar UV irradiance back to 1974,
Advances of Space Research 35, 365-369, 2005.
Foukal P., Harvey K., Hill F., Do changes in the photospheric
magnetic network cause the 11 year variation of
total solar irradiance?, ApJ 383, L89-92, 1991.
Krivova N.A., Solanki S.K., Modelling of irradiance
variations through atmosphere models. In: Memorie
della Societa Astronomica Italiana Vol. 76, p. 834, 2005
Schöll M., Schmutz W., Reconstructing the Spectral
Solar Irradiance: The Active Area Expansion.
In: PMOD/WRC 2007 Annual Report, p. 28, 2008.
Rozanov E., Egorova T., Fröhlich C., Haberreiter M.,
Peter T., Schmutz W., Estimation of the ozone and
temperature sensitivity to the variation of spectral solar
flux, In: From Solar Min to Max: Half a Solar Cycle with
SOHO, ESA SP-508, 181-184, 2002.
Irradiance (mW/m 2 /nm)
Figure 1. Reconstructed Lyman-α compared to SOLSTICE. The solid
black line uses the reconstructed filling factors for all regions, while the
dashed line displays the reconstruction using Wenzler’s MDI data and reconstructed
network regions.
Irradiance (mW/m 2 /nm)
10
9
8
7
6
1992 1994 1996 1998
Time (Year AD)
2000
10
9
8
7
6
5
SOLSTICE
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SOLSTICE
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1700 1800 1900 2000
Time (Year AD)
Figure 2. Reconstructed Lyman-α from 1610 to the present using the
open magnetic flux for the long-term trend network.
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