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<p>Coronal holes (CHs) are low-density regions in the solar atmosphere, characterised by open magnetic field lines driving the high-speed solar wind. While appearing dark in EUV/X-rays, their radio signatures are complex and frequency-dependent, showing either brightness depressions or enhancements relative to the quiet-Sun (QS). This variability reflects the complex inner temperature, density and magnetic structures of the solar atmosphere.</p><p>Within the SunDish project, we investigate the CH phenomenology using the National Institute for Astrophysics (INAF) large single-dish radio telescopes (Medicina ‘Gavril Grueff’ 32-m and Sardinia Radio Telescope ‘SRT’ 64-m), aiming to bridge the observational gap between the chromosphere and corona by exploring the under-investigated K-band (18 – <InlineEquation ID="IEq2"> <EquationSource Format="MATHML"><math> <mn>26</mn> <mtext>&#xa0;GHz</mtext> </math></EquationSource> <EquationSource Format="TEX">$26\text{ GHz}$</EquationSource> </InlineEquation>). This study analyses three representative scenarios: expected brightness temperature (<InlineEquation ID="IEq3"> <EquationSource Format="MATHML"><math> <msub> <mi>T</mi> <mi>b</mi> </msub> </math></EquationSource> <EquationSource Format="TEX">$T_{b}$</EquationSource> </InlineEquation>) depressions, radio-enhanced CHs, and radio-dark regions (RDRs) unrelated to standard CHs.</p><p>Our analysis reveals a non-uniform phenomenology where radio-enhanced CHs and RDRs identify deep-penetrating structures affecting the upper chromosphere, including the first radio counterpart to a dark halo. Spectral indices and weak circular polarisation confirm thermal free-free emission as the primary mechanism, excluding significant gyro-magnetic contributions. However, magnetic field estimates suggest that a purely thermal model in a simple stratified atmosphere cannot fully describe these regions, where radio enhancements likely arise from unresolved structures like coronal bright points and magnetic flux tubes. These results highlight multi-frequency diagnostics as an essential tool for probing the coupling between different layers of the solar atmosphere, demonstrating that CHs are dynamic environments where complex magnetic activity drives their vertical structure and evolution.</p>

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Exploring Coronal Holes Through K-Band Radio Observations (18 – \(26\text{ GHz}\)) with INAF Radio Telescopes

  • 期刊论文

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  • 发表日期：

    2026年8月4日

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  • Marco Marongiu,
  • Giulia Murtas,
  • Alberto Pellizzoni,
  • Sara Mulas,
  • Maria Noemi Iacolina,
  • Simona Righini,
  • Elise Egron,
  • Abimael Xavier Barbosa Amaro,
  • Adriana Marcucci,
  • Massimiliano Oranges,
  • Davide Napolitano,
  • Maddalena Sirigu,
  • Mauro Messerotti,
  • Salvatore Luigi Guglielmino,
  • Alessandra Zanichelli,
  • Vincenzo Andretta,
  • Serena Maria Lezzi,
  • Luca Schirru,
  • Adelaide Ladu,
  • Pierluigi Ortu,
  • Giuseppe Valente,
  • Mauro Pili,
  • Tonino Pisanu,
  • Giampaolo Serra,
  • Alessandro Cabras,
  • Andrea Melis,
  • Pasqualino Marongiu,
  • Alessandro Navarrini,
  • Giuseppe Pupillo,
  • Mattia Mancini,
  • Pietro Zucca

摘要

Coronal holes (CHs) are low-density regions in the solar atmosphere, characterised by open magnetic field lines driving the high-speed solar wind. While appearing dark in EUV/X-rays, their radio signatures are complex and frequency-dependent, showing either brightness depressions or enhancements relative to the quiet-Sun (QS). This variability reflects the complex inner temperature, density and magnetic structures of the solar atmosphere.

Within the SunDish project, we investigate the CH phenomenology using the National Institute for Astrophysics (INAF) large single-dish radio telescopes (Medicina ‘Gavril Grueff’ 32-m and Sardinia Radio Telescope ‘SRT’ 64-m), aiming to bridge the observational gap between the chromosphere and corona by exploring the under-investigated K-band (18 –  26  GHz $26\text{ GHz}$ ). This study analyses three representative scenarios: expected brightness temperature ( T b $T_{b}$ ) depressions, radio-enhanced CHs, and radio-dark regions (RDRs) unrelated to standard CHs.

Our analysis reveals a non-uniform phenomenology where radio-enhanced CHs and RDRs identify deep-penetrating structures affecting the upper chromosphere, including the first radio counterpart to a dark halo. Spectral indices and weak circular polarisation confirm thermal free-free emission as the primary mechanism, excluding significant gyro-magnetic contributions. However, magnetic field estimates suggest that a purely thermal model in a simple stratified atmosphere cannot fully describe these regions, where radio enhancements likely arise from unresolved structures like coronal bright points and magnetic flux tubes. These results highlight multi-frequency diagnostics as an essential tool for probing the coupling between different layers of the solar atmosphere, demonstrating that CHs are dynamic environments where complex magnetic activity drives their vertical structure and evolution.