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Three-dimensional mapping of fine structure in the solar atmosphere
Stockholm University, Faculty of Science, Department of Astronomy. (Solar Group)
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 [en]
Sun, sunspots, Swedish 1-meter Solar Telescope, active regions, image processing, sunspot penumbra
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-86798ISBN: 978-91-7447-634-7 (print)OAI: oai:DiVA.org:su-86798DiVA: diva2:589990
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
List of papers
1. A tilted interference filter in a converging beam
Open this publication in new window or tab >>A tilted interference filter in a converging beam
2011 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 533Article in journal (Refereed) Published
Abstract [en]

Context. Narrow-band interference filters can be tuned toward shorter wavelengths by tilting them from the perpendicular to the optical axis. This can be used as a cheap alternative to real tunable filters, such as Fabry-Perot interferometers and Lyot filters. At the Swedish 1-meter Solar Telescope, such a setup is used to scan through the blue wing of the Ca II H line. Because the filter is mounted in a converging beam, the incident angle varies over the pupil, which causes a variation of the transmission over the pupil, different for each wavelength within the passband. This causes broadening of the filter transmission profile and degradation of the image quality. Aims. We want to characterize the properties of our filter, at normal incidence as well as at different tilt angles. Knowing the broadened profile is important for the interpretation of the solar images. Compensating the images for the degrading effects will improve the resolution and remove one source of image contrast degradation. In particular, we need to solve the latter problem for images that are also compensated for blurring caused by atmospheric turbulence. Methods. We simulate the process of image formation through a tilted interference filter in order to understand the effects. We test the hypothesis that they are separable from the effects of wavefront aberrations for the purpose of image deconvolution. We measure the filter transmission profile and the degrading PSF from calibration data. Results. We find that the filter transmission profile differs significantly from the specifications. We demonstrate how to compensate for the image-degrading effects. Because the filter tilt effects indeed appear to be separable from wavefront aberrations in a useful way, this can be done in a final deconvolution, after standard image restoration with Multi-Frame Blind Deconvolution/Phase Diversity based methods. We illustrate the technique with real data.

Keyword
instrumentation: interferometers, methods: observational, techniques: image processing, techniques: imaging spectroscopy
National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-67266 (URN)10.1051/0004-6361/201117305 (DOI)000295168100082 ()
Note
authorCount :3Available from: 2011-12-30 Created: 2011-12-27 Last updated: 2017-12-08Bibliographically approved
2. Three-dimensional temperature mapping of solar photospheric fine structure using Ca II H filtergrams
Open this publication in new window or tab >>Three-dimensional temperature mapping of solar photospheric fine structure using Ca II H filtergrams
2012 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 548, A114- p.Article in journal (Refereed) Published
Abstract [en]

Context. The wings of the Ca II H and K lines provide excellent photospheric temperature diagnostics. At the Swedish 1-m Solar Telescope (SST), the blue wing of Ca II H is scanned with a narrowband interference filter mounted on a rotation stage. This provides up to 0 ''.10 spatial resolution filtergrams at high cadence that are concurrent with other diagnostics at longer wavelengths. Aims. The aim is to develop observational techniques that provide photospheric temperature stratification at the highest spatial resolution possible and use them to compare simulations and observations at different heights. Methods. We use filtergrams in the Ca II H blue wing that were obtained with a tiltable interference filter at the SST. Synthetic observations are produced from three-dimensional (3D) hydro and magneto-hydrodynamic numerical simulations and degraded to match the observations. The temperature structure obtained from applying the method to the synthetic data is compared with the known structure in the simulated atmospheres and with observations of an active region. Cross-correlation techniques using restored non-simultaneous continuum images are used to reduce high-altitude, small-scale seeing signal introduced from the non-simultaneity of the frames when differentiating data. Results. Temperature extraction using high-resolution filtergrams in the Ca II H blue wing works reasonably well when tested with simulated 3D atmospheres. The cross-correlation technique successfully compensates for the problem of small-scale seeing differences and provides a measure of the spurious signal from this source in differentiated data. Synthesized data from the simulated atmospheres (including pores) match well the observations morphologically at different observed heights and in vertical temperature gradients.

Keyword
techniques: image processing, Sun: faculae, plages, Sun: granulation, Sun: photosphere, Sun: magnetic topology, techniques: high angular resolution
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-86487 (URN)10.1051/0004-6361/201220344 (DOI)000311901200114 ()
Note

AuthorCount:1;

Available from: 2013-01-14 Created: 2013-01-14 Last updated: 2017-12-06Bibliographically approved
3. Ca II H sunspot tomography from the photosphere to the chromosphere
Open this publication in new window or tab >>Ca II H sunspot tomography from the photosphere to the chromosphere
(English)Manuscript (preprint) (Other academic)
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-86797 (URN)
Available from: 2013-01-18 Created: 2013-01-18 Last updated: 2013-01-21Bibliographically approved
4. Detection of Convective Downflows in a Sunspot Penumbra
Open this publication in new window or tab >>Detection of Convective Downflows in a Sunspot Penumbra
2011 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 333, no 6040, 316-319 p.Article in journal (Refereed) Published
Abstract [en]

The fine structure and dynamics of sunspots and the strong outflow in their outer filamentary part-the penumbra-have puzzled astronomers for more than a century. Recent theoretical models and three-dimensional numerical simulations explain the penumbral filaments and their radiative energy output as the result of overturning convection. Here, we describe the detection of ubiquitous, relatively dark downward flows of up to 1 kilometer per second (km/s) in the interior penumbra, using imaging spectropolarimetric data from the Swedish 1-meter Solar Telescope. The dark downflows are omnipresent in the interior penumbra, distinguishing them from flows in arched flux tubes, and are associated with strong (3 to 3.5 km/s) radial outflows. They are thus part of a penumbral convective flow pattern, with the Evershed flow representing the horizontal component of that convection.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-66577 (URN)10.1126/science.1206429 (DOI)000292732000035 ()
Note
authorCount :4Available from: 2011-12-21 Created: 2011-12-20 Last updated: 2017-12-08Bibliographically approved
5. SST/CRISP observations of convective flows in a sunspot penumbra
Open this publication in new window or tab >>SST/CRISP observations of convective flows in a sunspot penumbra
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.

Keyword
sunspots, convection, magnetic fields, magnetohydrodynamics (MHD)
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-80739 (URN)10.1051/0004-6361/201118026 (DOI)000303315400033 ()
Note

AuthorCount:2;

Available from: 2012-10-03 Created: 2012-09-27 Last updated: 2017-12-07Bibliographically approved

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