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The nature of solar brightness variations

Abstract

Determining the sources of solar brightness variations1,2, often referred to as solar noise3, is important because solar noise limits the detection of solar oscillations3, is one of the drivers of the Earth’s climate system4,5 and is a prototype of stellar variability6,7—an important limiting factor for the detection of extrasolar planets. Here, we model the magnetic contribution to solar brightness variability using high-cadence8,9 observations from the Solar Dynamics Observatory (SDO) and the Spectral And Total Irradiance REconstruction (SATIRE)10,11 model. The brightness variations caused by the constantly evolving cellular granulation pattern on the solar surface were computed with the Max Planck Institute for Solar System Research (MPS)/University of Chicago Radiative Magnetohydrodynamics (MURaM)12 code. We found that the surface magnetic field and granulation can together precisely explain solar noise (that is, solar variability excluding oscillations) on timescales from minutes to decades, accounting for all timescales that have so far been resolved or covered by irradiance measurements. We demonstrate that no other sources of variability are required to explain the data. Recent measurements of Sun-like stars by the COnvection ROtation and planetary Transits (CoRoT)13 and Kepler14 missions uncovered brightness variations similar to that of the Sun, but with a much wider variety of patterns15. Our finding that solar brightness variations can be replicated in detail with just two well-known sources will greatly simplify future modelling of existing CoRoT and Kepler as well as anticipated Transiting Exoplanet Survey Satellite16 and PLAnetary Transits and Oscillations of stars (PLATO)17 data.

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Fig. 1: Short-term TSI variability at three intervals of very different activity level and variability of the Sun.
Fig. 2: TSI variability on timescales from 4 min to 19 years.

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Acknowledgements

The authors received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 624817 and the European Research Council under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 715947). Financial support was also provided by the Brain Korea 21 plus program through the National Research Foundation funded by the Ministry of Education of Korea and by the German Federal Ministry of Education and Research under project 01LG1209A. We would like to thank the International Space Science Institute, Bern, for their support of science team 373 and the resulting helpful discussions.

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A.I.S., S.K.S. and N.A.K. designed the study. A.I.S. performed the calculations with contributions from R.H.C., who provided the MURaM time series, and K.L.Y., who prepared the HMI/SDO magnetograms and corrected them for noise. N.A.K. and S.K.S. led the development of the SATIRE code. R.H.C. actively contributed to the development of the MURaM code. W.K.S. provided the PREMOS data and expertise on the TSI data. A.I.S., S.K.S., N.A.K. and R.H.C. wrote the paper.

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Correspondence to A. I. Shapiro.

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Shapiro, A.I., Solanki, S.K., Krivova, N.A. et al. The nature of solar brightness variations. Nat Astron 1, 612–616 (2017). https://doi.org/10.1038/s41550-017-0217-y

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