Evidence of collisional depolarization of the Ba II ${\lambda}$4554 line in the low chromosphere

Derouich, M.

doi:10.1051/0004-6361:20078888

Cited by 11 publications

(24 citation statements)

References 14 publications

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“…In modeling the Ba ii D 2 line we account for the depolarizing effects of elastic collisions with hydrogen atoms but neglect the alignment transfer between the 2 P 3/2 level and the metastable 2 D 5/2 level. It has been shown by Derouich (2008) that this alignment transfer affects the line center polarization and is needed for magnetic field diagnostics. The purpose of the present paper is however not to determine magnetic fields but to clarify the physics of line formation.…”

Section: Discussionmentioning

confidence: 99%

MODELING THE QUANTUM INTERFERENCE SIGNATURES OF THE Ba II D₂4554 Å LINE IN THE SECOND SOLAR SPECTRUM

Smitha¹,

Nagendra²,

Stenflo³

et al. 2013

ApJ

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Quantum interference effects play a vital role in shaping the linear polarization profiles of solar spectral lines. The Ba ii D 2 line at 4554Å is a prominent example, where the F -state interference effects due to the odd isotopes produce polarization profiles, which are very different from those of the even isotopes that have no F -state interference. It is therefore necessary to account for the contributions from the different isotopes to understand the observed linear polarization profiles of this line. Here we do radiative transfer modeling with partial frequency redistribution (PRD) of such observations while accounting for the interference effects and isotope composition. The Ba ii D 2 polarization profile is found to be strongly governed by the PRD mechanism. We show how a full PRD treatment succeeds in reproducing the observations, while complete frequency redistribution (CRD) alone fails to produce polarization profiles that have any resemblance with the observed ones. However, we also find that the line center polarization is sensitive to the temperature structure of the model atmosphere. To obtain a good fit to the line center peak of the observed Stokes Q/I profile, a small modification of the FALX model atmosphere is needed, by lowering the temperature in the line-forming layers. Because of the pronounced temperature sensitivity of the Ba ii D 2 line it may not be a suitable tool for Hanle magnetic-field diagnostics of the solar chromosphere, because there is currently no straightforward way to separate the temperature and magnetic-field effects from each other.

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Section: Discussionmentioning

confidence: 99%

MODELING THE QUANTUM INTERFERENCE SIGNATURES OF THE Ba II D₂4554 Å LINE IN THE SECOND SOLAR SPECTRUM

Smitha¹,

Nagendra²,

Stenflo³

et al. 2013

ApJ

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“…We notice that in the line wings, which are not sensitive to the Hanle effect, the polarization obtained with the BOM value for the elastic collision rate is in slightly better agreement with the observed profile. As far as depolarizing collisions are concerned, recently Derouich (2008) has shown that in the low chromosphere, where (2) depolarizing collision rate is much smaller than the line radiative transition rate, the depolarization of the 2 P 3/2 level is mainly due to alignment transfer with the D 3/2,5/2 metastable levels of Ba ii. The corresponding depolarization increases when the hydrogen density increases, because the alignments of the metastable 2 D 3/2,5/2 levels are vulnerable to collisions.…”

Section: Sensitivity To Elastic Collisionsmentioning

confidence: 99%

“…This might affect the line polarization at the wavelengths of the two secondary peaks due to the odd isotopes, but not in the central peak nor in the line wings. Furthermore, it is very likely that any ground level polarization would be destroyed by elastic collisions with hydrogen atoms, as is the case for the metastable 2 D 5/2 levels (see Derouich 2008).…”

Section: Introductionmentioning

confidence: 99%

Hanle effect in the solar Ba II D2 line: a diagnostic tool for chromospheric weak magnetic fields

Faurobert¹,

Derouich²,

Bommier³

et al. 2008

A&A

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Context. The physics of the solar chromosphere depends in a crucial way on its magnetic structure. However there are presently very few direct magnetic field diagnostics available for this region.Aims. Here we investigate the diagnostic potential of the Hanle effect on the Ba ii D2 line resonance polarization for the determination of weak chromospheric turbulent magnetic fields. Methods. The line formation is described with a non-LTE polarized radiative transfer model taking into account partial frequency redistribution with an equivalent two-level atom approximation, in the presence of depolarizing collisions and the Hanle effect. We investigate the line sensitivity to temperature variations in the low chromosphere and to elastic collision with hydrogen atoms. We compare center-to-limb variations of the intensity and linear polarization profiles observed at THEMIS in 2007 to our numerical results. Results. We show that the line resonance polarization is very strongly affected by partial frequency redistribution effects both in the line central peak and in the wings. Complete frequency redistribution cannot reproduce the polarization observed in the line wings. The line is weakly sensitive to temperature differences between warm and cold components of the chromosphere. The effects of elastic collisions with hydrogen atoms and of alignment transfer due to multi-level coupling with the metastable 2 D 5/2 levels have been studied in a recent paper showing that they depolarize the 2 P 3/2 level of the line. In the conditions where the line is formed we estimate the amount of depolarization due to this mechanism as a factor of 0.7 to 0.65. If we first neglect this effect and determine the turbulent magnetic field strength required to account for the observed line polarization, we obtain values between 20 G and 30 G. We show that this overestimates the magnetic strength by a factor between 1.7 and 2. Applying these correction factors to our previous estimates, we find that the turbulent magnetic field strength is between 10 G and 18 G.Conclusions. Because of its low sensitivity to temperature variations, the solar Ba ii D2 line appears as a very good candidate for the diagnosis of weak magnetic fields in the low chromosphere (z ≥ 900 km) by means of the Hanle effect.

