The Temperature – Magnetic Field Relation in Observed and Simulated Sunspots

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Abstract

Observations of the relation between continuum intensity and magnetic field strength in sunspots have been made for nearly five decades. This work presents full-Stokes measurements of the full-split (\(g = 3\)) line Fe i 1564.85 nm with a spatial resolution of \(0.5^{\prime\prime}\) obtained with the GREGOR Infrared Spectrograph in three large sunspots. The continuum intensity is corrected for instrumental scattered light, and the brightness temperature is calculated. Magnetic field strength and inclination are derived directly from the line split and the ratio of Stokes components. The continuum intensity (temperature) relations to the field strength are studied separately in the umbra, light bridges, and penumbra. The results are consistent with previous studies, and it was found that the scatter of values in the relations increases with increasing spatial resolution thanks to resolved fine structures. The observed relations show trends common for the umbra, light bridges, and the inner penumbra, while the outer penumbra has a weaker magnetic field than the inner penumbra at equal continuum intensities. This fact can be interpreted in terms of the interlocking comb magnetic structure of the penumbra. A comparison with data obtained from numerical simulations was made. The simulated data generally have a stronger magnetic field and a weaker continuum intensity than the observations, which may be explained by stray light and limited spatial resolution of the observations, and also by photometric inaccuracies of the simulations.

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  1. 1.

    See http://download.hao.ucar.edu/pub/rempel/sunspot_models/Fine_Structure/ .

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Acknowledgements

This work was supported by the grant 14-04338S of the Czech Science Foundation, the FP-7 Capacities Project No. 312495 SOLARNET, and the institutional support RVO:67985815 of the Czech Academy of Sciences. R.R. acknowledges financial support by the Spanish Ministry of Economy and Competitiveness through the project AYA2014-60476-P. We thank J.M. Borrero for synthetic spectra computed in the frame of the international working group Extracting Information from Spectropolarimetric Observations: Comparison of Inversion Codes at the International Space Science Institute (ISSI) in Bern (Switzerland). We use data provided by M. Rempel at the National Center for Atmospheric Research (NCAR). The National Center for Atmospheric Research is sponsored by the National Science Foundation. The 1.5-meter GREGOR solar telescope was built by a German consortium under the leadership of the Kiepenheuer Institute for Solar Physics in Freiburg with the Leibniz Institute for Astrophysics Potsdam, the Institute of Astrophysics Göttingen, and the Max Planck Institute for Solar System Research in Göttingen as partners, and with contributions by the Instituto de Astrofísica de Canarias and the Astronomical Institute of the Czech Academy of Sciences. We thank the referee for the comments that led to a substantial improvement of the paper.

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Correspondence to Michal Sobotka.

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Sobotka, M., Rezaei, R. The Temperature – Magnetic Field Relation in Observed and Simulated Sunspots. Sol Phys 292, 188 (2017). https://doi.org/10.1007/s11207-017-1220-2

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Keywords

  • Sunspots
  • Magnetic fields