Understanding and modeling of solar-irradiance variability is important not only for solar physics but also for solar-terrestrial and solar-stellar studies. The latest irradiance measurements call into question aspects of currently-available empirical and semi-empirical models of solar-irradiance variability. A new generation of significantly more realistic physics-based irradiance models can now be created to incorporate recent advances in modeling and observing the solar atmosphere. This next generation of irradiance models will include new advances in MHD, surface flux transport, and radiative transfer simulations as well as new state-of-the-art solar data. By relying on physics-based understandings rather than merely empirical relationships established for the Sun, these new models will also allow more direct and physical extrapolations to other stars, opening a new regime for solar-stellar connection studies, as well as improved long-term estimates of historical solar variability.

1.  Overview of existing solar irradiance datasets (models and observations).
2.  Proxies of long-term solar magnetic activity.
3.  State-of-the-art in solar irradiance modeling.
4.  Simulations of solar surface magnetic field distribution with surface flux transport models.
5.  Structure of solar magnetic features: what can we learn from the MHD simulations?
6.  Radiative transfer calculations for next generation of irradiance models.
7.  Brightness contrasts of solar magnetic features from high-resolution solar imagery
(SUNRISE, SDO, HINODE, etc.).
8.  Solar irradiance variability on timescales less than a day: magnetic and non-magnetic components.
9.  Can we use solar models to explain brightness variations of Sun-like stars?
10. Climate research needs for solar-irradiance time series: temporal and spectral coverage, critical issues, and priorities.

Will Ball (Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland)
Serena Criscuoli (National Solar Observatory, Boulder, USA)
Thierry Dudok de Wit (CNRS Orleans Campus, France)
Ricky Egeland (High Altitude Observatory,  Boulder, USA)
Hideyuki Hotta (Chiba University, Japan)
Emre Isik (Max Planck Institute for Solar System Research, Göttingen, Germany)
Richard Radick (National Solar Observatory, Sacramento Peak, United States)
Rob Rutten (Institute for Theoretical Astrophysics, Oslo, Norway)
Tatiana Ryabchikova (Institute for Astronomy RAS, Moscow, Russia)
Hauke Schmidt (Max Planck Institute for Meteorology, Hamburg, Germany)
Sami Solanki (Max Planck Institute for Solar System Research, Göttingen, Germany)

• Robert Cameron - Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany
• Paul Charbonneau - Université de Montréal, Quebec (QC), Canada
• Ilaria Ermolli - Osservatorio Astronomico di Roma, Roma, Italy
• *Juan Fontenla - NorthWest Research Associates, Boulder, USA  (passed away in Jan. 2018)
• Mark Giampapa - National Solar Observatory, USA
• Jie Jiang - Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences, Beijing, China
• Greg Kopp - Laboratory for Atmospheric and Space Physics, Boulder, USA (co-chair and co-editor of proceedings)
• Matthieu Kretzschmar - Univ. of Orleans & CNRS, Orléans, France
• Natalie Krivova - Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany
• Werner Schmutz - Physikalisch-Meteorologishes Observatorium Davos, World Radiation Center, Switzerland
• Alexander Shapiro - Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany (co-chair & co-editor of proceedings)
• Yvonne Unruh - Blackett Laboratory Imperial College London, UK
• Ilya Usoskin - University of Oulu, Oulu, Finland
• Aline Vidotto - Trinity College Dublin, Ireland

