<i>Solar Magnetic Fields: Polarized Radiation Diagnostics</i>

Stenflo, J. O.; Rabin, D. M.

doi:10.1063/1.2808137

Cited by 102 publications

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“…We show that the polarized light arises from selective absorption from the ground level of the triplet system of helium, and that it implies the presence of magnetic fields of the order of a few gauss that are highly inclined with respect to the solar radius vector. This disproves the common belief 4,10,11 that population imbalances in long-lived atomic levels are insignificant in the presence of inclined fields with strengths in the gauss range, and demonstrates the operation of the ground-level Hanle effect in an astrophysical plasma.…”

supporting

confidence: 64%

“…They can be studied through their effects on atomic energy levels, which produce polarized spectral lines 4,5 .…”

mentioning

confidence: 99%

“…Since some of the many "enigmatic" signals of scattering polarization that have been observed within the edge of the solar disc 13,14,15 have been shown to be due to ground state atomic polarization 16,17,9 , and because milligauss (or weaker) magnetic fields are believed to be very rare in the highly conductive solar plasma, it has been concluded that the field must be lines: the first is due to the emission process (that is, to the atomic polarization of the upper level), while the second is subtly related with the absorption process (that is, to the atomic polarization of the lower level). In general, the first mechanism (caused exclusively by the spontaneous emission events that follow the anisotropic radiative excitation) is the only one that is taken into account 4,19,10,14,11 . The role played by the atomic polarization of the lower level in producing linear polarization via the absorption process 20 has never been considered seriously.…”

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confidence: 99%

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Selective absorption processes as the origin of puzzling spectral line polarization from the Sun

Bueno

Degl’Innocenti

Collados

et al. 2002

Nature

139

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Magnetic fields play a key role in most astrophysical systems, from the Sun to active galactic nuclei 1,2,3 . They can be studied through their effects on atomic energy levels, which produce polarized spectral lines 4,5 .In particular, anisotropic radiation pumping processes 6,7 (which send electrons to higher atomic levels) induce population imbalances that are modified by weak magnetic fields 8,9 . Here we report peculiarly polarized light in the He i 10,830-Å multiplet observed in a coronal filament located at the centre of the solar disk. We show that the polarized light arises from selective absorption from the ground level of the triplet system of helium, and that it implies the presence of magnetic fields of the order of a few gauss that are highly inclined with respect to the solar radius vector. This disproves the common belief 4,10,11 that population imbalances in long-lived atomic levels are insignificant in the presence of inclined fields with strengths in the gauss range, and demonstrates the operation of the ground-level Hanle effect in an astrophysical plasma.The Zeeman effect and optical pumping are two mechanisms capable of inducing polarization signals in the spectral lines that originate in the outer layers of stellar atmospheres.The Zeeman effect 12 requires the presence of a magnetic field, which causes the atomic energy levels to split into different magnetic sublevels. This splitting produces local sources and sinks of light polarization because of the ensuing wavelength shifts of transitions between levels. The Zeeman effect is most sensitive in circular polarization, with a magnitude that scales with the ratio between the Zeeman splitting and the width of the spectral line (which is very much larger than the natural width of the atomic levels). This so-called longitudinal Zeeman effect responds to the line-of-sight component of the magnetic field. In contrast, the transverse Zeeman effect responds to the component of the magnetic field perpendicular to the line of sight, but produces linear polarization signals that are normally negligible for magnetic strengths B < 100 gauss.Anisotropic radiation pumping 6,7 produces atomic level polarization-that is, population imbalances and quantum interferences between the sublevels of degenerate atomic levels (Fig. 1). The presence of a magnetic field is not necessary for the operation of such optical pumping processes, which can be particularly efficient in creating atomic polarization if the depolarizing rates from elastic collisions are sufficiently low. As clarified below, the mere presence of atomic polarization of the type illustrated in Fig. 1 implies local sources and sinks of linear polarization. The Hanle effect 8,9 (Fig. 1) modifies the atomic polarization of the unmagnetized reference case, and gives rise to a complicated magnetic field dependence of the linear polarization of the scattered light, which is being increasingly applied as a diagnostic tool for weak magnetic fields in astrophysics.It is often assumed that the observable eff...

