| 1. |
- Bjørgen, Johan P., et al.
(författare)
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Three-dimensional modeling of the Ca II H and K lines in the solar atmosphere
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 611
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Judge, Philip G., et al.
(författare)
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New Light on an Old Problem of the Cores of Solar Resonance Lines
- 2020
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 901:1
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Tidskriftsartikel (refereegranskat)abstract
- We reexamine a 50+ yr old problem of deep central reversals predicted for strong solar spectral lines, in contrast to the smaller reversals seen in observations. We examine data and calculations for the resonance lines of H i, Mg ii, and Ca ii, the self-reversed cores of which form in the upper chromosphere. Based on 3D simulations, as well as data for the Mg ii lines from the Interface Region Imaging Spectrograph (IRIS), we argue that the resolution lies not in velocity fields on scales in either of the micro- or macroturbulent limits. Macroturbulence is ruled out using observations of optically thin lines formed in the upper chromosphere, and by showing that it would need to have unreasonably special properties to account for critical observations of the Mg ii resonance lines from the IRIS mission. The power in "turbulence" in the upper chromosphere may therefore be substantially lower than earlier analyses have inferred. Instead, in 3D calculations horizontal radiative transfer produces smoother source functions, smoothing out intensity gradients in wavelength and in space. These effects increase in stronger lines. Our work will have consequences for understanding the onset of the transition region, for understanding the energy in motions available for heating the corona, and for the interpretation of polarization data in terms of the Hanle effect applied to resonance line profiles.
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| 3. |
- Schmit, D., et al.
(författare)
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Comparison of Solar Fine Structure Observed Simultaneously in Ly alpha and MgII h
- 2017
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 847:2
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Tidskriftsartikel (refereegranskat)abstract
- The Chromospheric Lyman Alpha Spectropolarimeter (CLASP) observed the Sun in H I Ly alpha during a suborbital rocket flight on 2015 September 3. The Interface Region Imaging Telescope (IRIS) coordinated with the CLASP observations and recorded nearly simultaneous and co-spatial observations in the Mg II h and k lines. The Mg II h and Lya lines are important transitions, energetically and diagnostically, in the chromosphere. The canonical solar atmosphere model predicts that these lines form in close proximity to each other and so we expect that the line profiles will exhibit similar variability. In this analysis, we present these coordinated observations and discuss how the two profiles compare over a region of quiet Sun at viewing angles that approach the limb. In addition to the observations, we synthesize both line profiles using a 3D radiation-MHD simulation. In the observations, we find that the peak width and the peak intensities are well correlated between the lines. For the simulation, we do not find the same relationship. We have attempted to mitigate the instrumental differences between IRIS and CLASP and to reproduce the instrumental factors in the synthetic profiles. The model indicates that formation heights of the lines differ in a somewhat regular fashion related to magnetic geometry. This variation explains to some degree the lack of correlation, observed and synthesized, between Mg II and Lya. Our analysis will aid in the definition of future observatories that aim to link dynamics in the chromosphere and transition region.
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| 4. |
- Shchukina, N. G., et al.
(författare)
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A Si I atomic model for NLTE spectropolarimetric diagnostics of the 10 827 angstrom line
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 603
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Tidskriftsartikel (refereegranskat)abstract
- Aims. The Si I 10 827 angstrom line is commonly used for spectropolarimetric diagnostics of the solar atmosphere. First, we aim at quantifying the sensitivity of the Stokes profiles of this line to non-local thermodynamic equilibrium (NLTE) effects. Second, we aim at facilitating NLTE diagnostics of the Si I 10 827 angstrom line. To this end, we propose the use of a relatively simple silicon model atom, which allows a fast and accurate computation of Stokes profiles. The NLTE Stokes profiles calculated using this simple model atom are very similar to those obtained via the use of a very comprehensive silicon model atom. Methods. We investigate the impact of the NLTE effects on the Si I 10 827 angstrom line by means of multilevel radiative transfer calculations in a three-dimensional (3D) model atmosphere taken from a state-of-the-art magneto-convection simulation with small-scale dynamo action. We calculate the emergent Stokes profiles for this line at the solar disk center and for every vertical column of the 3D snapshot model, neglecting the effects of horizontal radiative transfer. Results. We find significant departures from LTE in the Si I 10 827 angstrom line, not only in the intensity but also in the linearly and circularly polarized profiles. At wavelengths around 0.1 angstrom, where most of the Stokes Q, U, and V peaks of the Si I 10 827 angstrom line occur, the differences between the NLTE and LTE profiles are comparable with the Stokes amplitudes themselves. The deviations from LTE increase with increasing Stokes Q, U, and V signals. Concerning the Stokes V profiles, the NLTE effects correlate with the magnetic field strength in the layers where such circular polarization signals are formed. Conclusions. The NLTE effects should be taken into account when diagnosing the emergent Stokes I profiles as well as the Stokes Q, U, and V profiles of the Si I 10 827 angstrom line. The sixteen-level silicon model atom proposed here, with six radiative bound-bound transitions, is suitable to account for the physics of formation of the Si I 10 827 angstrom line and for modeling and inverting its Stokes profiles without assuming LTE.
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| 5. |
- Sukhorukov, Andrii V., et al.
(författare)
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Partial redistribution in 3D non-LTE radiative transfer in solar-atmosphere models
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 597
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Tidskriftsartikel (refereegranskat)abstract
- Context. Resonance spectral lines such as H I Ly alpha, Mg II h&k, and Ca II H& K that form in the solar chromosphere, are influenced by the effects of 3D radiative transfer as well as partial redistribution (PRD). So far no one has modeled these lines including both effects simultaneously owing to the high computing demands of existing algorithms. Such modeling is, however, indispensable for accurate diagnostics of the chromosphere. Aims. We present a computationally tractable method to treat PRD scattering in 3D model atmospheres using a 3D non-local thermodynamic equilibrium (non-LTE) radiative transfer code. Methods. To make the method memory-friendly, we use the hybrid approximation for the redistribution integral. To make the method fast, we use linear interpolation on equidistant frequency grids. We verify our algorithm against computations with the RH code and analyze it for stability, convergence, and usefulness of acceleration using model atoms of Mg i i with the h&k lines and H i with the Ly alpha line treated in PRD. Results. A typical 3D PRD solution can be obtained in a model atmosphere with 252 x 252 x 496 coordinate points in 50 000-200 000 CPU hours, which is a factor ten slower than computations assuming complete redistribution. We illustrate the importance of the joint action of PRD and 3D effects for the Mg II h&k lines for disk-center intensities, as well as the center-to-limb variation. Conclusions. The proposed method allows for the simulation of PRD lines in a time series of radiation-magnetohydrodynamic models, in order to interpret observations of chromospheric lines at high spatial resolution.
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