Solar Physics

, 292:195 | Cite as

Solar Radius at Subterahertz Frequencies and Its Relation to Solar Activity

  • Fabian MenezesEmail author
  • Adriana Valio


The Sun emits radiation at several wavelengths of the electromagnetic spectrum. In the optical band, the solar radius is 695 700 km, and this defines the photosphere, which is the visible surface of the Sun. However, as the altitude increases, the electromagnetic radiation is produced at other frequencies, causing the solar radius to change as a function of wavelength. These measurements enable a better understanding of the solar atmosphere, and the radius dependence on the solar cycle is a good indicator of the changes that occur in the atmospheric structure. We measure the solar radius at the subterahertz frequencies of 0.212 and 0.405 THz, which is the altitude at which these emissions are primarily generated, and also analyze the radius variation over the 11-year solar activity cycle. For this, we used radio maps of the solar disk for the period between 1999 and 2017, reconstructed from daily scans made by the Solar Submillimeter-wave Telescope (SST), installed at El Leoncito Astronomical Complex (CASLEO) in the Argentinean Andes. Our measurements yield radii of \(966.5'' \pm2.8''\) for 0.2 THz and \(966.5'' \pm2.7''\) for 0.4 THz. This implies a height of \(5.0 \pm2.0 \times10^{6}\) m above the photosphere. Furthermore, we also observed a strong anticorrelation between the radius variation and the solar activity at both frequencies.


Solar radius Solar cycle Solar atmosphere 



We would like to acknowledge the visionary insight of the late Pierre Kaufmann, who envisioned the many possibilities of monitoring the Sun at high submillimeter-frequencies and built the SST telescope in Argentina. Moreover, we thank CASLEO and CRAAM for the data provided. The authors also thank C. Selhorst, J. Valle, and D. Cornejo for fruitful discussions and the anonymous referee for valuable input. F. M. thanks CAPES for the graduate scholarship.

Disclosure of Potential Conflicts of Interest

The authors declare that they have no conflicts of interest.


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© Springer Science+Business Media B.V., part of Springer Nature 2017

Authors and Affiliations

  1. 1.Center for Radio Astronomy and Astrophysics (CRAAM)Mackenzie Presbyterian UniversitySão PauloBrazil

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