Solar Radius at Subterahertz Frequencies and Its Relation to 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 Submillimeterwave Telescope (SST), installed at El Leoncito Astronomical Complex (CASLEO) in the Argentinean Andes. Our measurements yield radii of 966.5 ′′ ± 2.8 ′′ for 0.2 THz and 966.5 ′′ ± 2.7 ′′ for 0.4 THz. This implies a height of (5.0 ± 2.0 × 10 6 ) m above the photosphere. Furthermore, we also observed strong anti-correlation between the radius variation and the solar activity at both frequencies.

The subterahertz solar cycle: Polar and equatorial radii derived from SST and ALMA

“…Wavelength Frequency Radius Altitude (arcsec) (10 6 m) Fürst et al (1979) 1 dm 3 GHz 1070 ± 17 80 ± 12 Fürst et al (1979) 6 cm 5 GHz 1020 ± 9 44 ± 7 Bachurin (1983) 3.3 cm 9 GHz 989 ± 2 21 ± 1 Fürst et al (1979) 2.7 cm 11 GHz 991 ± 5 23 ± 4 Bachurin (1983) 2.3 cm 13 GHz 989 ± 2 21 ± 1 Wrixon (1970) 1.9 cm 16 GHz 990 ± 4 22 ± 3 Selhorst et al (2004) 1.8 cm 17 GHz 976.6 ± 1.5 12.3 ± 1.1 Costa et al (1986) 1.4 cm 22 GHz 981.7 ± 0.8 16.0 ± 0.6 Fürst et al (1979) 1.2 cm 25 GHz 979 ± 4 14 ± 3 Wrixon (1970) 1 cm 30 GHz 979 ± 4 14 ± 3 Pelyushenko & Chernyshev (1983) 8.6 mm 35 GHz 979 ± 3 14 ± 2 Selhorst et al (2019a) 8.1 mm 37 GHz 979 ± 5 14 ± 4 Costa et al (1986) 6.8 mm 44 GHz 978.1 ± 1.3 13.4 ± 0.9 Costa et al (1999) 6.2 mm 48 GHz 983.6 ± 1.9 17.4 ± 1.4 Pelyushenko & Chernyshev (1983) 6.2 mm 48 GHz 973.1 ± 2.9 9.8 ± 2.1 Coates (1958) 4.3 mm 70 GHz 969 ± 5 7 ± 4 Kislyakov et al (1975) 4 mm 74 GHz 967 ± 4 5 ± 3 Swanson (1973) 3.2 mm 94 GHz 972 ± 5 9 ± 4 Alissandrakis et al (2017) 3 mm 100 GHz 964.1 ± 4.5 3.2 ± 3.3 Labrum et al (1978) 3 mm 100 GHz 966 ± 1 5.0 ± 0.7 Selhorst et al (2019b) 3 mm 100 GHz 965.9 ± 3.2 4.5 ± 2.3 Wannier et al (1983) 2.6 mm 115 GHz 969.3 ± 1.6 7.0 ± 1.2 Menezes & Valio (2017) 1.4 mm 212 GHz 966.5 ± 2.8 5.0 ± 2.0 Alissandrakis et al (2017) 1.3 mm 230 GHz 961.1 ± 2.5 1.1 ± 1.8 Selhorst et al (2019b) 1.3 mm 230 GHz 961.6 ± 2.1 1.4 ± 1.5 Horne et al (1981) 1.3 mm 231 GHz 968.2 ± 1.0 6.2 ± 7.3 Menezes & Valio (2017) 0.7 mm 405 GHz 966.5 ± 2.7 5.0 ± 2.0…”

“…The study of the radio solar radius provides important information about the solar atmosphere and activity cycle (Swanson 1973;Costa et al 1999;Menezes & Valio 2017;Selhorst et al 2019b). Using radio measurements of the solar radius derived from eclipse and direct observations one can probe the solar atmosphere, since these measurements show the height above the photosphere at which most of the emission at determined observation frequency is generated (Swanson 1973;Menezes & Valio 2017;Selhorst et al 2019b). However, as the observation frequency changes, the height changes as well (Selhorst et al 2004(Selhorst et al , 2019bMenezes & Valio 2017).…”

“…Observations at radio wavelengths began to take place after 1950 (Coates 1958), and many authors had been using measurements of the solar disk size -i.e. the center-to-limb distance -as ways to determine the solar disk radius (hereafter solar radius) at different radio wavelengths (Coates 1958;Wrixon 1970;Swanson 1973;Kislyakov et al 1975;Labrum et al 1978;Fürst et al 1979;Horne et al 1981;Bachurin 1983;Pelyushenko & Chernyshev 1983;Wannier et al 1983;Costa et al 1986Costa et al , 1999Selhorst et al 2004;Alissandrakis et al 2017;Menezes & Valio 2017;Selhorst et al 2019b,a). There are different techniques to measure the solar radius at radio frequencies, such as the determination from total solar eclipse observations (Kubo 1993;Kilcik et al 2009), and from direct observations as the inflection point method (Alissandrakis et al 2017) and the the half-power method (Costa et al 1999;Selhorst et al 2011;Menezes & Valio 2017).…”

Subterahertz Radius and Limb Brightening of the Sun Derived from SST and ALMA

“…Therefore at a specific wavelength, the difference of ★ E-mail: menezes.astroph@gmail.com the measured radius from the optical radius can be understood as the height above the photosphere where most of the emission is being created in the limb regions. Moreover, the study of the solar radius at different wavelengths, derived from eclipse and direct observations, enables probing the solar atmosphere throughout the solar activity cycle (Swanson 1973;Costa et al 1999;Menezes & Valio 2017;Selhorst et al 2019b).…”

“…At radio wavelengths, the radius can be measured by the inflectionpoint method (Alissandrakis et al 2017;Menezes et al 2021), and by the half-power method (Costa et al 1999;Selhorst et al 2011;Menezes & Valio 2017). The measurement of the solar radius by any of theses two methods is considerably affected by the presence of limb brightening in the solar disk.…”