The Dark Halos (DHs) are regions of reduced emission in the neighborhood of solar active regions (ARs). Since their discovery (Hale & Ellerman, 1903), DHs have been called with various names, the first one being circumfacules (Deslanders, 1930), as they were first identified in the chromosphere as faint areas surrounding plages in the Ca II K-line, and later also in the Ca II 8542 Å line (D’Azumbuja, 1930). Bumba & Howard (1965) proposed that the Ca II circumfacules are composed of dark features corresponding to dark Hα fibrils, and this suggestion was supported by different authors (e.g. Rutten et al. 2007, Pietarila et al. 2009).
A typical chromospheric fibrillar DH normally has a counterpart also in the other layers of the solar atmosphere. Indeed, with the advent of UV and EUV observations, it has been possible to observe dark regions around ARs also in a wide range of spectral lines originating in Transition Region and low corona. DHs around ARs are seen as areas of reduced emission in many wavelengths, for instance in the SOHO/SUMER images in O V at 630 Å and S VI at 933 Å lines, as pointed out by Andretta et al. (2014), and in the SDO/AIA 171 Å waveband, as noted by Wang et al. (2011).
DHs are very common solar features, yet we do not fully understand what their structure is, what determines their appearance, nor the physical mechanism that creates and sustains them. Moreover, because they are large-scale structures, of an extent comparable, if not larger, than that of the associated ARs, they may influence the irradiance of the Sun, especially during the maximum of the solar activity, when several ARs are present on the solar disk.
Furthermore, DHs are sometimes mistaken for Coronal Holes (CHs) when seen in the UV and EUV. Nevertheless, to date there has not been a quantitative characterization that allows a clear distinction between these two solar features. In particular, the connectivity of DHs with the outer solar atmosphere and the solar wind must be different from the one of CHs in a way that is not yet clear.
In this work we report preliminary results of the analysis of a comprehensive set of imaging and spectroscopic observations of the DH around AR 12706, observed on the solar disk on 22 April 2018, by using IRIS full-disk mosaics and SDO/AIA images, that together allow a tomography of the DH from the chromosphere up to the low corona. The first results highlight the main observational characteristics (line intensities, Mg II h & k fibrils) that can help distinguish between DHs and CHs. These characteristics can be used in future works aimed at systematically studying DHs.
Co-authors: Vincenzo Andretta, Mariarita Murabito