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Imaging spectropolarimetry of solar active regions
Stockholm University, Faculty of Science, Department of Astronomy. (Institute for Solar Physics)
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Solar magnetic fields span a wide range of spatial scales from sunspots and plages to magnetic bright points. A clear understanding of the physical processes underlying the evolution of these magnetic features requires high-resolution spectropolarimetric observations of solar active regions and comparisons with synthetic data from simulations. This thesis is based on observations with the Swedish 1-m Solar Telescope (SST) and the CRISP imaging spectropolarimeter which, processed with a sophisticated image restoration technique, produce data of unsurpassed quality. The Fe I 630.25 nm line is used for all the spectropolarimetric observations.

It appears likely that present telescopes resolve the fundamental scales of penumbral filaments. However, the penumbrae of sunspots are still not fully understood, with various theoretical models competing to explain their fine structure and flows. We analyze spectropolarimetric observations with a resolution close to the SST diffraction limit of 0.16 arcsecond. Using inversion techniques, we map the line-of-sight velocities and the magnetic-field configuration of dark-cored penumbral filaments.

Over the past decade, sunspots and quiet sun magnetic fields have received considerable attention, with intermediate plage regions being somewhat neglected. We perform a detailed analysis of a plage region and present the first observational evidence of a small-scale granular magneto-convection pattern associated with a plage region.

Magnetic bright points are believed to be formed due to magnetic field intensification caused by flux-tube collapse involving strong downflows. Although magneto-hydrodynamic (MHD) simulations agree with this view, only a few observations with adequate spatial resolution exist in support of the simulations. We present several cases of bright-point formation associated with strong downflows, which qualitatively agree with simulations and past observations. However, we find the field intensification to be transient rather than permanent.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University , 2011. , 51 p.
Keyword [en]
Sun, Spectropolarimetry, Solar magnetic fields, Sunspots, Plages
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-54687ISBN: 978-91-7447-212-7 (print)OAI: oai:DiVA.org:su-54687DiVA: diva2:396781
Public defence
2011-03-16, FB42, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:15 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following paper was unpublished: Paper 3: Accepted.Available from: 2011-02-22 Created: 2011-02-11 Last updated: 2011-02-22Bibliographically approved
List of papers
1. CRISP spectropolarimetric imaging of penumbral fine structure
Open this publication in new window or tab >>CRISP spectropolarimetric imaging of penumbral fine structure
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2008 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, Vol. 689, no 1, L69- p.Article in journal (Refereed) Published
Abstract [en]

We discuss penumbral fine structure in a small part of a pore, observed with the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope (SST), close to its diffraction limit of 0.16 . Milne-Eddington inversions applied to these Stokes data reveal large variations of field strength and inclination angle over dark-cored penumbral intrusions and a dark-cored light bridge. The mid-outer part of this penumbra structure shows 0.3 wide spines, separated by 1.6 (1200 km) and associated with 30° inclination variations. Between these spines, there are no small-scale magnetic structures that easily can be identified with individual flux tubes. A structure with nearly 10° more vertical and weaker magnetic field is seen midway between two spines. This structure is cospatial with the brightest penumbral filament, possibly indicating the location of a convective upflow from below.

Keyword
sunspot, polarimetry, sun, spectropolarimetry
National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-16337 (URN)10.1086/595744 (DOI)000262733800018 ()
Available from: 2008-12-17 Created: 2008-12-17 Last updated: 2011-02-11Bibliographically approved
2. Small-scale convection signatures associated with a strong plage solar magnetic field
Open this publication in new window or tab >>Small-scale convection signatures associated with a strong plage solar magnetic field
2010 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 524, A3- p.Article in journal (Refereed) Published
Abstract [en]

Context. Solar convection in a strong plage, in which the magnetic field is vertical and strong over extended regions, but much weaker than in the umbrae of large sunspots, has so far not been well studied. This has been mostly because of a lack of spectropolarimetric data at adequate spatial resolution. The combination of a large solar telescope, such as the Swedish 1-m Solar Telescope, adaptive optics, powerful image reconstruction techniques, and a high-fidelity imaging spectropolarimeter is, however, capable of producing such data. Aims. In this work, we study and quantify the properties of strong-field small-scale convection and compare these observed properties with those predicted by numerical simulations. Methods. We analyze spectropolarimetric 630.25 nm data from a unipolar ephemeral region near the Sun center. We use line-of-sight velocities and magnetic field measurements obtained with Milne-Eddington inversion techniques along with measured continuum intensities and Stokes V amplitude asymmetry at a spatial resolution of 0.'' 15 to establish statistical relations between the measured quantities. We also study these properties for different types of distinct magnetic features, such as micropores, bright points, ribbons, flowers, and strings. Results. We present the first direct observations of a small-scale granular magneto-convection pattern within extended regions of a strong (more than 600 G on average) magnetic field. Along the boundaries of the flux concentrations, we see mostly downflows and asymmetric Stokes V profiles, consistent with synthetic line profiles calculated from MHD simulations. We note the frequent occurrence of bright downflows along these boundaries. In the interior of the flux concentrations, we observe an up/down flow pattern that we associate with small-scale magnetoconvection, appearing similar to that of field-free granulation but with scales 4 times smaller. Measured rms velocities are 70% of those of nearby field-free granulation, even though the average radiative flux is not lower than that of the quiet Sun. The interiors of these flux concentrations are dominated by upflows.

Keyword
Convection, Sun: faculae, plages, Sun: granulation, magnetic fields, polarization
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-51223 (URN)10.1051/0004-6361/201014956 (DOI)000284625300049 ()
Note
authorCount :2Available from: 2011-01-14 Created: 2011-01-10 Last updated: 2017-12-11Bibliographically approved
3. Transient downflows associated with the intensification of small-scale magnetic features and bright point formation
Open this publication in new window or tab >>Transient downflows associated with the intensification of small-scale magnetic features and bright point formation
(English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746Article in journal (Refereed) Accepted
Place, publisher, year, edition, pages
IOPScience
Keyword
sun – photosphere, sun - granulation, faculae, plage
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-54665 (URN)
Available from: 2011-02-10 Created: 2011-02-10 Last updated: 2017-12-11Bibliographically approved

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