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

II. Evidence of coronal emission

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 INAF – Astrophysical Observatory of Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁴ University of Florence, Department of Physics and Astronomy, Via Giovanni Sansone 1, 50019 Sesto Fiorentino (FI), Italy
⁵ ASI – c/o Cagliari Astronomical Observatory, Via della Scienza 5, 09047 Selargius (CA), Italy
⁶ Los Alamos National Laboratory, Bikini Atoll Rd, Los Alamos, NM 87545, USA
⁷ INAF – Catania Astrophysical Observatory, Via Santa Sofia 78, 95123 Catania, Italy
⁸ ASTRON – The Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
⁹ INAF – Turin Astrophysical Observatory, Via Osservatorio 20, 10025 Pino Torinese (TO), Italy
¹⁰ INAF – Trieste Astronomical Observatory, Via Giambattista Tiepolo 11, 34131 Trieste, Italy
¹¹ Department of Physics, University of Trieste, Via Alfonso Valerio 2, 34127 Trieste, Italy
¹² NRAO – Central Development Laboratory, 1180 Boxwood Estate Rd, Charlottesville, VA 22903, USA
¹³ INAF – Astronomical Observatory of Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy

Received: 28 November 2023
Accepted: 10 February 2024

Abstract

Context. One of the most important objectives of solar physics is to gain a physical understanding of the solar atmosphere, whose structure can also be described in terms of the density (N) and temperature (T) distributions of the atmospheric matter. Several multi-frequency analyses have shown that the characteristics of these distributions are still under debate, especially for outer coronal emission.

Aims. We aim to constrain the T and N distributions of the solar atmosphere through observations in the centimetric radio domain. We employed single-dish observations from two of the INAF radio telescopes at the K-band frequencies (18–26 GHz). We investigated the origin of the significant brightness temperature (T_B) detected up to the upper corona (at an altitude of ∼800 Mm with respect to the photospheric solar surface).

Methods. To probe the physical origin of the atmospheric emission and to constrain instrumental biases, we reproduced the solar signal by convolving specific 2D antenna beam models. We 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. We compared the modelled T_B profiles with those observed by averaging solar maps obtained at 18.3 and 25.8 GHz during the minimum of solar activity (2018–2020).

Results. We 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.

Conclusions. Our 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 T_B profiles. Our results suggest that the modelled T and N distributions are in good agreement (within 25% of uncertainty) with our solar maps up to altitudes of ≲100 Mm. 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.