Solar Radius at Subterahertz Frequencies and Its Relation to Solar Activity


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.

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  • 12 June 2018

    Correction to: Solar Phys (2017) 292:195

    In this article we forgot to acknowledge the financial support for operation of the Solar Submillimeter Telescope (SST) from FAPESP (Proc. #2013/24155-3) and AFOSR Grant #FA9550-16-1-0072. The authors apologize for this negligence.


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    ADC: analog-to-digital converter units that are linearly proportional to temperature.


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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.

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Correspondence to Fabian Menezes.

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Menezes, F., Valio, A. Solar Radius at Subterahertz Frequencies and Its Relation to Solar Activity. Sol Phys 292, 195 (2017).

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  • Solar radius
  • Solar cycle
  • Solar atmosphere