FM09: Solar Irradiance — Physics-Based Advances

Solar Spectrum

An artificially created representation of the solar spectrum. Each of the 50 slices covers 6 nanometers in wavelength, together spanning the spectrum across the visual range from 400 to 700 nm. The wavelength increases from top to bottom and from left to right along each strip. (N. A. Sharp, NOAO / NSO / Kitt Peak FTS / AURA / NSF)

Solar irradiance varies on all timescales over which it has been observed and, presumably, also over longer timescales. Interest in solar irradiance variability extends well beyond the solar-physics community. Terrestrial atmospheric and climate systems respond to variations in solar radiative output on timescales from days to decades, and there is 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.

Recent controversies and debates have shown that currently available empirical and semi-empirical models of solar irradiance variability are not yet ready to answer all critical questions posed by available measurements. At the same time, recent advances in the modeling and observing of the solar atmosphere make it possible to create a new generation of significantly more realistic, physics-based irradiance models.

Having benefited from significant recent progress in solar observations and models, it is now possible to develop a new generation of irradiance models based on current state-of-the-art 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 observations. These simulations make it possible to replace 1D representations of the solar atmosphere with realistic 3D simulations; they also enable the 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 opacities in the solar atmosphere. The irradiance variability in the ultraviolet, violet, blue and green spectral domains is fully controlled by millions of Fraunhofer lines. Recent advances in laboratory astrophysics and in collecting 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) more realistically simulate the evolution of the large-scale surface magnetic field during the solar cycle. This allows the reconstruction of 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. 
  • Magnetic features on the solar surface, which are the main drivers 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, and Hinode. SDO in particular provides frequent space-based magnetograms, which are needed inputs to the newest physics-based solar irradiance models.

Focus Meeting 9 brings together experts within these areas of research who will discuss progress within their individual fields and the means of incorporating these advances into the irradiance models.

Our list of invited speakers includes the following (in alphabetical order):

  • Will Ball (Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland);
  • Christoffer Karoff (Aarhus University, Denmark);
  • 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);
  • Ansgar Reiners (Institute for Astrophysics, University of Goettingen, Germany);
  • 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).
GREG KOPP, a senior research scientist at the University of Colorado, is the instrument scientist for the Total Irradiance Monitor instruments aboard several satellites. He studies solar variability and Earth’s climate. ALEXANDER SHAPIRO works at the Max Planck Institute for Solar System Research. He is a Leader of the SOLVe Research Group funded by an ERC Starting Grant. He studies solar and stellar variability.