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supporting

confidence: 64%

“…They can be studied through their effects on atomic energy levels, which produce polarized spectral lines 4,5 .…”

mentioning

confidence: 99%

mentioning

confidence: 99%

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Selective absorption processes as the origin of puzzling spectral line polarization from the Sun

Bueno

Degl’Innocenti

Collados

et al. 2002

Nature

139

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“…In contrast to Hanle effect where interference occurs between magnetic substates with the same J n , interference effects between states with different J n have also been observed in polarization studies of solar Ca II H-K and Na I D 1 and D 2 lines [13], wherein it is mentioned that this can take place even when the lines are 3.5 nm apart. The general terminology, 'Raman scattering' has been employed [12,13] …”

Section: Article In Pressmentioning

confidence: 99%

“…The concept of scattering matrix connecting the Stokes vector S 0 of incident radiation to the Stokes vector S of scattered radiation was introduced quite early in the context of Rayleigh scattering [2]. Polarized radiation in spectral lines formed in the presence of an external magnetic field has been studied widely and a comprehensive theoretical framework has been developed [3][4][5][6][7][8][9][10][11][12][13][14][15]. The Hanle effect is a depolarizing phenomenon which arises due to 'partially ARTICLE IN PRESS k 0 .…”

Section: Introductionmentioning

confidence: 99%

Scattering polarization in the presence of magnetic and electric fields

Sampoorna

Nagendra

et al. 2007

Journal of Quantitative Spectroscopy and Radiative Transfer

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The polarization of radiation by scattering on an atom embedded in combined external quadrupole electric and uniform magnetic fields is studied theoretically. Limiting cases of scattering under Zeeman effect, and Hanle effect in weak magnetic fields are discussed. The theory is general enough to handle scattering in intermediate magnetic fields (Hanle-Zeeman effect) and for arbitrary orientation of magnetic field. The quadrupolar electric field produces asymmetric line shifts, and causes interesting level-crossing phenomena either in the absence of an ambient magnetic field, or in its presence. It is shown that the quadrupolar electric field produces an additional depolarization in the Q=I profiles and rotation of the plane of polarization in the U=I profile over and above that arising from magnetic field itself. This characteristic may have a diagnostic potential to detect steady-state and time-varying electric fields that surround radiating atoms in solar atmospheric layers. r

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Meso‐structured magnetic atmospheres: Stochastic polarized radiative transfer and Stokes profile inversion

Carroll

Staude

2003

Astron Nachr

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We present a general radiative transfer model which allows the Zeeman diagnostics of complex and unresolved solar magnetic fields. Present modeling techniques still rely to a large extent on a-priori assumptions about the geometry of the underlying magnetic field. In an effort to obtain a more flexible and unbiased approach we pursue a rigorous statistical description of the underlying atmosphere. Based on a Markov random field model the atmospheric structures are characterized in terms of probability densities and spatial correlations. This approach allows us to derive a stochastic transport equation for polarized light valid in a regime with an arbitrary fluctuating magnetic field on finite scales. One of the key ingredients of the derived stochastic transfer equation is the correlation length which provides an additional degree of freedom to the transport equation and can be used as a diagnostic parameter to estimate the characteristic length scale of the underlying magnetic field. It is shown that the stochastic transfer equation represents a natural extension of the (polarized) line formation under the microand macroturbulent assumption and contains both approaches as limiting cases. In particular, we show how in an inhomogeneous atmosphere asymmetric Stokes profiles develop and that the correlation length directly controls the degree of asymmetry and net circular polarization (NCP). In a number of simple numerical model calculations we demonstrate the importance of a finite correlation length for the polarized line formation and its impact on the resulting Stokes line profiles.

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Solar Magnetic Fields: Polarized Radiation Diagnostics

Cited by 102 publications

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Selective absorption processes as the origin of puzzling spectral line polarization from the Sun

Selective absorption processes as the origin of puzzling spectral line polarization from the Sun

Scattering polarization in the presence of magnetic and electric fields

Meso‐structured magnetic atmospheres: Stochastic polarized radiative transfer and Stokes profile inversion

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