Model Selection for Spectropolarimetric Inversions

Ramos, A. Asensio; Sainz, R. Manso; González, M. J. Martínez; Viticchiè, B.; Suárez, D. Orozco; Socas‐Navarro, H.

doi:10.1088/0004-637x/748/2/83

Cited by 18 publications

(5 citation statements)

References 60 publications

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“…We identified the pixels whose He I Stokes profiles are better represented by multiple atmosphere components using the Milne-Eddington model (HELIX + ) to fit oneand two-component model fits to the Stokes I and V spectra alone. We applied the Bayesian information criterion for model selection (see Asensio Ramos et al, 2012) and found that less than one percent of the profiles in the observed region needs to be fit with multicomponent atmospheres. We did not perform more fits of the these pixels and eliminated them from the remaining analysis.…”

Section: Multimodel Inversionsmentioning

confidence: 99%

He i Vector Magnetic Field Maps of a Sunspot and Its Superpenumbral Fine-Structure

et al. 2015

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Advanced inversions of high-resolution spectropolarimetric observations of the He I triplet at 1083 nm are used to generate unique maps of the chromospheric magnetic field vector across a sunspot and its superpenumbral canopy. The observations were acquired by the Facility Infrared Spectropolarimeter (FIRS) at the Dunn Solar Telescope (DST) on 29 January 2012. Multiple atmospheric models are employed in the inversions because superpenumbral Stokes profiles are dominated by atomic-level polarization, while sunspot profiles are Zeeman-dominated, but also exhibit signatures that might be induced by symmetrybreaking effects of the radiation field incident on the chromospheric material. We derive the equilibrium magnetic structure of a sunspot in the chromosphere and furthermore show that the superpenumbral magnetic field does not appear to be finely structured, unlike the observed intensity structure. This suggests that fibrils are not concentrations of magnetic flux, but are instead distinguished by individualized thermalization. We also directly compare our inverted values with a current-free extrapolation of the chromospheric field. With improved measurements in the future, the average shear angle between the inferred magnetic field and the potential field may offer a means to quantify the non-potentiality of the chromospheric magnetic field to study the onset of explosive solar phenomena. T.A. Schad: Previously at the Department of Planetary Sciences of the University of Arizona, with joint affiliation with the National Solar Observatory. B T.A. Schad

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Section: Multimodel Inversionsmentioning

confidence: 99%

He i Vector Magnetic Field Maps of a Sunspot and Its Superpenumbral Fine-Structure

et al. 2015

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“…Each fibril is fit to an appropriately ordered polynomial selected according to the Bayesian Information Criterion (Schwarz 1978;Asensio Ramos et al 2012):…”

Section: Fibril Tracingmentioning

confidence: 99%

He I VECTOR MAGNETOMETRY OF FIELD-ALIGNED SUPERPENUMBRAL FIBRILS

Schad

Penn

Lin

2013

ApJ

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Atomic-level polarization and Zeeman effect diagnostics in the neutral helium triplet at 10830 Å in principle allow full vector magnetometry of fine-scaled chromospheric fibrils. We present high-resolution spectropolarimetric observations of superpenumbral fibrils in the He i triplet with sufficient polarimetric sensitivity to infer their full magnetic field geometry. He i observations from the Facility Infrared Spectropolarimeter are paired with high-resolution observations of the Hα 6563 Å and Ca ii 8542 Å spectral lines from the Interferometric Bidimensional Spectrometer from the Dunn Solar Telescope in New Mexico. Linear and circular polarization signatures in the He i triplet are measured and described, as well as analyzed with the advanced inversion capability of the "Hanle and Zeeman Light" modeling code. Our analysis provides direct evidence for the often assumed field alignment of fibril structures. The projected angle of the fibrils and the inferred magnetic field geometry align within an error of ±10• . We describe changes in the inclination angle of these features that reflect their connectivity with the photospheric magnetic field. Evidence for an accelerated flow (∼40 m s −2 ) along an individual fibril anchored at its endpoints in the strong sunspot and weaker plage in part supports the magnetic siphon flow mechanism's role in the inverse Evershed effect. However, the connectivity of the outer endpoint of many of the fibrils cannot be established.

