On the validity of the 630 nm Fe I lines for magnetometry of the internetwork quiet Sun

González, M. J. Martínez; Collados, M.; Cobo, B. Ruiz

doi:10.1051/0004-6361:20065008

Cited by 67 publications

(79 citation statements)

References 21 publications

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“…The distribution of the nearly horizontal fields (red curve at bottom left of Fig. 8) shows an enhanced fraction of these weak fields below 200 G, which implies that the detection limit due to the noise is not yet reached at 200 G. In the question of the seemingly preference for kG fields in the analysis of 630 nm data, Martínez González et al (2006b) cautioned that the thermodynamics must be treated consistently in an analysis of the 630 nm line pair in the weak field limit. Rezaei et al (2007) were able to derive a distribution without the bump using a SIR inversion of 630 nm data of lower spatial resolution than available nowadays in HINODE observations.…”

Section: Inversion Resultsmentioning

confidence: 97%

The magnetic flux of the quiet Sun internetwork as observed with the Tenerife infrared polarimeter

Beck¹,

Rezaei²

2009

A&A

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Context. Observations made with the spectropolarimeter onboard the HINODE satellite have detected abundant horizontal magnetic fields in the internetwork quiet Sun. Aims. We compare the results for the horizontal fields obtained at 630 nm with ground-based observations at 1.56 μm, where the sensitivity to magnetic fields is higher than in the visible. Methods. We obtained 30-s integrated spectropolarimetric data of the quiet Sun on disc centre during a period of extremely stable and good seeing. The data have a rms noise in polarization of around 2 × 10 −4 of the continuum intensity. The low noise level allows the spectra to be inverted with the SIR code. We compare the inversion results with proxies to determine the magnetic flux. Results. We confirm the presence of the horizontal fields in the quiet Sun internetwork as reported for the satellite data, including voids without linear polarization signal that extend over an area of a few granules. Voids in the circular polarization signal are only of granular scale. More than 60% of the surface show polarization signals of above four times the rms noise level. We find that the total magnetic flux contained in the more inclined to horizontal fields (γ > 45 • ) is lower by a factor of around 2 than that of the less inclined fields. The proxies for flux determination are strongly affected by the thermodynamic state of the atmosphere, and hence, seem to be unreliable. Conclusions. During spells of good seeing conditions, adaptive optics can render ground-based slit-spectrograph observations at a 70-cm telescope equivalent to the seeing-free space-based data of half-meter class telescopes. We suggest that the difference in the ratio of horizontal to transversal flux between the ground-based infrared data and the satellite-based visible data is due to the different formation heights of the respective spectral lines. We emphasize that the true amount of magnetic flux cannot be derived directly from the spectra. For purely horizontal flux, one would need its vertical extension that has to be estimated by explicit modeling, using the observed spectra as boundary conditions, or be taken from MHD simulations. Time-series of the evolution of the magnetic flux and chromospheric diagnostics are needed to address its possible contribution to chromospheric heating.

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Section: Inversion Resultsmentioning

confidence: 97%

The magnetic flux of the quiet Sun internetwork as observed with the Tenerife infrared polarimeter

Beck¹,

Rezaei²

2009

A&A

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“…Martínez González et al 2006). Effective magnetic filling factors are about 10% for ground-based observations at 1 resolution (e.g.…”

Section: Inversion Setupmentioning

confidence: 99%

Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations

Beck

Fabbian

Moreno‐Insertis

et al. 2013

A&A

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Context. Numerical three-dimensional (3D) radiative (magneto-)hydrodynamical [(M)HD] simulations of solar convection are nowadays used to understand the physical properties of the solar photosphere and convective envelope, and, in particular, to determine the Sun's photospheric chemical abundances. To validate this approach, it is important to check that no excessive thermodynamic fluctuations arise as a consequence of the partially incomplete treatment of radiative transfer causing radiative damping that is too modest. Aims. We investigate the realism of the thermodynamics in recent state-of-the-art 3D convection simulations of the solar atmosphere carried out with the Stagger code.Methods. We compared the characteristic properties of several Fe i lines (557.6 nm, 630 nm, 1565 nm) and one Si i line at 1082.7 nm in solar disc-centre observations of different spatial resolution with spectra synthesized from 3D convection simulations. The observations were taken with ground-based (Echelle spectrograph, Göttingen Fabry-Pérot Interferometer (GFPI), POlarimetric LIttrow Spectrograph, Tenerife Infrared Polarimeter, all at the Vacuum Tower Telescope on Tenerife) and space-based instruments (Hinode/Spectropolarimeter). We degraded the synthetic spectra to the spatial resolution of the observations, based on the distribution of the continuum intensity I c . We estimated the spectral degradation to be applied to the simulation results by comparing atlas spectra with averaged observed spectra. In addition to deriving a set of line parameters directly from the intensity profiles, we used the SIR (Stokes Inversion based on Response functions) code to invert the spectra. Results. The spatial degradation kernels yield a similar generic spatial stray-light contamination of about 30% for all instruments. The spectral stray light inside the different spectrometers is found to be between 2% and 20%. Most of the line parameters from the observational data are matched by the degraded HD simulation spectra. The inversions predict a macroturbulent velocity v mac below 10 m s −1 for the HD simulation spectra at full spatial resolution, whereas they yield v mac 1000 m s −1 at a spatial resolution of 0. 3. The temperature fluctuations in the inversion of the degraded HD simulation spectra do not exceed those from the observational data (of the order of 100−200 K rms for −2 log τ 500 nm −0.5). The comparison of line parameters in spatially averaged profiles with the averaged values of line parameters in spatially resolved profiles indicates a significant change in (average) line properties on a spatial scale between 0. 13 and 0. 3. Conclusions. Up to a spatial resolution of 0. 3 (GFPI spectra), we find no indications of excessive thermodynamic fluctuations in the 3D HD simulation. To definitely confirm that simulations without spatial degradation contain fully realistic thermodynamic fluctuations requires observations at even higher spatial resolution (i.e. <0. 13).

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“…Unfortunately, the signals are barely above the noise level, thus it has not yet been possible to fully characterize the magnetic field vector in these regions. This has led to a long-standing controversy about the distribution of the module of the magnetic field vector in the internetwork (Domínguez Cerdeña et al 2003;Socas-Navarro & Lites 2004;Sánchez Almeida 2005;Martínez González et al 2006;Asensio Ramos et al 2007;López Ariste et al 2007;Socas-Navarro et al 2008; see also references therein).…”

Section: Introductionmentioning

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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On the validity of the 630 nm Fe I lines for magnetometry of the internetwork quiet Sun

Cited by 67 publications

References 21 publications

The magnetic flux of the quiet Sun internetwork as observed with the Tenerife infrared polarimeter

The magnetic flux of the quiet Sun internetwork as observed with the Tenerife infrared polarimeter

Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations

Inferring the magnetic field vector in the quiet Sun

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