The determination of vector magnetic fields from stokes profiles

Auer, L. H.; Heasley, J. N.; House, L. L.

doi:10.1007/bf00150873

Cited by 111 publications

(100 citation statements)

References 10 publications

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“…The UNNOFIT inversion code is based on the Marquardt algorithm to reach the minimum theory/observation discrepancy with the theoretical profiles given by the Unno-Rachkowski solution. Pionereed by Harvey et al (1972) and Auer et al (1977), this technique has been improved by Landolfi and by Landi Degl'Innocenti (1982) and Landolfi et al (1984) to allow for magneto-optical and damping effects.…”

Section: Validation Of the Methodsmentioning

confidence: 99%

Vector magnetic field map at the photospheric level below and around a solar filament (neutral line)

Bommier¹,

Rayrole²,

Eff-Darwich³

2005

A&A

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Abstract. We present a vector magnetic field map obtained on 7 December 2003, below and around a filament located not so far from the active region NOAA 517, whose one spot is also found on the map of 240 × 340 arcsec. This region was itself located near the disk center, so that the longitudinal (resp. transverse) field is nearly the vertical (resp. horizontal) one. The THEMIS telescope was used in its spectropolarimetric multiline mode MTR ("MulTiRaies"). The noise level is 5−10 Gauss in the longitudinal field and 50−100 Gauss in the transverse field, while the pixel size is 0.45 arcsec. Fundamental ambiguity is not solved, and the atmosphere is assumed to be homogeneous. The magnetic field derivation method described in this paper was validated on eight test points submitted to the UNNOFIT inversion code, and the results are found in agreement within 14% discrepancy. Two main results appear on the map: (i) a strong spatial correlation between the longitudinal and transverse field resulting in an inclined field vector (making a most probable angle of 60 • or 120 • with the line-of-sight in the filament region); and (ii) homogeneity of the field direction (inclination and azimuth) in the filament region. Parasitic polarities were also detected: first those located at the filament feet, as theoretically expected, on the one hand; and then weak opposite polarity regular patterns that appear between the network field (strong field at the frontiers of supergranules), on the other.The exact superimposition of the magnetic field map derived from the Fe  6302.5 Å line and of the Hα map, which enabled association of the parasitic polarities with the filament feet, was possible because these two maps were simultaneously obtained, thanks to a unique facility available in the multiline mode of THEMIS.

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Section: Validation Of the Methodsmentioning

confidence: 99%

Vector magnetic field map at the photospheric level below and around a solar filament (neutral line)

Bommier¹,

Rayrole²,

Eff-Darwich³

2005

A&A

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“…1e-4e match the dashed-black ones. To a first approximation (Auer et al 1977;Jeferries & Mickey 1991), the azimuthal angle of the magnetic field vector is given by φ = (1/2) tan −1 (U/Q). Thus, that the inversion of the Stokes profiles selected with the S /R quv -criterion also retrieves the correct distribution for φ comes as a rather surprising result, since here most of the selected Stokes Q and U profiles are below the 4.5σ-level (see Sect.…”

Section: Azimuth Of the Magnetic Field Vector: φmentioning

confidence: 99%

Inferring the magnetic field vector in the quiet Sun

Borrero

Kobel

2012

A&A

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In a previous paper, we argued that the inversion of Stokes profiles applied to spectropolarimetric observations of the solar internetwork yield unrealistically large values of the inclination of the magnetic field vector (γ). This is because photon noise in Stokes Q and U are interpreted by the inversion code as valid signals, that leads to an overestimation of the transverse component B ⊥ , thus the inclination γ. However, our study was based on the analysis of linear polarization signals that featured only uncorrelated noise. In this paper, we develop this idea further and study this effect in Stokes Q and U profiles that also show correlated noise. In addition, we extend our study to the three components of the magnetic field vector, as well as the magnetic filling factor α. With this, we confirm the tendency to overestimate γ when inverting linear polarization profiles that, although non-zero, are still below the noise level. We also establish that the overestimation occurs mainly for magnetic fields that are nearly vertical γ 20 • . This indicates that a reliable inference of the inclination of the magnetic field vector cannot be achieved by analyzing only Stokes I and V. In addition, when inverting Stokes Q and U profiles below the noise, the inversion code retrieves a randomly uniform distribution of the azimuth of the magnetic field vector φ. To avoid these problems, we propose only inverting Stokes profiles for which the linear polarization signals are sufficiently above the noise level. However, this approach is also biased because, in spite of allowing for a very accurate retrieval of the magnetic field vector from the selected Stokes profiles, it selects only profiles arising from highly inclined magnetic fields.

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“…VSM is a full disk Stokes polarimeter. Auer et al (1977). Beginning January 2012, QL vector magnetograms are created using weak-field approximation (Ronan et al 1987).…”

Section: Synoptic Optical Long-term Investigations Of the Sun (Solis)mentioning

confidence: 99%

Full-disk nonlinear force-free field extrapolation of SDO/HMI and SOLIS/VSM magnetograms

Tadesse

Wiegelmann

Inhester

et al. 2013

A&A

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Context. The magnetic field configuration is essential for understanding solar explosive phenomena, such as flares and coronal mass ejections. To overcome the unavailability of coronal magnetic field measurements, photospheric magnetic field vector data can be used to reconstruct the coronal field. Two complications of this approach are that the measured photospheric magnetic field is not force-free and that one has to apply a preprocessing routine to achieve boundary conditions suitable for the force-free modeling. Furthermore the nonlinear force-free extrapolation code should take uncertainties into account in the photospheric field data. They occur due to noise, incomplete inversions, or azimuth ambiguity-removing techniques. Aims. Extrapolation codes in Cartesian geometry for modeling the magnetic field in the corona do not take the curvature of the Sun's surface into account and can only be applied to relatively small areas, e.g., a single active region. Here we apply a method for nonlinear force-free coronal magnetic field modeling and preprocessing of photospheric vector magnetograms in spherical geometry using the optimization procedure to full disk vector magnetograms. We compare the analysis of the photospheric magnetic field and subsequent force-free modeling based on full-disk vector maps from Helioseismic and Magnetic Imager (HMI) onboard the solar dynamics observatory (SDO) and Vector Spectromagnetograph (VSM) of the Synoptic Optical Long-term Investigations of the Sun (SOLIS). Methods. We used HMI and VSM photospheric magnetic field measurements to model the force-free coronal field above multiple solar active regions, assuming magnetic forces to dominate. We solved the nonlinear force-free field equations by minimizing a functional in spherical coordinates over a full disk and excluding the poles. After searching for the optimum modeling parameters for the particular data sets, we compared the resulting nonlinear force-free model fields. We compared quantities, such as the total magnetic energy content, free magnetic energy, the longitudinal distribution of the magnetic pressure, and surface electric current density, using our spherical geometry extrapolation code. Results. The magnetic field lines obtained from nonlinear force-free extrapolation based on HMI and VSM data show good agreement. However, the nonlinear force-free extrapolation based on HMI data contain more total magnetic energy, free magnetic energy, the longitudinal distribution of the magnetic pressure, and surface electric current density than do the VSM data.

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The determination of vector magnetic fields from stokes profiles

Cited by 111 publications

References 10 publications

Vector magnetic field map at the photospheric level below and around a solar filament (neutral line)

Vector magnetic field map at the photospheric level below and around a solar filament (neutral line)

Inferring the magnetic field vector in the quiet Sun

Full-disk nonlinear force-free field extrapolation of SDO/HMI and SOLIS/VSM magnetograms

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