High flow speeds and transition-region like temperatures in the solar chromosphere during flux emergence

Abstract

Context: Flux emergence in the solar atmosphere is a complex process that causes a release of magnetic energy as heat and acceleration of solar plasma at a variety of spatial scales. Methods: We analysed imaging spectropolarimetric data taken in the He 1083 nm line. This line is sensitive to temperatures larger than 15 kK, unlike diagnostics such as \MgIIhk, \CaIIHK, and \Halpha, which lose sensitivity already at 15 kK. The He I data is complemented by imaging spectropolarimetry in the \CaIIK, \Feline, and \Caline\ lines. We employed inversions to determine the magnetic field and vertical velocity in the solar atmosphere. We computed He 1083 nm profiles from a radiation-MHD simulation of the solar atmosphere to help interpretation of the observations. Results: We find fast-evolving blob-like emission features in the He 1083 nm triplet at locations where the magnetic field is rapidly changing direction, and these are likely sites of magnetic reconnection. We fit the line with a model consisting of an emitting layer located below a cold layer representing the fibril canopy. The modelling provides evidence that this model, while simple, catches the essential characteristics of the line formation. The morphology of the emission in the He 1083 nm line is localized and blob-like, unlike the emission in the \CaIIK\ line, which is more filamentary. Conclusions: The modelling shows that the \Heline\ emission features and their Doppler shifts can be caused by opposite-polarity reconnection and/or horizontal current sheets below the canopy layer in the chromosphere. Based on the high observed Doppler width and the blob-like appearance of the emission features, we conjecture that at least a fraction of them are produced by plasmoids. We conclude that transition-region-like temperatures in the deeper layers of the active region chromosphere are more common than previously thought.

arXiv commentAbstract abridged. Submitted to A&A, movies available at https://dubshen.astro.su.se/~jleen/hespmov/

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• On the center-to-limb variations of the He I 10830 Å triplet
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