Solar irradiance varies on all timescales at which it has ever been observed and presumably also at longer ones. Irradiance variability on timescales longer than a day is attributed to solar surface magnetic field. On shorter timescales the magnetic-caused irradiance variability is complemented by the constantly evolving granulation pattern and solar oscillations.  Interest in solar irradiance variability extends well beyond solar community. The terrestrial atmospheric and climate systems respond to variations in solar radiative output on timescales from days to decades, and there is also evidence for solar influences on climate over longer timescales. The variability of solar irradiance is also of interest to stellar astronomers, who have been comparing it with the variability of other lower main sequence stars. Understanding the physics behind solar variability helps assess stellar brightness variations (and vice-versa) and the resulting effects on the detectability and habitability of exoplanets.  Early models of solar irradiance variability were purely empirical. These were primarily based on observed correlations between measured solar irradiance and solar magnetic activity proxies. Successive generations of models utilized semi-empirical 1D models of solar magnetic features and surrounding quiet regions. The brightness contrasts of magnetic features have been calculated with a simplified radiative transfer, e.g. often ignoring effects of non-local thermodynamic equilibrium. These semi-empirical models provided insights into the causes of irradiance variability and enabled understanding of the physics behind the empirical correlations.
While the semi-empirical approach has been at the forefront of irradiance models for many years, it is largely based on foregone concepts and does not incorporate recent achievements in solar physics. Most semi-empirical models are based on 1D representations of the solar atmosphere that were developed more than two decades ago before the advent of high-resolution solar data and reliable MHD simulations. Since these representations depend on a number of free parameters, semi-empirical models do not unambiguously constrain solar irradiance variability.
This became particularly conspicuous when data from the Solar Irradiance Monitor (SIM) on the Solar Radiation and Climate Experiment (SORCE) suggested a pattern of solar irradiance variability completely different from that yielded by empirical and semi-empirical models as well as by preceding observations. While it is now believed that the SIM results are due to high instrumental uncertainties, the intense discussion and controversy prompted by these data  revealed significant gaps in the present understandings of solar irradiance variability. Furthermore, recent debate on the historical solar forcing recommendation for the Coupled Model Intercomparison Project Phase 6 (CMIP6) to be used in the upcoming International Panel on Climate Change (IPCC-6) assessment showed that these gaps hinder the progress in modeling of the Sun-Earth connection by potentially misleading climate researchers.  Benefitting from the enormous recent progress in solar observations and models, it is now possible to develop a third generation of irradiance models based on the current state-of-the-art in solar physics. In particular:

-  3D magneto-hydrodynamic (MHD) simulations of flows and magnetic fields in the near-surface layers of the Sun and stars have reached a high level of realism, and can now reproduce many sensitive observational tests. These simulations make it possible to replace 1D representations of the solar atmosphere with realistic 3D simulations and also enable assessment of the contributions of granulation to short-term solar irradiance variability.

-  New time-efficient radiative transfer codes and approaches have been developed. These allow calculated emergent spectra from 3D MHD cubes to account for effects from millions of atomic and molecular lines as well as deviations from local thermodynamic equilibrium, giving more accurate estimates of outgoing radiation as a function of position on the solar disk.

-  New atomic and molecular data allow more reliable computation of the opacities in the solar atmosphere. The irradiance variability in the UV, violet, blue, and green spectral domains is fully controlled by millions of the Fraunhofer lines. Recent advances in laboratory astrophysics and in collecting the data (e.g. a major upgrade of the Vienna atomic line database, which now also includes molecular data) make possible significantly more accurate calculations of solar irradiance variability.

-  Surface flux transport models (SFTM) now more realistically simulate the evolution of the large- scale surface magnetic field over solar cycle. This allows reconstructing the evolution of the solar surface magnetic field and irradiance over long timescales, which is crucial to understanding the pre-anthropogenic solar contributions to climate change from which natural sensitivities of climate can best be determined.

-  The magnetic features on the solar surface, which are the main driver of solar irradiance variability, can now be directly studied with high-resolution imagery from recent solar missions such as the Solar Dynamics Observatory (SDO), STEREO, SUNRISE, HINODE, etc. SDO in particular provides frequent space-based magnetograms, which are needed inputs to the newest physics-based solar irradiance models.

The incorporation of the aforementioned solar modeling and measurement improvements into irradiance models will provide more reliable and self-consistent solar irradiance records to the climate community. These physics-based models are also more straightforward to extrapolate to other Sun-like stars and will thus provide a more direct link to and understanding of stellar variability. In particular, models will facilitate distinguishing between typical photometric signatures of intrinsic stellar variations and exoplanet transits. The interest in this topic has been recently raised to a new level by the CoRoT and Kepler space missions, and with anticipation of the upcoming NASA TESS and ESA PLATO missions.  The challenge to incorporate recent advances in solar physics into this next generation of irradiance models requires communication and collaboration between the irradiance community and researchers working in the fields of solar and stellar MHD simulations, radiative transfer, and surface-flux transport. Since IAU General Assemblies are widely attended by all these communities, a Focus Meeting during the 2018 IAU GA is an excellent opportunity to push the models of solar irradiance variability to a next level.