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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. 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. 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_ Ly alpha image. We also analyzed the Spectral Imaging of the Coronal Environment (SPICE) Lybeta C III N VI O VI and Ne VIII lines, which sample the TR in the logT (K) sim 4. 0 - 5. 8 range. This analysis was complemented with a simultaneous B_ LOS magnetogram taken by the High Resolution Telescope (HRT). 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_ Ly alpha band and present photospheric B_ LOS footprints in the HRT magnetogram. This spatial correlation indicates that the origin of the 174 A DH may lie in the low atmosphere: the photosphere and chromosphere.
Lezzi et al. (Mon,) studied this question.