Rationale

Regular spaceborne measurements of the total solar irradiance (TSI) started in 1978 with the launch of the NIMBUS 7 mission. These sensitive measurements revealed that the TSI varies on multiple timescales. Two striking features of the TSI record are 1) a ∼0.1% modulation that is in-phase with the 11-year solar activity cycle, and 2) more irregular variations with amplitudes of up to 0.3% on the multiple-day timescales. Measurements of the spectral solar irradiance (SSI) indicate variability on the same timescales. The SSI variability strongly depends on wavelength and generally increases towards shorter wavelengths. It is widely accepted now that the solar irradiance variability on timescales greater than a day is caused by the evolution of solar surface magnetic fields, whereas the variability on shorter periods is due to the global superposition of convection and oscillations. 

While the TSI variability on multiple-day and 11-year time scales is relatively well constrained, there are several issues, particularly regarding the newer SSI measurements, that the Commission E1 Working Group will address: 

• The magnitude (primarily in the UV part of the spectrum) and even the phase (particularly in the visible) of the 11-year solar cycle SSI variations remain highly controversial;
  
  
• Reliable estimates of uncertainties in many irradiance datasets are missing;
  
  
• There are no direct irradiance measurements prior to the satellite era to help constrain the variability of solar irradiance on centennial time scales, which is needed for Earth climate studies;
  
  
• Coordinated efforts for data intercomparisons between various independent measurements as well as between measurements and models are needed. 

Some of these issues can be addressed at the instrument level, while others will benefit from comparisons between data and solar models or between solar and stellar observations. The E1 Working Group brings together experts representing all these fields. 

Over the last decade a number of solar irradiance variability models have been created to better understand the mechanisms of solar irradiance variability and to provide extended and uninterrupted irradiance records. While most of the models yield a qualitatively similar picture of solar irradiance variability, there are a number of significant systematic discrepancies between their results. In particular, proxy-based and semi-empirical models indicate different solar irradiance variability in the UV. Also the mechanisms of solar irradiance variability on centennial timescales are poorly understood, indicating a need for the models to incorporate more physics-based constraints. This requires a close collaboration between solar irradiance observers and modellers as well as coordinated assistance from experts in MHD simulations and radiative transfer. 

In this context the main goal of the Group will be to improve collaboration between different solar irradiance modellers and observers, promote further progress in the field by addressing the primary issues listed above, and, since the interest in solar irradiance variability extends well beyond the solar community, recommend which solar irradiance datasets to use for such external researchers. By coordinating the efforts of and facilitating collaborations between the Working Group members, one result of this group will be a database (in the form of links to original sources) of different irradiance datasets, and, most importantly, recommendations on applicability and uncertainties of each of the datasets for various external data users. 

The group also includes several experts in MHD simulations, solar magnetic fields, and radiative transfer. These members will provide valuable constraints for the SSI modellers and help them to understand the uncertainties of the modelled irradiance as well as to improve the irradiance models. Comparisons between the Sun and stellar analogs, enabled by E1 Working Group members with solar-stellar expertise, will improve physics-based constraints on solar variability over centennial timescales, which are critical for long-term Earth climate studies.