Two publications about the size and the atmosphere of the Sun are nearing publication in Astronomy & Astrophysics. These works ("Study of solar brightness profiles in the 18-26 GHz frequency range with INAF radio telescopes I. Solar radius" and "Study of solar brightness profiles in the 18-26 GHz frequency range with INAF radio telescopes II. Evidence of coronal emission") bear the signature of Marco Marongiu, post-doc of the Cagliari Astronomical Observatory, in collaboration with the SunDish team. These papers 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.

In the first paper they determine the mean, equatorial, and polar radii of the Sun in the frequency range 18.1 – 26.1 GHz. 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, they also analysed the possible degrading effects of the antenna beam pattern on our solar maps using two 2D models (ECB and 2GECB). They 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. They 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. Their 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 (equatorial radius > polar radius), although the equatorial and polar radii 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 the mean radius (and the equatorial radius) at all observing frequencies, and (2) a weak anti-correlation between the temporal variation of the polar radius and solar activity at 25.8 GHz.

In the second paper they aim to constrain the temperature (T) and density (N) distributions of the solar atmosphere through observations in the centimetric radio domain. They investigated the origin of the significant brightness temperature (TB) detected up to the upper corona (at an altitude of ∼ 800 Mm with respect to the photospheric solar surface). To probe the physical origin of the atmospheric emission and to constrain instrumental biases, they reproduced the solar signal by convolving specific 2D antenna beam models. They performed an analysis of the solar atmosphere by adopting a physical model that assumes the thermal bremsstrahlung as the emission mechanism, with specific T and N distributions. They compared the modelled TB profiles with those observed by averaging solar maps obtained at 18.3 and 25.8 GHz during the minimum of solar activity (2018 – 2020). They probed any possible discrepancies between the T and N distributions assumed from the model and those derived from our measurements. The T and N distributions are compatible (within a 25% of uncertainty) with the model up to ∼ 60 Mm and ∼ 100 Mm in altitude, respectively. Their analysis of the role of the antenna beam pattern on our solar maps proves the physical nature of the atmospheric emission in our images up to the coronal tails seen in our TB profiles. A subsequent, more challenging analysis of the coronal radio emission at higher altitudes, together with the data from satellite instruments, will require further multi-frequency measurements.

Read on ArXiv: “Study of solar brightness profiles in the 18-26 GHz frequency range with INAF radio telescopes I. Solar radius" by Marco Marongiu, Alberto Pellizzoni, Sara Mulas, Simona Righini, Renzo Nesti et al.

Read on ArXiv: “Study of solar brightness profiles in the 18-26 GHz frequency range with INAF radio telescopes II. Evidence for coronal emission" by Marco Marongiu, Alberto Pellizzoni, Simona Righini, Sara Mulas, Renzo Nesti et al.

Our first pubblication "Solar observations with single-dish INAF radio telescopes: continuum imaging in the 18-26 GHz range" about a new solar radio imaging system in the context of the SunDish project is now available at this link. This system is implemented through the upgrade of the large single-dish telescopes of the Italian National Institute for Astrophysics (INAF), not originally conceived for solar observations.

During the development and early science phase of the project (2018-2020), about 170 maps of the entire solar disk are obtained in the 18-26 GHz band, filling the observational gap in the field of solar imaging at these frequencies. These solar images have typical resolutions in the 0.7-2 arcmin range and a brightness temperature sensitivity <10 K. Accurate calibration adopting the Supernova Remnant Cas A as a flux reference, provided typical errors <3% for the estimation of the quiet-Sun level components and for active regions flux measurements.

As the first early science result of the project, this work includes a catalog of radio continuum solar imaging observations with Medicina 32-m and SRT 64-m radio telescopes, including the multi-wavelength identification of active regions, their brightness and spectral characterization. The interpretation of the observed emission as thermal bremsstrahlung components combined with gyro-magnetic variable emission pave the way to the use of the imaging system of this work for long-term monitoring of the Sun. Also, this paper focuses on useful outcomes both for solar physics (e.g. study of the chromospheric network dynamics) and space weather applications (e.g. flare precursors studies).

Read on Solar Physics: “Solar observations with single-dish INAF radio telescopes: continuum imaging in the 18-26 GHz range"  by Alberto Pellizzoni, Simona Righini, Maria Noemi Iacolina, Marco Marongiu, Sara Mulas, Giulia Murtas, Giuseppe Valente, Elise Egron, et al.