Solar Radius at Subterahertz Frequencies and its Relation to Solar Activity

TL;DR

This study measures the solar radius at subterahertz frequencies using radio maps from 1999 to 2017, revealing altitude-dependent radius variations and a strong anti-correlation with solar activity, enhancing understanding of the solar atmosphere.

Contribution

It provides new measurements of the solar radius at subterahertz frequencies and analyzes its variation over the solar cycle, linking atmospheric structure changes to solar activity.

Findings

Measured radii of 966.5" at 0.2 and 0.4 THz.

Estimated altitude of emission sources at about 5 million meters above photosphere.

Found strong anti-correlation between radius variation and solar activity.

Abstract

The Sun emits radiation at several wavelengths of the electromagnetic spectrum. In the optical band, the solar radius is 695,700 km and this is what defines the photosphere, 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 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 -- i.e., the altitude where these emissions are primarily generated -- and also analyse 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" $\pm$ 2.8" for 0.2 THz and 966.5" $\pm$ 2.7" for 0.4 THz. This implies a height of $(5.0 \pm 2.0 \times 10^6)$ m above the photosphere. Furthermore, we also observed strong anti-correlation between the radius variation and the solar activity at both frequencies.