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Chromospheric condensations and magnetic field in a C3.6-class flare studied via He I D-3 spectro-polarimetry
Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Faculty of Science, Department of Astronomy.ORCID iD: 0000-0002-4640-5658
Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Faculty of Science, Department of Astronomy.
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Number of Authors: 52019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 621, article id A35Article in journal (Refereed) Published
Abstract [en]

Context. Magnetic reconnection during flares takes place in the corona, but a substantial part of flare energy is deposited in the chromosphere. However, high-resolution spectro-polarimetric chromospheric observations of flares are very rare. The most used observables are Ca II 8542 angstrom and He I 10830 angstrom.

Aims. We aim to study the chromosphere during a C3.6 class flare via spectro-polarimetric observations of the He I D-3 line.

Methods. We present the first SST/CRISP spectro-polarimetric observations of He I D3. We analyzed the data using the inversion code HAZEL, and estimate the line-of-sight velocity and the magnetic field vector.

Results. Strong He I D-3 emission at the flare footpoints, as well as strong He I D(3 )absorption profiles tracing the flaring loops are observed during the flare. The He I D-3 traveling emission kernels at the flare footpoints exhibit strong chromospheric condensations of up to similar to 60 km s(-1) at their leading edge. Our observations suggest that such condensations result in shocking the deep chromosphere, causing broad and modestly blueshifted He I D-3 profiles indicating subsequent upflows. A strong and rather vertical magnetic field of up to similar to 2500 G is measured in the flare footpoints, confirming that the He I D-3 line is likely formed in the deep chromosphere at those locations. We provide chromospheric line-of-sight velocity and magnetic field maps obtained via He I D-3 inversions. We propose a fan-spine configuration as the flare magnetic field topology.

Conclusions. The He I D-3 line is an excellent diagnostic to study the chromosphere during flares. The impact of strong condensations on the deep chromosphere has been observed. Detailed maps of the flare dynamics and the magnetic field are obtained.

Place, publisher, year, edition, pages
2019. Vol. 621, article id A35
Keywords [en]
Sun: flares, Sun: atmosphere, Sun: activity, Sun: magnetic fields, radiative transfer, line: formation
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-165689DOI: 10.1051/0004-6361/201833610ISI: 000454875500009OAI: oai:DiVA.org:su-165689DiVA, id: diva2:1285989
Available from: 2019-02-05 Created: 2019-02-05 Last updated: 2019-03-12Bibliographically approved
In thesis
1. The diagnostic potential of the He I D3 spectral line in the solar atmosphere
Open this publication in new window or tab >>The diagnostic potential of the He I D3 spectral line in the solar atmosphere
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The research question of my PhD is in a way a simple one: what can observations of the He I Dline teach us about the solar chromosphere? This optical spectral line at 5876 Å is generally formed in the upper chromosphere, and is sensitive to the local magnetic field. The He I D3 line is also indirectly sensitive to heating of the transition region and corona, since it is resulting from a transition that occurs between levels in the triplet system of neutral helium. These levels are generally populated via an ionization-recombination mechanism under the influence of EUV radiation originating in the transition region and corona.

The He I D3 line was used as a flare diagnostic in the seventies and in the subsequent decades also to measure magnetic fields in prominences. However, due to the poor spatial resolution and low signal-to-noise of that data, almost exclusively off-limb targets have been studied. The on-disk absorption of He I D3 is very weak and localized. Recent instrumental developments allow for the acquisition of high spatial resolution on-disk spectroscopic and spectro-polarimetric data of He I D3 with different instruments at the SST, opening the possibility of studying all types of targets in the chromosphere in a new light. During my PhD, I have focused on the study of reconnection targets via high-resolution observations of He I D3 with TRIPPEL and CRISP at the SST, in co-observation with space-borne instruments. Subsequently, a theoretical study has aimed at in-depth understanding of He I D3 line formation in small-scale reconnection events.

The data which I have obtained and analyzed during my PhD has provided new insights in Ellerman bombs and flares. Our He I D3 observations have suggested that the temperature of Ellerman Bombs is higher than 2×104 K based on the discovery of helium emission signatures in these events. This result is unexpected, since previous modeling in the literature estimates the temperatures of Ellerman Bombs below 104 K. Subsequently, 3D non-LTE radiative transfer calculations have revealed the detailed physical mechanisms to generate He I D3 emission in these events. The calculations also confirmed that temperatures between 2×104 - 106 K are required to populate the helium triplet levels.

In the context of flares, we measured strong downflows in the chromosphere via He I D3, revealing detailed dynamics in the deep atmosphere during a flare. Spectro-polarimetry was used to measure the magnetic field during a flare and to propose its magnetic topology. In conclusion, the He I D3 line is an excellent probe for reconnection targets in the solar atmosphere. Detailed dynamics as well as the magnetic field configuration can be derived using the line. Our findings encourage the use of the He I D3 spectral line as a diagnostic for the chromosphere and open up a range of applications that is yet to be exploited.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University, 2019
Keywords
Sun, Line formation, Chromosphere, Spectro-Polarimetry, Flares, Reconnection
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-166996 (URN)978-91-7797-602-8 (ISBN)978-91-7797-603-5 (ISBN)
Public defence
2019-04-26, FB52, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
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Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Manuscript.

Available from: 2019-04-03 Created: 2019-03-12 Last updated: 2019-03-27Bibliographically approved

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