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The synthetic chromosphere: Results and techniques with a numerical approach
Stockholm University, Faculty of Science, Department of Astronomy. (Institute för Solfysik)ORCID iD: 0000-0001-6770-6305
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
##### Abstract [en]

Realistic numerical simulations of the solar atmosphere can be used to interpret different phenomena observed on the solar surface. To gain insight into the atmospheric physical conditions, we compare the observations with 3D radiative magnetohydrodynamic models combined with forward modeling (radiative transfer). This thesis focuses particularly on the less understood chromospheric layer between the photosphere and the transition region. Only a few and complex spectral lines can probe the chromosphere making its observations a real challenge.The chromospheric environment is strongly influenced by departures from local thermodynamic equilibrium (non-LTE), horizontal radiative transfer (3D effects), and partially-coherent scattering of photons (partial redistribution effects). All these effects make the detailed 3D non-LTE radiative transfer very computationally demanding.In paper I, we focus on increasing the efficiency of non-LTE modeling of spectral lines in realistic solar models. We implemented a non-linear multigrid solver into the Multi3D code and showed that the method can handle realistic model atmospheres produced by radiative-MHD simulations. We obtained a speed-up of a factor 4.5-6 compared to multilevel accelerated lambda iteration.In paper II, we studied the chromospheric resonance lines Ca \textsc{ii} H\&K. Understanding their formation is crucial to interpreting the observations from the new imaging spectrometer CHROMIS, recently installed at the Swedish 1-m Solar Telescope. We investigated how the synthetic observables of Ca \textsc{ii} H\&K lines are related to atmospheric parameters.In paper III, we investigated a simulated active region including flux emergence that produced a flare. We modeled strong chromospheric lines, such as Ca \textsc{ii} H\&K, 8542 \AA, Mg \textsc{ii} h\&k, and H-$\alpha$, to investigate how it appears in synthetic images and spectra.

##### Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University , 2019. , p. 74
##### Keywords [en]
Sun, chromosphere, radiative transfer, numerical method
##### National Category
Astronomy, Astrophysics and Cosmology
Astronomy
##### Identifiers
ISBN: 978-91-7797-550-2 (print)ISBN: 978-91-7797-551-9 (electronic)OAI: oai:DiVA.org:su-162512DiVA, id: diva2:1268625
##### Public defence
2019-02-01, FB55, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
##### Note

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

Available from: 2019-01-09 Created: 2018-12-06 Last updated: 2018-12-21Bibliographically approved
##### List of papers
1. Numerical non-LTE 3D radiative transfer using a multigrid method
Open this publication in new window or tab >>Numerical non-LTE 3D radiative transfer using a multigrid method
2017 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 599, article id A118Article in journal (Refereed) Published
##### Abstract [en]

Context. 3D non-LTE radiative transfer problems are computationally demanding, and this sets limits on the size of the problems that can be solved. So far, multilevel accelerated lambda iteration (MALI) has been the method of choice to perform high-resolution computations in multidimensional problems. The disadvantage of MALI is that its computing time scales as O(n(2)), with n the number of grid points. When the grid becomes finer, the computational cost increases quadratically. Aims. We aim to develop a 3D non-LTE radiative transfer code that is more efficient than MALI. Methods. We implement a non-linear multigrid, fast approximation storage scheme, into the existing Multi3D radiative transfer code. We verify our multigrid implementation by comparing with MALI computations. We show that multigrid can be employed in realistic problems with snapshots from 3D radiative magnetohydrodynamics (MHD) simulations as input atmospheres. Results. With multigrid, we obtain a factor 3.3-4.5 speed-up compared to MALI. With full-multigrid, the speed-up increases to a factor 6. The speed-up is expected to increase for input atmospheres with more grid points and finer grid spacing. Conclusions. Solving 3D non-LTE radiative transfer problems using non-linear multigrid methods can be applied to realistic atmospheres with a substantial increase in speed.

##### Keywords
radiative transfer, Sun: chromosphere, methods: numerical
##### National Category
Astronomy, Astrophysics and Cosmology
Astronomy
##### Identifiers
urn:nbn:se:su:diva-142717 (URN)10.1051/0004-6361/201630237 (DOI)000395821900130 ()
Available from: 2017-05-04 Created: 2017-05-04 Last updated: 2018-12-10Bibliographically approved
2. Three-dimensional modeling of the Ca II H and K lines in the solar atmosphere
Open this publication in new window or tab >>Three-dimensional modeling of the Ca II H and K lines in the solar atmosphere
2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 611, article id A62Article in journal (Refereed) Published
##### Abstract [en]

Context. CHROMIS, a new imaging spectrometer at the Swedish 1-m Solar Telescope (SST), can observe the chromosphere in the H and K lines of Ca II at high spatial and spectral resolution. Accurate modeling as well as an understanding of the formation of these lines are needed to interpret the SST/CHROMIS observations. Such modeling is computationally challenging because these lines are influenced by strong departures from local thermodynamic equilibrium, three-dimensional radiative transfer, and partially coherent resonance scattering of photons. Aims. We aim to model the Ca II H and K lines in 3D model atmospheres to understand their formation and to investigate their diagnostic potential for probing the chromosphere. Methods. We model the synthetic spectrum of Ca II using the radiative transfer code Multi3D in three different radiation-magnetohydrodynamic model atmospheres computed with the Bifrost code. We classify synthetic intensity profiles according to their shapes and study how their features are related to the physical properties in the model atmospheres. We investigate whether the synthetic data reproduce the observed spatially-averaged line shapes, center-to-limb variation and compare this data with SST/CHROMIS images. Results. The spatially-averaged synthetic line profiles show too low central emission peaks, and too small separation between the peaks. The trends of the observed center-to-limb variation of the profiles properties are reproduced by the models. The Ca II H and K line profiles provide a temperature diagnostic of the temperature minimum and the temperature at the formation height of the emission peaks. The Doppler shift of the central depression is an excellent probe of the velocity in the upper chromosphere.

##### Keywords
Sun: chromosphere, methods: numerical, radiative transfer
##### National Category
Astronomy, Astrophysics and Cosmology
Astronomy
##### Identifiers
urn:nbn:se:su:diva-156084 (URN)10.1051/0004-6361/201731926 (DOI)000428423300006 ()
Available from: 2018-05-18 Created: 2018-05-18 Last updated: 2018-12-10Bibliographically approved
3. Three-dimensional modeling of chromospheric spectral lines in a simulated active region
Open this publication in new window or tab >>Three-dimensional modeling of chromospheric spectral lines in a simulated active region
##### National Category
Astronomy, Astrophysics and Cosmology
Astronomy
##### Identifiers
urn:nbn:se:su:diva-162681 (URN)
Available from: 2018-12-10 Created: 2018-12-10 Last updated: 2018-12-10Bibliographically approved

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Cite
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