EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Accounting for small bipolar magnetic regions in solar irradiance reconstructions

Bernhard Hofer1,2, Natalie A. Krivova1, Sami K. Solanki1,3, Robert Cameron1, Chi-Ju Wu1, and Ilya G. Usoskin4
Bernhard Hofer et al.
  • 1Max-Planck-Institut für Sonnensystemforschung, Sun and Heliosphere, Göttingen, Germany (
  • 2Georg-August-Universität Göttingen, Institute for Astrophysics, Göttingen, Germany
  • 3School of Space Research, Kyung Hee University, Yongin, Republic of Korea
  • 4Space Climate Research Unit and Sodankylä Geophysical Observatory, University of Oulu, Oulu, Finland

Historical solar irradiance is a critical input to climate models. As no direct measurements are available before 1978, reconstructions of past irradiance changes are employed instead. Such reconstructions are based on the knowledge that solar irradiance on time scales of interest to climate studies is modulated by the evolution of the solar surface magnetic structures, such as sunspots and faculae. This calls for historical records or proxies of such features. The longest direct, and thus mostly used, record is the sunspot number. It allows a reasonable description of the emergence and evolution of active regions, which are larger magnetic regions containing sunspots. At the same time, a significant amount of the magnetic flux on the Sun emerges in the form of the so-called ephemeral magnetic regions, which are weaker short-lived bipolar regions that do not contain sunspots. Due to their high frequency, ephemeral regions are an important source of the irradiance variability, especially on time scales longer than the solar cycle. Difficulties in their proper accounting are a main reason for the high uncertainty in the secular irradiance variability. Existing models either do not account for their evolution at all or link them linearly to active regions. We use a new, more realistic model of the ephemeral region emergence, relying on recent independent solar observations, as input to a surface flux transport model (SFTM) to simulate the evolution of the magnetic field in such regions. The latter can then be used to reconstruct the solar irradiance since the Maunder minimum.

How to cite: Hofer, B., Krivova, N. A., Solanki, S. K., Cameron, R., Wu, C.-J., and Usoskin, I. G.: Accounting for small bipolar magnetic regions in solar irradiance reconstructions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8323,, 2021.


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