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“…Our aim is to provide a correction factor on the theoretical value of the line linear polarization obtained by solving the non-LTE radiative transfer problem in the framework of two-level modeling. In this sense, we recall that Faurobert et al (2009) used a correction factor determined by Derouich (2008) for the Ba  λ4554 line; they found that, using a correction factor, the accuracy on the magnetic field has been improved by ∼50%.…”

Section: The Correction Factor F Cmentioning

confidence: 96%

“…(3) given in Derouich (2008). Physically, the collisional relaxation rates R k (J) correspond to the loss of atomic polarization and, in contrast, the transfer rates C k I (J l → J) and C k S (J u → J) correspond to the gain of atomic polarization coming from other levels.…”

Section: Summary Of the Theory Of Depolarizing Collisionsmentioning

confidence: 99%

Collisional effects on the formation of the second solar spectrum of the Sr IIλ4078 line

Deb

Derouich

2014

A&A

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Context. One of the challenging theoretical problems in solar physics is the modeling of polarimetric observations in view of diagnostics of magnetic fields in the solar atmosphere. Aims. This work aims to provide key elements to gain better understanding of the formation of the second solar spectrum of the Sr  λ4078 line.Methods. The atomic states are quantified by the density matrix elements expressed on the basis of the irreducible tensorial operators. We perform accurate computation of the collisional depolarization and polarization transfer rates for all levels involved in the Sr  4078 Å line. We solve the statistical equilibrium equations to calculate the linear polarization degree where: (a) collisions are completely ignored; (b) collisions are taken into account in the framework of a simplified atomic model; and (c) collisions are taken into account in the framework of a 5-levels and 5-lines atomic model. Results.We provide all collisional rates needed for Sr  λ4078 line modeling. Although Sr  λ4078 line is the resonance line of Sr  connecting the ground state 2 S 1/2 to the excited state 2 P 3/2 , we show that its linear polarization is sensitive to collisions with neutral hydrogen mainly because the metastable D-levels are vulnerable to collisions. Thus, a 5-levels and 5-lines atomic model is needed to study this line. We determine a correction factor that one must apply to the value of the linear polarization derived from a simplified two-level model. Conclusions.In a certain range of hydrogen density, the effect of isotropic collisions between Sr  ions with hydrogen atoms is important in determining the polarization of the Sr  λ4078 line. The use of an atomic model that neglects the metastable level 4d of Sr  can induce errors of up to 25% in the value of the scattering polarization of the solar Sr  λ4078 line.

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Evidence of collisional depolarization of the Ba II ${\lambda}$4554 line in the low chromosphere

Cited by 11 publications

References 14 publications

MODELING THE QUANTUM INTERFERENCE SIGNATURES OF THE Ba II D₂4554 Å LINE IN THE SECOND SOLAR SPECTRUM

MODELING THE QUANTUM INTERFERENCE SIGNATURES OF THE Ba II D₂4554 Å LINE IN THE SECOND SOLAR SPECTRUM

Hanle effect in the solar Ba II D2 line: a diagnostic tool for chromospheric weak magnetic fields

Collisional effects on the formation of the second solar spectrum of the Sr IIλ4078 line

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Evidence of collisional depolarization of the Ba II ${\lambda}$4554 line in the low chromosphere

Cited by 11 publications

References 14 publications

MODELING THE QUANTUM INTERFERENCE SIGNATURES OF THE Ba II D24554 Å LINE IN THE SECOND SOLAR SPECTRUM

MODELING THE QUANTUM INTERFERENCE SIGNATURES OF THE Ba II D24554 Å LINE IN THE SECOND SOLAR SPECTRUM

Hanle effect in the solar Ba II D2 line: a diagnostic tool for chromospheric weak magnetic fields

Collisional effects on the formation of the second solar spectrum of the Sr IIλ4078 line

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MODELING THE QUANTUM INTERFERENCE SIGNATURES OF THE Ba II D₂4554 Å LINE IN THE SECOND SOLAR SPECTRUM

MODELING THE QUANTUM INTERFERENCE SIGNATURES OF THE Ba II D₂4554 Å LINE IN THE SECOND SOLAR SPECTRUM