Solar Orbiter's First Observation of Dark Halo in Solar Atmosphere Reveals New Insights
The Solar Orbiter mission has made a significant observation of a phenomenon known as a dark halo in the solar atmosphere. This observation, detailed in a recent paper by Serena Maria Lezzi and colleagues, marks the first time that the fine structure of a dark halo has been studied using the Extreme Ultraviolet High Resolution Imager (HRIEUV) onboard the Solar Orbiter.
Dark halos (DHs) are large areas surrounding solar active regions (ARs) that exhibit reduced emission compared to the surrounding quiet Sun. Despite their presence, the mechanisms behind their darker emission have not been thoroughly understood until now.
The study utilized a one-hour dataset collected on March 19, 2022, which included high-resolution observations of NOAA 12967 and the surrounding dark halo. The researchers analyzed the dynamics of the DH's fine structure and its appearance in various spectral lines, including Lyα, C III, N VI, O VI, and Ne VIII, which sample the transition region (TR) at temperatures between approximately 4,000 K and 5,800 K.
A notable finding from this research is the identification of a peculiar fine structure within the dark halo, characterized by bright EUV bundles and dark regions that are interconnected, forming a spatial continuum that extends radially from the core of the active region. This suggests a deep connection between the dark halo and the active region itself. Furthermore, the bright EUV bundles were observed across all spectral lines analyzed, indicating that the origin of the 174 Å dark halo may be linked to the low atmosphere, specifically the photosphere and chromosphere.
These findings contribute to a better understanding of solar dynamics and the behavior of solar active regions, which can have implications for space weather forecasting and our overall comprehension of solar phenomena. The full paper can be accessed at arXiv:2408.17172.