| 1. |
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| 2. |
- Rachmeler, L. A., et al.
(författare)
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Quiet Sun Center to Limb Variation of the Linear Polarization Observed by CLASP2 Across the Mg ıı h and k Lines
- 2022
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 936:1
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Tidskriftsartikel (refereegranskat)abstract
- The CLASP2 (Chromospheric LAyer Spectro-Polarimeter 2) sounding rocket mission was launched on 2019 April 11. CLASP2 measured the four Stokes parameters of the Mg ıı h and k spectral region around 2800 Å along a 200'' slit at three locations on the solar disk, achieving the first spatially and spectrally resolved observations of the solar polarization in this near-ultraviolet region. The focus of the work presented here is the center-to-limb variation of the linear polarization across these resonance lines, which is produced by the scattering of anisotropic radiation in the solar atmosphere. The linear polarization signals of the Mg ıı h and k lines are sensitive to the magnetic field from the low to the upper chromosphere through the Hanle and magneto-optical effects. We compare the observations to theoretical predictions from radiative transfer calculations in unmagnetized semiempirical models, arguing that magnetic fields and horizontal inhomogeneities are needed to explain the observed polarization signals and spatial variations. This comparison is an important step in both validating and refining our understanding of the physical origin of these polarization signatures, and also in paving the way toward future space telescopes for probing the magnetic fields of the solar upper atmosphere via ultraviolet spectropolarimetry.
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| 3. |
- Pozuelo, S. Esteban, et al.
(författare)
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Estimating the longitudinal magnetic field in the chromosphere of quiet-Sun magnetic concentrations
- 2023
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 672
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Tidskriftsartikel (refereegranskat)abstract
- Context. Details of the magnetic field in the quiet-Sun chromosphere are key to our understanding of essential aspects of the solar atmosphere. However, the strength and orientation of this magnetic field have not been thoroughly studied at high spatial resolution.Aims. We aim to determine the longitudinal magnetic field component (B∥) of quiet-Sun regions depending on their size.Methods. We estimated B∥ by applying the weak-field approximation to high-spatial-resolution Ca II 854.2 nm data taken with the Swedish 1 m Solar Telescope. Specifically, we analyzed the estimates inferred for different spectral ranges using the data at the original cadence and temporally integrated signals.Results. The longitudinal magnetic field in each considered plasma structure correlates with its size. Using a spectral range restricted to the line core leads to chromospheric longitudinal fields varying from ∼50 G at the edges to 150–500 G at the center of the structure. These values increase as the spectral range widens due to the photospheric contribution. However, the difference between this contribution and the chromospheric one is not uniform for all structures. Small and medium-sized concentrations show a steeper height gradient in B∥ compared to their chromospheric values, so estimates for wider ranges are less trustworthy. Signal addition does not alleviate this situation as the height gradients in B∥ are consistent with time. Finally, despite the amplified noise levels that deconvolving processes may cause, data restored with the destretching technique show similar results, though are affected by smearing.Conclusions. We obtained B∥ estimates similar to those previously found, except for large concentrations and wide spectral ranges. In addition, we report a correlation between the height variation of B∥ compared to the chromospheric estimates and the concentration size. This correlation affects the difference between the photospheric and chromospheric magnetic flux values and the reliability of the estimates for wider spectral ranges.
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| 4. |
- 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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| 5. |
- Stepan, J., et al.
(författare)
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THREE-DIMENSIONAL RADIATIVE TRANSFER SIMULATIONS OF THE SCATTERING POLARIZATION OF THE HYDROGEN LY alpha LINE IN A MAGNETOHYDRODYNAMIC MODEL OF THE CHROMOSPHERE-CORONA TRANSITION REGION
- 2015
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 803:2
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Tidskriftsartikel (refereegranskat)abstract
- Probing the magnetism of the upper solar chromosphere requires measuring and modeling the scattering polarization produced by anisotropic radiation pumping in UV spectral lines. Here we apply PORTA (a novel radiative transfer code) to investigate the hydrogen Ly alpha line in a three-dimensional model of the solar atmosphere resulting from a state of the art magnetohydrodynamic (MHD) simulation. At full spatial resolution the linear polarization signals are very significant all over the solar disk, with a large fraction of the field of view (FOV) showing line-center amplitudes well above the 1% level. Via the Hanle effect the line-center polarization signals are sensitive to the magnetic field of the model's transition region, even when its mean field strength is only 15 G. The breaking of the axial symmetry of the radiation field produces significant forward-scattering polarization in Ly alpha, without the need of an inclined magnetic field. Interestingly, the Hanle effect tends to decrease such forward-scattering polarization signals in most of the points of the FOV. When the spatial resolution is degraded, the line-center polarization of Ly alpha drops below the 1% level, reaching values similar to those previously found in one-dimensional (1D) semi-empirical models (i.e., up to about 0.5%). The center to limb variation (CLV) of the spatially averaged polarization signals is qualitatively similar to that found in 1D models, with the largest line-center amplitudes at mu = cos theta approximate to 0.4 (theta being the heliocentric angle). These results are important, both for designing the needed space-based instrumentation and for a reliable interpretation of future observations of the Ly alpha polarization.
