First Solar Orbiter observation of a dark halo in the solar atmosphere

¹ Università di Napoli “Federico II”, C.U. Monte Sant’Angelo, Via Cinthia, I-80126 Napoli, Italy
² INAF Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, I-80131 Napoli, Italy
³ School of Physical Sciences, Dublin City University, Glasnevin Campus, Dublin D09V209, Ireland
⁴ University College London, Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
⁵ Institut d’Astrophysique Spatiale, Bâtiment 121, Rue Jean Dominique Cassini, Université Paris Saclay, 91405 Orsay, France
⁶ INAF Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Roma, I-00078 Monte Porzio Catone, Roma, Italy
⁷ Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico, s.n.c. I-00133, Roma, Italy

Received: 17 June 2024
Accepted: 29 August 2024

Abstract

Context. Solar active regions (ARs) are often surrounded by dark large areas of reduced emission compared to the quiet Sun, observed at various wavelengths corresponding to the chromosphere, transition region (TR), and corona, known as dark halos (DHs). The mechanisms behind the darker emission of DHs remain unclear and merit a wider scope of study.

Aims. This study aims to investigate for the first time the fine structure of a DH observed by the EUV High Resolution Imager (HRI_EUV) on board the ESA’s Solar Orbiter (SO) mission and its appearance in the TR.

Aims. We utilized the extensive 1 hour dataset from SO on 19 March 2022, which includes high-resolution observations of NOAA 12967 and part of the surrounding DH. We analyzed the dynamics of the HRI_EUV DH fine structure and its appearance in the HRI_Lyα image. We also analyzed the Spectral Imaging of the Coronal Environment (SPICE) Lyβ, C III, N VI, O VI, and Ne VIII lines, which sample the TR in the log T(K) ∼ 4.0–5.8 range. This analysis was complemented with a simultaneous B_LOS magnetogram taken by the High Resolution Telescope (HRT).

Methods. We report the presence of a peculiar fine structure that has not been observed for the quiet Sun. It is characterized by combined bright EUV bundles and dark regions, arranged and interconnected in such a way that they cannot be clearly separated. They form a spatial continuum extending approximately radially from the AR core, suggesting a deep connection between the DH and the AR. Additionally, we find that the bright EUV bundles are observed in all the SPICE TR lines and the HRI_Lyα band and present photospheric B_LOS footprints in the HRT magnetogram. This spatial correlation indicates that the origin of the 174 Å DH may lie in the low atmosphere: the photosphere and chromosphere.