Solar Irradiance is known to vary on temporal scales ranging from minutes to centuries and beyond. The amplitudes of these variations are strongly dependent on the wavelength and on the time range considered. In the last decades several studies have focused on the understanding and modeling of irradiance variations, especially those measured on the solar-activity time scale. Apart from improving our understanding of the physical processes that determine the radiative output of the Sun, and, more in general, of Sun-like stars, the interest in this area of investigation has also been driven by the effects that solar spectral irradiance variations have on the Earth’s atmosphere and climate.
How the physical and chemical processes taking place in the Earth’s atmosphere are affected by solar spectral variability is still poorly understood. These studies are also strongly hampered by the difficulties in providing the Earth atmosphere modelers with long and homogeneous irradiance measurements. Such measurements are in fact obtained with space-based instruments, which are affected by a fast (compared to the temporal scales one would like to investigate) deterioration induced by the harsh space environment. As a result, calibrating these instruments is extremely difficult and often relies on measurements obtained with different instruments or on independent irradiance reconstructions.  The latter are generally performed by combining measures of the variation of surface magnetism, as derived by the analysis of full-disk data, with estimates of the radiative output as either synthesized with atmosphere models or derived from regression techniques (see Ermolli et al. 2013; Yeo et al. 2014 for recent reviews). These reconstruction techniques have been proven successful so far in matching more than 90% of the variability of the measured Total Irradiance (i.e., the irradiance integrated over the whole spectrum), but the agreement is worse when studying time scales longer than one decade and/or restricting the analysis to finite spectral ranges. Moreover, these techniques are often targeted to reproduce certain parts of the spectrum at certain temporal scales, so that they may fail when employed to reconstruct different temporal and spectral ranges. Finally, the uncertainties in the reconstructions are still too large to conclusively assess the influence of solar spectral irradiance on Earth’s atmosphere and climate (e.g. Thuillier et al. 2014; Ball et al. 2014).    
The solar irradiance variability is influenced by a multitude of magnetohydrodynamic processes at different spatial and temporal scales. Consequently, the atmosphere modeling and radiative transfer calculations needed to advance our understanding of this problem are extremely complex and challenging. Spectral irradiance reconstruction techniques developed so far take into account such complexity only partially at best. In this context, a step forward in modeling such processes would be the use of three-dimensional magnetohydrodynamic (3D MHD) simulations of the solar photosphere and chromosphere, instead of the one-dimensional static atmosphere models employed so far for irradiance reconstruction purposes. These simulations, which typically represent small areas of the solar atmosphere (linear dimensions up to few tens of Mm) with high spatial resolution (few tens of kilometers or better), have indeed been proven to reproduce the observed properties of the solar spectrum with a higher degree of accuracy with respect to one-dimensional models (e.g. Asplund et al. 2009; Wedemeyer-Böhm & Rouppe van der Voort 2009; Pereira et al. 2013). Moreover, Uitenbroek & Criscuoli (2011) showed that, because of the three-dimensional nature of radiative transfer processes, semi-empirical one-dimensional models often employed in irradiance reconstructions, derived to reproduce certain ranges of the solar spectrum, do not describe physical average properties of the solar atmosphere, so that it is not surprising that these models may fail when employed to reproduce other ranges of the spectrum. Therefore, there is now a general consensus among the scientific community that the use of 3D MHD simulations of the lower levels of the solar atmosphere for spectral irradiance reconstructions should be investigated. Nonetheless, the literature on this subject is still scarce. So far, most of the analysis focused on deriving the photometric properties of ‘bright’ magnetic regions (network and faculae) from the simulations and commenting the results in the context of irradiance (e.g. Afram et al. 2011; Criscuoli 2013; Thaler & Spruit 2014). Recently, Criscuoli & Uitenbroek (2014) made a step forward by using outputs from 3D MHD simulations and a simple model of the distribution of magnetic features over the cycle to interpret irradiance measurements obtained by SIM radiometers. In particular, they showed that the solar cycle counter-phase signal measured in the visible and infrared, which is not reproduced by most of irradiance reconstruction techniques, may have a solar origin.
Up to now, the use of 3D MHD simulations for irradiance reconstruction studies has been hampered by various factors, such as the different spatial scales at which simulations and observations are performed, the highly demanding computational resources for both the atmosphere modeling and the radiative output estimation and, last but not least, the separation between the community specialized in irradiance studies and the one specialized in 3D MHD modeling with multi-dimensional radiative transfer. The formation of an international ISSI team will be a unique opportunity to bring the two communities together to discuss open questions related to irradiance measuring and modeling and, though the synergies that will be thus established, to foster irradiance reconstruction models based on 3D MHD simulations.


The team will have three main goals:
  1. Review achievements and limitations of irradiance reconstruction techniques and measurements on one side, and achievements and limitations of numerical simulations on the other.
  2.  Investigate the potential and the future development of numerical simulations and their use for irradiance modeling. In particular, the team will address, among other aspects, how small-scale simulations with limited spatial extent can be employed to model the global variations of spectral irradiance, the treatment of multidimensional radiative transfer, the influence of numerical schemes and resolution on the radiative output, computational limitations, the validation of simulations against observations, the role of a local dynamo, and sunspot modeling.
  3.   Initiate the creation of new state-of-the-art irradiance reconstruction models based on 3D MHD simulations.


The team will meet twice, about 12 months apart. During the first meeting the discussion will focus on points 1 and 2 listed above. In particular, specific questions regarding the improvement and validation of 3D MHD simulations for irradiance reconstruction purposes will be formulated.  Sub-groups of 3 / 4 people will then work on answering these questions and the results will be discussed during the second meeting. The outcome of the discussions and the results obtained by the team will be employed to help develop new spectral irradiance reconstruction techniques. We require support for two one-week (5 days) meetings.


The development, or at least the definition of the fundaments for the development, of one or more spectral irradiance techniques based on the use of 3D MHD simulations will be an important achievement for the scientific community. We also expect to publish a minimum of two scientific papers in peer review journals on issues listed under number 2) of the ‘Goals’ section.


ISSI has supported in the past other teams that focused on the measurement and reconstruction of solar irradiance variations measured by space-based instrumentation. The purpose of the proposed team is to explore a new approach, based on the use of 3D MHD simulations, for the modeling and interpretation of such measurements. One of the main goals of the project is to bring together experts on solar irradiance and experts on numerical simulations of the solar photosphere/chromosphere. The modus operandi of the ISSI international Teams offers a unique opportunity for cross-disciplinary discussions and therefore for the realization of this project. Moreover, the home institutes of team members are in three different continents and the ISSI in Bern is in a very accessible location from most EU and non-EU countries.


The team members that have agreed participate to the meeting are listed below. The number of experts on numerical simulations and on solar irradiance variability is approximately equal, with some overlap. The members’ expertise covers three of the most successful numerical codes developed so far to model solar and stellar atmospheres. The direct expertise of the members in irradiance reconstruction is, given the variety of available models, necessarily more limited. Nevertheless, many of the members have long experience and vast knowledge about solar irradiance (some of them are Principal Investigators for International Projects related to solar irradiance) so that they will be able to provide insight on techniques that they have not directly worked on. Two external experts will participate in the discussions: Katja Matthes, expert on the effects of solar spectral irradiance on the Earth’s atmosphere and climate; and Jerry Harder, expert on irradiance measurements. Finally, if the proposal will be accepted, up to two young scientists will be invited to join the team.