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“…We refer the reader to Del Toro Iniesta and Martínez Pillet (2012) for a discussion on polarimetric accuracy and signal-to-noise ratio. For Bayesian selection among model atmospheres, see Asensio Ramos et al (2012).…”

Section: Degrees Of Approximation In the Model Atmospheresmentioning

confidence: 99%

“…The educated successive sampling grows linearly with the number of free parameters instead of exponentially. The decrease in computational cost has allowed the authors to deal both with ME atmospheres and with general LTE atmospheres where the physical quantities vary with depth (Asensio Ramos et al, 2012).…”

Section: Bayesian Inversionsmentioning

confidence: 99%

“…The interested reader can complement this chronological overview with the reviews by Del Toro Iniesta and Ruiz Cobo (1995,1996,1997), Socas-Navarro (2001), Del Toro Iniesta (2003a), Bellot Rubio (2006, and Asensio Ramos et al (2012) and the didactical introductions and discussions by Stenflo (1994), Del Toro Iniesta (2003b), and Landi Degl'Innocenti and Landolfi (2004). A critical discussion on the different techniques and the specific implementations will be developed throught the paper, which is structured as follows: the basic assumptions of radiative transfer are discussed in Section 2; the following two Sections discuss the approximations used for the model atmospheres and the Stokes profiles; an analysis of the forward problem, namely the synthesis of the Stokes spectrum, is presented in Section 5, which is followed by an analysis of the sensitivities of spectral lines to physical quantities (Section 6); the basics of inversion techniques are analyzed in Section 7 and a discussion on inversion results presented in Section 8; finally, Section 9 summarizes the conclusions.…”

Section: Introductionmentioning

confidence: 99%

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Inversion of the radiative transfer equation for polarized light

Iniesta

Cobo

2016

Living Rev. Sol. Phys.

116

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Since the early 1970s, inversion techniques have become the most useful tool for inferring the magnetic, dynamic, and thermodynamic properties of the solar atmosphere. The intrinsic model dependence makes it necessary to formulate specific means that include the physics in a properly quantitative way. The core of this physics lies in the radiative transfer equation (RTE), where the properties of the atmosphere are assumed to be known while the unknowns are the four Stokes profiles. The solution of the (differential) RTE is known as the direct or forward problem. From an observational point of view, the problem is rather the opposite: the data are made up of the observed Stokes profiles and the unknowns are the solar physical quantities. Inverting the RTE is therefore mandatory. Indeed, the formal solution of this equation can be considered an integral equation. The solution of such an integral equation is called the inverse problem. Inversion techniques are automated codes aimed at solving the inverse problem. The foundations of inversion techniques are critically revisited with an emphasis on making explicit the many assumptions underlying each of them. An incremental complexity procedure is advised for the implementation in practice. Coarse details of the profiles or coarsely sam- pled profiles should be reproduced first with simple model atmospheres (with, for example, a few physical quantities that are constant with optical depth). If the Stokes profiles are well sampled and differences between synthetic and observed ones are greater than the noise, then the inversion should proceed by using more complex models (that is, models where physical quantities vary with depth or, eventually, with more than one component). Significant im- provements are expected as well from the use of new inversion techniques that take the spatial degradation by the instruments into account.Comment: Invited review accepted for publication in Living Reviews in Solar Physics. 84 pages, 30 figure

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Model Selection for Spectropolarimetric Inversions

Cited by 18 publications

References 60 publications

He i Vector Magnetic Field Maps of a Sunspot and Its Superpenumbral Fine-Structure

He i Vector Magnetic Field Maps of a Sunspot and Its Superpenumbral Fine-Structure

He I VECTOR MAGNETOMETRY OF FIELD-ALIGNED SUPERPENUMBRAL FIBRILS

Inversion of the radiative transfer equation for polarized light

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