Magnetic structure of coronal dark halos

Abstract

At low coronal temperatures around or below 1 MK distinct areas in the surroundings of active regions (AR) 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. To better understand the nature of dark halos we study the connection between the photospheric magnetic field and coronal emission at different temperatures. Combining Solar Orbiter data from the high-resolution Polarimetric and Helioseismic Imager (SO/PHI) and Extreme Ultraviolet Imager (EUI) instruments allows us to identify these areas that are dark in the extreme ultraviolet (EUV) in the immediate vicinity of an AR. We probe both the photospheric magnetic field as well as the coronal intensities as a function of distance to the AR NOAA 12893. 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 $\leq$1 MK is $\sim$40 % below QS and shows no dependence on distance to the AR centre. In contrast, at $\geq$1.6 MK, emission exceeds QS levels, but declines outward toward 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. 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 loops structures detected for hotter and cooler coronal temperatures likely play a crucial role in understanding coronal dark halos.

Figures (13)

• Figure 1: The $\mathrm{HRI_{EUV}}$-image (left) and SO/PHI-HRT-magnetogram (right). The 75% intensity contours of the dark halo are plotted in white on the $\mathrm{HRI_{EUV}}$-image and are projected onto the SO/PHI-HRT-magnetogram. The individual dark halo patches are labeled in white on the $\mathrm{HRI_{EUV}}$ image. The green rectangle on the bottom indicates the area of the reference QS.
• Figure 2: The EUV emissions for three different SDO/AIA passbands: 171Å (left), 193Å (middle), and 211Å (right). The contours of the dark halo (white), same as in Fig. \ref{['overview_EUI+PHI']}, have been projected onto each of the images and the orange square shows the FOV of the $\mathrm{HRI_{EUV}}$ observation. In the 193Å image we also show the extent of the equatorial CH (blue contour) derived by an intensity-threshold of 40% of the channels mean disc-intensity.
• Figure 3: The relative mean $\mathrm{HRI_{EUV}}$-intensity inside rings around the AR obtained by dividing the mean intensity in a ring by the mean value of the reference QS. The panels on the left and on the right show the relative mean intensity inside the dark halo (i.e., we averaged over all dark halo patches at a certain distance to the AR and do not treat them separately) and for the brighter areas in-between the dark halo, respectively.
• Figure 4: Open magnetic field originating from the CH adjacent to the AR (blue contours), including from within the dark halo patch DH3 (white contours). The image shows the SDO/AIA 193Å channel on 2021 November 6 12:00 with the footpoints of open field lines in green.
• Figure 5: The relative mean $\mathrm{HRI_{EUV}}$-intensity as a function of distance to the AR. The mean intensity has been normalised by the mean value derived for the reference QS. The relative mean intensity inside the dark halo (blue) and for the brighter areas in-between the dark halo (dark-red) are shown. The green line denotes the value for the reference QS.