Team Purpose

The effect of solar spectral irradiance variations on different time scales on the Earth’s atmosphere and climate is still poorly known. Apart from the severe difficulty of modeling the physical and chemical processes that regulate the Sun-Earth atmosphere interaction, these investigations are hampered by uncertainties in the measurement of solar irradiance variations. To overcome the limitations of the available measurements and to understand the processes that generate irradiance variations, various proxy-based techniques have been developed to reproduce irradiance from observations of surface magnetism as derived from solar full-disk observations. However, discrepancies between estimates obtained from different reconstruction techniques are still too large to assess the effects of irradiance variations on the terrestrial atmosphere with confidence.

Observations of the solar atmosphere reveal the presence of different types of features whose radiative output can be properly understood and modelled only by taking into account the multidimensional nature of convective and radiative processes. On the other hand, many irradiance reconstruction techniques employ one-dimensional static atmosphere models to synthesize the radiative output of quiet and magnetic regions. A natural step forward in modeling solar irradiance would therefore be the use of three-dimensional magnetohydrodynamic (3D MHD) simulations of the solar atmosphere, which indeed have been shown to be successful in reproducing many of the observed properties of the solar atmosphere with unprecedented degree of realism, particularly compared to traditional one-dimensional models. While there presently is a general consensus among the solar physics community that the employment of such 3D simulations should be further exploited, the literature concerning the application of MHD simulations to irradiance reconstruction studies is still quite scarce. At the moment, one major difficulty is that the researchers specializing in irradiance reconstructions and those specializing in 3D MHD modeling with multi-dimensional radiative transfer are part of two very distinct communities. The formation of an ISSI international team on the proposed topic would therefore be a unique opportunity to bring the two communities together and make them aware of the difficulties inherent in irradiance measurements and reconstruction on one hand, and in multi-dimensional modeling of the solar atmosphere on the other, and foster development of irradiance models based on 3D MHD simulations.

The team is composed of 12 members from 9 different countries, and two external experts. The expertise of the members is equally balanced between solar irradiance variability and numerical simulations, with some overlap. By focusing on specific issues, the team will answer the question whether, and how, 3D MHD simulations of the solar atmosphere can be employed to improve our understanding of solar spectral irradiance variations, and reproduce them more faithfully for the purpose of improving our knowledge of the Sun-Earth atmosphere interaction.