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| 6. |
- Ishikawa, Ryohko, et al.
(författare)
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Mapping solar magnetic fields from the photosphere to the base of the corona
- 2021
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Ingår i: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 7:8
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Tidskriftsartikel (refereegranskat)abstract
- Routine ultraviolet imaging of the Sun’s upper atmosphere shows the spectacular manifestation of solar activity; yet, we remain blind to its main driver, the magnetic field. Here, we report unprecedented spectropolarimetric observations of an active region plage and its surrounding enhanced network, showing circular polarization in ultraviolet (Mg ii h & k and Mn i) and visible (Fe i) lines. We infer the longitudinal magnetic field from the photosphere to the very upper chromosphere. At the top of the plage chromosphere, the field strengths reach more than 300 G, strongly correlated with the Mg ii k line core intensity and the electron pressure. This unique mapping shows how the magnetic field couples the different atmospheric layers and reveals the magnetic origin of the heating in the plage chromosphere.
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| 7. |
- 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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| 8. |
- Shchukina, N., et al.
(författare)
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Impact of surface dynamo magnetic fields on the solar abundance of the CNO elements
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 586
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Tidskriftsartikel (refereegranskat)abstract
- Most studies of the solar metallicity, based on abundance determinations of the CNO elements, ignore the fact that the quiet solar photosphere is significantly magnetized by a small-scale magnetic field with a mean field strength of similar to 100 G. Here we quantify how this significant magnetization affects determinations of the abundances of these chemical elements. To this end, we used two three-dimensional models of the solar photosphere taken from a magneto-convection simulation with small-scale dynamo action, one virtually unmagnetized, and the other characterized by a mean field strength of 160 G in the low photosphere. We performed local thermodynamic equilibrium spectral synthesis for a large set of C I, N I, and O I lines to derive abundance corrections. We included the magnetic broadening of the lines (direct effect) and the magnetically induced changes of the photospheric temperature stratification (indirect effect). We find that these small-scale dynamo magnetic fields only negligibly affect the determination of the solar abundances of carbon, nitrogen, and oxygen.
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| 9. |
- Socas-Navarro, H., et al.
(författare)
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An open-source, massively parallel code for non-LTE synthesis and inversion of spectral lines and Zeeman-induced Stokes profiles
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 577
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Tidskriftsartikel (refereegranskat)abstract
- With the advent of a new generation of solar telescopes and instrumentation, interpreting chromospheric observations (in particular, spectropolarimetry) requires new, suitable diagnostic tools. This paper describes a new code, NICOLE, that has been designed for Stokes non-LTE radiative transfer, for synthesis and inversion of spectral lines and Zeeman-induced polarization profiles, spanning a wide range of atmospheric heights from the photosphere to the chromosphere. The code features a number of unique features and capabilities and has been built from scratch with a powerful parallelization scheme that makes it suitable for application on massive datasets using large supercomputers. The source code is written entirely in Fortran 90/2003 and complies strictly with the ANSI standards to ensure maximum compatibility and portability. It is being publicly released, with the idea of facilitating future branching by other groups to augment its capabilities.
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| 10. |
- Song, Donguk, et al.
(författare)
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Polarization Accuracy Verification of the Chromospheric LAyer SpectroPolarimeter
- 2022
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Ingår i: Solar Physics. - : Springer Science and Business Media LLC. - 0038-0938 .- 1573-093X. ; 297:10
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Tidskriftsartikel (refereegranskat)abstract
- We have developed an advanced UV spectropolarimeter called Chromospheric LAyer SpectroPolarimeter (CLASP2), aimed at achieving very high accuracy measurements (<0.1% at 3σ) of the linear (Q/I and U/I) and circular (V/I) polarizations of the Mg II h and k lines (280 nm). CLASP2 was launched on board a NASA sounding rocket on April 11, 2019. It successfully detected the full Stokes vector in an active-region plage and in the quiet Sun near the limb across the Mg II h and k lines for the first time. To verify the polarization characteristics of CLASP2, the response matrix is estimated by combining the results obtained from the preflight calibration on the ground, with the results of the inflight calibration acquired at the solar-disk center. We find that the response matrix of CLASP2 in the Mg II h and k lines is notably close to an ideal response matrix, i.e., the scale factor and the crosstalk terms are close to 1 and 0, respectively. Moreover, the uncertainty of each Stokes parameter estimated by the repeatability of the measurements is verified to be within the required tolerance. Based on our investigation, we conclude that CLASP2 achieves 0.1% polarization accuracy at a 3σ3σ level.
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