Lezzi, Serena Maria (2024) Dark Halos around active regions in the Solar Atmosphere. [Tesi di dottorato]

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Abstract

When observed at certain wavelengths mapping the chromosphere, the transition region and the corona, active regions (ARs) on the Sun often appear surrounded by large areas of reduced emission compared to the quiet Sun. These large-scale regions are long lasting and very common on the Sun. Nevertheless they are decidedly under-studied, mainly because it is difficult to observe them in their entirety with high resolution spectrographs. If in the chromosphere it is understood that these areas, historically known as circumfacular regions, correspond to opaque chromospheric AR fibrils, in the upper layers of the solar atmosphere the physical mechanism that originates them is still debated. For simplicity, I call these solar structures dark halos. This is a generic expression that should always be accompanied by the observed wavelength or waveband of the specific studied case. This thesis is devoted to studying dark halos from an observational point of view by following a multi-wavelength, multi-instrument approach that tries to comprehensively involve all the atmospheric layers at which these solar structures are observed. In the first part of the work a diverse dataset of both spectroscopic and narrow-band images is exploited to characterize, from chromosphere to corona, the dark halo around AR NOAA 12706 and the coronal hole observed on 22 April 2018 by the Interface Region Imaging Spectrograph (IRIS), the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI). The emission properties of these structures over the different layers of the solar atmosphere are investigated. The results of the analysis suggest a substantial difference between the dark halo and the coronal hole, that henceforth should be considered two different types of dark solar structures. Moreover, this analysis shows that the dark halo’s spatial extent seems to go trough a spatial enlargement when passing from the chromosphere to the low corona. The main focus of the second part of the work is the study of the morphology of the extreme ultraviolet (EUV) fine structure of the dark halo in the proximity of AR NOAA 12967 and its appearance at cooler (chromospheric and transition region) temperatures by analyzing data taken on 19 March 2022 by the High Resolution Imager (HRI), the Spectral Imaging of the Coronal Environment (SPICE) and the High Resolution Telescope (HRT) onboard Solar Orbiter. In particular, HRI’s EUV observations at unprecedented spatial resolution show that the dark halo has a fine structure, made up of dark regions and bright EUV bundles, that cannot be separated from the plage of the AR itself. The bright EUV bundles of the DH’s fine structure can be traced back to bright patches observed in SPICE transition region lines in the logT [K]= 4.0 - 5.8 range and in chromospheric HRILyα images, and even to photospheric magnetic patches in the HRT magnetogram. These footprints place the origin of the EUV dark halo in the lower atmosphere, i.e. chromosphere and/or photosphere. In the last part of the work, monochromatic spectroscopic observations of a sample of dark halos in corona taken on 16 January 2013 by the Extreme ultraviolet Imaging Spectrometer (EIS) onboard Hinode are examined and analyzed. The analysis shows that dark halos can be detected up to logT= 6.3, and that no clear radial Doppler and non-thermal velocity patterns can be distinguished inside the dark halo areas. Finally, in the light of these findings, I propose a physical scenario in which the dark halo is composed of closed low-lying loops whose fainter EUV emission from temperatures below logT= 6.3 may be the consequence of acoustic power suppression due to the heavily inclined chromospheric magnetic field at the edges of the AR plage. This power suppression would cause reduced heating that propagates up to the low corona, causing a depletion in EUV intensity. In this scenario, the origin of the coronal DH is chromospheric, but it affects the solar atmosphere up to the low corona. The observational evidence obtained during the work of this thesis demonstrates that dark halos are primarily a transition region phenomenon clearly associated with the AR’s magnetic configuration. The thesis is organized as follows: in Chapter 1 I provide a brief introduction on the relevant concepts and terms used in Solar Physics and in this work; in Chapter 2 I provide a review of the literature on dark halos and the specific questions that this thesis aims to answer; the instruments used in the investigation have been described in Chapter 3; Chapter 4 reports the analysis concerning the emission properties of the dark halo around NOAA 12706 from chromosphere to corona; the study described in Chapter 5 focuses on the fine structure of the dark halo around NOAA 12967; a spectroscopic study of a sample of dark halos observed in the solar corona on 16 January 2013 is presented in Chapter 6; the conclusions drawn from this thesis are reported in Chapter 7.

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