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SST/CRISP observations of convective flows in a sunspot penumbra
Stockholm University, Faculty of Science, Department of Astronomy. Royal Swedish Academy of Sciences, Sweden.
Stockholm University, Faculty of Science, Department of Astronomy. Royal Swedish Academy of Sciences, Sweden.
2012 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 540, A19Article in journal (Refereed) Published
Abstract [en]

Context. Recent discoveries of intensity correlated downflows in the interior of a sunspot penumbra provide direct evidence for overturning convection, adding to earlier strong indications of convection from filament dynamics observed far from solar disk center, and supporting recent simulations of sunspots. Aims. Using spectropolarimetric observations obtained at a spatial resolution approaching 0''.1 with the Swedish 1-m Solar Telescope (SST) and its spectropolarimeter CRISP, we investigate whether the convective downflows recently discovered in the C I line at 538.03 nm can also be detected in the wings of the Fe I line at 630.15 nm. Methods. We make azimuthal fits of the measured LOS velocities in the core and wings of the 538 nm and 630 nm lines to disentangle the vertical and horizontal flows. To investigate how these depend on the continuum intensity, the azimuthal fits are made separately for each intensity bin. By using spatially high-pass filtered measurements of the LOS component of the magnetic field, the flow properties are determined separately for magnetic spines (relatively strong and vertical field) and inter-spines (weaker and more horizontal field). Results. The dark convective downflows discovered recently in the 538.03 nm line are evident also in the 630.15 nm line, and have similar strength. This convective signature is the same in spines and inter-spines. However, the strong radial (Evershed) outflows are found only in the inter-spines. Conclusions. At the spatial resolution of the present SST/CRISP data, the small-scale intensity pattern seen in continuum images is strongly related to a convective up/down flow pattern that exists everywhere in the penumbra. Earlier failures to detect the dark convective downflows in the interior penumbra can be explained by inadequate spatial resolution in the observed data.

Place, publisher, year, edition, pages
2012. Vol. 540, A19
Keyword [en]
sunspots, convection, magnetic fields, magnetohydrodynamics (MHD)
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:su:diva-80739DOI: 10.1051/0004-6361/201118026ISI: 000303315400033OAI: oai:DiVA.org:su-80739DiVA: diva2:558425
Note

AuthorCount:2;

Available from: 2012-10-03 Created: 2012-09-27 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Three-dimensional mapping of fine structure in the solar atmosphere
Open this publication in new window or tab >>Three-dimensional mapping of fine structure in the solar atmosphere
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The effects on image formation through a tilted interference filter in a converging beam are investigated and an adequate compensation procedure is established. A method that compensates for small-scale seeing distortions is also developed with the aim of co-aligning non-simultaneous solar images from different passbands. These techniques are applied to data acquired with a narrow tiltable filter at the Swedish 1-meter Solar Telescope. Tilting provides a way to scan the wing of the Ca II H line. The resulting images are used to map the temperature stratification and vertical temperature gradients in a solar active region containing a sunspot at a resolution approaching 0''10. The data are compared with hydro-dynamical quiet sun models and magneto-hydrodynamic models of plage. The comparison gives credence to the observational techniques, the analysis methods, and the simulations. Vertical temperature gradients are lower in magnetic structures than in non-magnetic.

Line-of-sight velocities and magnetic field properties in the penumbra of the same sunspot are estimated using the CRISP imaging spectropolarimeter and straylight compensation adequate for the data. These reveal a pattern of upflows and downflows throughout the entire penumbra including the interior penumbra. A correlation with intensity positively identifies these flows as convective in origin. The vertical convective signatures are observed everywhere, but the horizontal Evershed flow is observed to be confined to areas of nearly horizontal magnetic field. 

The relation between temperature gradient and total circular polarization in magnetically sensitive lines is investigated in different structures of the penumbra. Penumbral dark cores are prominent in total circular polarization and temperature gradient maps. These become longer and more contiguous with increasing height. Dark fibril structures over bright regions are observed in the Ca II H line core, above both the umbra and penumbra.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University, 2013. 41 p.
Keyword
Sun, sunspots, Swedish 1-meter Solar Telescope, active regions, image processing, sunspot penumbra
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-86798 (URN)978-91-7447-634-7 (ISBN)
Public defence
2013-02-15, sal FB52, Albanova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
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: 2013-01-24 Created: 2013-01-18 Last updated: 2013-01-24Bibliographically approved

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