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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 chromosphere, transition region (TR) and corona, and known as Dark Halos (DHs). DHs have been insufficiently studied, and the mechanisms behind their darker emission remain unclear. This study aims to investigate for the first time the fine structure of a DH observed by the EUV High Resolution Imager (HRI$_{EUV}$) onboard 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 and the Spectral Imaging of the Coronal Environment (SPICE) Ly$\beta$, 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 which is not observed in the quiet Sun, 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 \r{A} DH may lie in the low atmosphere, i.e. photosphere/chromosphere., Comment: 9 pages, 8 figures; accepted for A&A

Dark areas around active regions (ARs) have been first observed in chromospheric lines more than a century ago and are now associated to the H{\alpha} fibril vortex around ARs. Nowadays, large areas surrounding ARs with reduced emission relative to the Quiet Sun (QS) are also observed in spectral lines emitted in the transition-region (TR) and low-corona. For example, they are clearly seen in the SDO/AIA 171 {\AA} images. We name these chromospheric and TR/coronal dark regions as Dark Halos (DHs). Coronal DHs are poorly studied and, because their origin is still unknown, to date it is not clear if they are related to the chromospheric fibrillar ones. Furthermore, they are often mistaken for Coronal Holes (CHs). Our goal is to characterize the emission properties of a DH by combining, for the first time, chromospheric, TR and coronal observations in order to provide observational constraints for future studies on the origin of DHs. This study also aims at investigating the different properties of DHs and CHs and at providing a quick-look recipe to distinguish between them. We study the DH around AR NOAA 12706 and the southern CH, that were on the disk on 2018 April 22, by analyzing IRIS full-disk mosaics, SDO/AIA filtergrams and SDO/HMI magnetograms. Fibrils are observed all around the AR core in the chromospheric Mg II h&k IRIS mosaics, most clearly in the h3 and k3 features. The TR emission in the DH is much lower compared to QS area, unlike in the CH. Moreover, the DH is much more extended in the low-corona than in the chromospheric Mg II h3 and k3 images. Finally, the intensities, emission measure, spectral profile, non-thermal velocity and average magnetic field strength measurements clearly show that DHs and CHs exhibit different characteristics and therefore should be considered as distinct types of structures on the Sun., Comment: 13 pages, 12 figures; accepted for A&A; degraded images to accomplish size limits