Study of solar brightness profiles in the 18–26 GHz frequency range with INAF radio telescopes

I. Solar radius

¹ INAF – Cagliari Astronomical Observatory, Via della Scienza 5, 09047 Selargius, CA, Italy
e-mail: marco.marongiu@inaf.it
² INAF – Institute of Radio Astronomy, Via Gobetti 101, 40129 Bologna, Italy
³ Istituto Nazionale di Astrofisica (INAF/OAA), Largo Enrico Fermi 5, 50125 Firenze, Italy
⁴ Los Alamos National Laboratory, Bikini Atoll Rd, Los Alamos, NM 87545, USA
⁵ ASI – c/o Cagliari Astronomical Observatory, Via della Scienza 5, 09047 Selargius, CA, Italy
⁶ INAF – Catania Astrophysical Observatory, Via Santa Sofia 78, 95123 Catania, Italy
⁷ INAF – Turin Astrophysical Observatory, Via Osservatorio 20, 10025 Pino Torinese, TO, Italy
⁸ INAF – Trieste Astronomical Observatory, Via Giambattista Tiepolo 11, 34131 Trieste, Italy
⁹ Department of Physics, University of Trieste, Via Alfonso Valerio 2, 34127 Trieste, Italy
¹⁰ NRAO – Central Development Laboratory, 1180 Boxwood Estate Rd, Charlottesville, VA 22903, USA
¹¹ ASTRON – The Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands

Received: 28 November 2023
Accepted: 20 January 2024

Abstract

Context. The Sun is an extraordinary workbench, on which several fundamental astronomical parameters can be measured with high precision. Among these parameters, the solar radius R_⊙ plays an important role in several aspects, for instance, in evolutionary models. Moreover, it conveys information about the structure of the different layers that compose the solar interior and its atmosphere. Despite the efforts to obtain accurate measurements of R_⊙, the subject is still debated, and measurements are puzzling and/or lacking in many frequency ranges.

Aims. We determine the mean, equatorial, and polar radii of the Sun (R_c, R_eq, and R_pol) in the frequency range 18.1 − 26.1 GHz. We employed single-dish observations from the newly appointed Medicina Gavril Grueff Radio Telescope and the Sardinia Radio Telescope (SRT) in five years, from 2018 to mid-2023, in the framework of the SunDish project for solar monitoring.

Methods. Two methods for calculating the radius at radio frequencies were employed and compared: the half-power, and the inflection point. To assess the quality of our radius determinations, we also analysed the possible degrading effects of the antenna beam pattern on our solar maps using two 2D models (ECB and 2GECB). We carried out a correlation analysis with the evolution of the solar cycle by calculating Pearson’s correlation coefficient ρ in the 13-month running means.

Results. We obtained several values for the solar radius, ranging between 959 and 994 arcsec, and ρ, with typical errors of a few arcseconds. These values constrain the correlation between the solar radius and solar activity, and they allow us to estimate the level of solar prolatness in the centimeter frequency range.

Conclusions. Our R_⊙ measurements are consistent with the values reported in the literature, and they provide refined estimates in the centimeter range. The results suggest a weak prolateness of the solar limb (R_eq > R_pol), although R_eq and R_pol are statistically compatible within 3σ errors. The correlation analysis using the solar images from the Grueff Radio Telescope shows (1) a positive correlation between solar activity and the temporal variation in R_c (and R_eq) at all observing frequencies, and (2) a weak anti-correlation between the temporal variation of R_pol and solar activity at 25.8 GHz.