ROMIC/MUSIC - Role Of the MIddle atmosphere in Climate: Modeling and Understanding Solar Irradiance Changes
Overarching goal of ROMIC, a research program funded by the German Federal Ministry of Education and Research (BMBF), is the investigation of the role of the middle atmosphere in climate. For this important processes in the middle atmosphere will be analyzed in order to understand their relevance for climate. The impact of solar variability is one of the central topics and the focus of the subproject MUSIC (Modeling and Understanding Solar Irradiance Changes).
Changes in solar radiation are the main external drivers of the Earth’s climate system. Whereas total solar irradiance (TSI, i.e. irradiance integrated over the entire spectrum) is the prime source of energy for the atmosphere, variations in the solar spectral irradiance (SSI), in particular in the ultraviolet part of the spectrum have a pronounced effect on the chemistry and dynamics of the Earth’s atmosphere. However, the strength of and the physical processes governing the solar influence on climate remain uncertain. This is due to:
- The uncertainties in the magnitude and details of the solar variability
- The uncertainties in the mechanisms by which varying solar radiation influences climate.
Main aim of MUSIC, as part of the ROMIC project, is to deal with the first part of the problem. Its main product will be the reconstructed solar irradiance (both TSI and SSI) that will later be used by Earth atmosphere modelers, in particular by those involved in the ROMIC program, as an input to their models.
It is generally agreed that the dominant part of the variations of TSI and SSI on time scales longer than a day are due to the appearance, disappearance and evolution of the magnetic field at the solar surface, namely its manifestations in the form of dark sunspots and bright faculae. The measured TSI has been best reproduced by models that make this assumption. We have developed the SATIRE (Spectral And Total Irradiance REconstructions) model that computes the radiance spectrum of magnetic features at different parts of the solar disc by solving the radiative transfer equations in semi-empirical model atmospheres describing sunspot umbrae, penumbrae, faculae and the quiet Sun. The version of SATIRE relevant to this project, SATIRE-S (i.e. SATIRE for the Satellite era) uses observations (spatially resolved full-disc solar magnetograms) to describe the evolution of magnetic structures on the solar surface.
The SATIRE-S model captures over 90% of the observed TSI variability  when compared with the PMOD composite of the TSI measurements obtained by space-based radiometers since 1978 (Fig. 1). SATIRE-S has in the past also been successful in reproducing SSI variations both on short (days to months) and on long (years to decades) timescales [2,3,4,5,6,7]. However, the situation has changed with the advent of data taken by instruments onboard SORCE (Solar Radiation and Climate Experiment) since 2003/2004. The new data agree with the models on short time scales (see left-hand panel of Fig. 2)[5,6], but display a strikingly different behavior on longer time scales (right-hand panel of Fig. 2)[6,7]. This is a problem faced not just by SATIRE, but by all models that can reproduce TSI variations (see Ermolli et al. 2013  for a review and Fig. 2). Currently, it is not clear whether the problem lies with the models or with the data.
Space for improvement
The main shortcoming of the present version of SATIRE is usage of a few 1D semi-empirical model atmospheres to describe the wide variety of features appearing at the solar surface. So far, a free parameter is used to compensate for the missing link between the semi-empirical model atmospheres and the amount of magnetic field measured in magnetograms. Obviously it would be more satisfactory for the model to work without a free parameter. This is only possible if the semi-empirical model atmospheres are replaced by physically self-consistent ones. Since the Sun shows a very complex surface structure (produced by the interplay of convection, oscillations, magnetic fields and radiation), such models must take into account the three-dimensional structuring of the atmosphere, as represented by state-of-the-art 3D radiation MHD simulations. Figure 3 shows the variety of solar structures reproduced by the MHD simulations [9,10] carried out with the MURaM, a radiative MHD code developed at the MPS .
Our basic approach is to improve the self-consistency, physical realism and reliability of the irradiance models by extending them in two directions:
- Introduce significantly more physical realism into the modelling, in a first step, by employing state-of-the-art 3D radiation MHD simulations of the solar atmosphere, in particular MURaM simulations, to compute the spectrum of solar regions with different amounts of magnetic flux. The available simulations have local thermodynamic equilibrium (LTE) radiative transfer and allow the spectrum to be computed down to 300 nm.
- Further develop the MHD simulations. The most important extension is in height, so that the models also cover the solar chromosphere, where the cores of the stronger spectral lines in the UV spectrum (and some in the visible and infrared) are formed. In order to do so we need to take non-LTE into account in the radiative energy transport in the simulation. This extension of the model will significantly improve the critical UV fluxes below 300 nm resulting from the MHD simulations.
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