Issue |
A&A
Volume 680, December 2023
|
|
---|---|---|
Article Number | A61 | |
Number of page(s) | 13 | |
Section | The Sun and the Heliosphere | |
DOI | https://doi.org/10.1051/0004-6361/202347414 | |
Published online | 08 December 2023 |
Dark halos around solar active regions
I. Emission properties of the dark halo around NOAA 12706
1
Università di Napoli “Federico II”, C.U. Monte Sant’Angelo, Via Cinthia, 80126 Naples, Italy
2
INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Naples, Italy
e-mail: serena.lezzi@inaf.it
3
DAMTP, University of Cambridge, Wilberforce Rd, Cambridge, CB3 0WA, UK
Received:
10
July
2023
Accepted:
19
September
2023
Context. Dark areas around active regions (ARs) were first observed in chromospheric lines more than a century ago and are now associated with the Hα 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 the low corona. For example, they are clearly seen in the SDO/AIA 171 Å images. We name these chromospheric and TR-coronal dark regions “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).
Aims. 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 to investigate the different properties of DHs and CHs and provide a quick-look recipe to distinguish between them.
Methods. We studied the DH around AR NOAA 12706 and the southern CH that were on the disk on April 22, 2018 by analyzing IRIS full-disk mosaics and SDO/AIA filtergrams to evaluate their average intensities, normalized to the QS. In addition, we used the AIA images to derive the DH and CH emission measure (EM) and the IRIS Si IV 1393.7 Å line to estimate the nonthermal velocities of plasma in the TR. We also employed SDO/HMI magnetograms to study the average magnetic field strength inside the DH and the CH.
Results. 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 than in the 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, EM, spectral profile, nonthermal 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.
Key words: Sun: corona / Sun: atmosphere / Sun: chromosphere / Sun: transition region / Sun: activity / Sun: UV radiation
© The Authors 2023
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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