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Observations of Ellerman bomb emission features in He I D-3 and He I 10 830 angstrom
Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Faculty of Science, Department of Astronomy.ORCID iD: 0000-0003-0585-7030
Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Faculty of Science, Department of Astronomy.ORCID iD: 0000-0003-4936-4211
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Number of Authors: 52017 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 598, article id A33Article in journal (Refereed) Published
Abstract [en]

Context. Ellerman bombs (EBs) are short-lived emission features, characterised by extended wing emission in hydrogen Balmer lines. Until now, no distinct signature of EBs has been found in the He I 10 830 angstrom line, and conclusive observations of EBs in He I D-3 have never been reported.

Aims. We aim to study the signature of EBs in neutral helium triplet lines.

Methods. The observations consisted of ten consecutive SST/TRIPPEL raster scans close to the limb, featuring the H beta, He I D-3 and He I 10 830 angstrom spectral regions. We also obtained raster scans with IRIS and made use of the SDO/AIA 1700 angstrom channel. We used HAZEL to invert the neutral helium triplet lines.

Results. Three EBs in our data show distinct emission signatures in neutral helium triplet lines, most prominently visible in the He I D-3 line. The helium lines have two components: a broad and blueshifted emission component associated with the EB, and a narrower absorption component formed in the overlying chromosphere. One of the EBs in our data shows evidence of strong velocity gradients in its emission component. The emission component of the other two EBs could be fitted using a constant slab. Our analysis hints towards thermal Doppler motions having a large contribution to the broadening for helium and IRIS lines. We conclude that the EBs must have high temperatures to exhibit emission signals in neutral helium triplet lines. An order of magnitude estimate places our observed EBs in the range of T similar to 2 x 10(4) 10(5) K.

Place, publisher, year, edition, pages
2017. Vol. 598, article id A33
Keywords [en]
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-142522DOI: 10.1051/0004-6361/201629266ISI: 000394465000033OAI: oai:DiVA.org:su-142522DiVA, id: diva2:1094272
Available from: 2017-05-09 Created: 2017-05-09 Last updated: 2020-01-30Bibliographically 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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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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