Magnetic structure of coronal dark halos

¹ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
² Institut für Sonnenphysik (KIS), Georges-Köhler-Allee 401A, 79110 Freiburg, Germany
³ Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Braunschweig, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it. , This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 27 August 2025
Accepted: 14 March 2026

Abstract

Context. At low coronal temperatures around or below 1 MK distinct areas in the surroundings of active regions (ARs) show emission at a level significantly below the emission coming from the quiet Sun (QS). These areas are referred to as dark halos, dark canopies, or dark moats.

Aims. To better understand the nature of dark halos, we studied the connection between the photospheric magnetic field and coronal emission at different temperatures.

Methods. Combining Solar Orbiter data from the high-resolution Polarimetric and Helioseismic Imager (SO/PHI) and Extreme Ultraviolet Imager (EUI) instruments allowed us to identify the areas that are dark in the extreme ultraviolet in the immediate vicinity of an AR. We probed the photospheric magnetic field as well as the coronal intensities as a function of distance to the AR NOAA 12893.

Results. The dark halo has an unsigned magnetic flux density similar to the QS, but shows a strong radial dependence with distance from the AR centre. It drops by 38% from 6.1 G at the inner boundary to 3.8 G at the outer, shifting from above to below QS levels. Coronal emission ≤1 MK is ∼40% below QS and shows no dependence on distance to the AR centre. In contrast, at ≥1.6 MK, emission exceeds QS levels, but declines outwards towards QS values. A few hot loops extend from the AR periphery across the halo, while at lower temperatures no such loops appear and short loops dominate the corona.

Conclusions. The reduced unsigned magnetic flux density in the outermost parts of the dark halo, below QS level, suggests that reduced coronal heating due to weak underlying magnetic flux heating could be partially responsible for the reduced emission around 1 MK. Closer to the AR, other mechanisms might lead to reduced heating. The different loop structures detected for hotter and cooler coronal temperatures likely play a crucial role in understanding coronal dark halos.