A polarization model for the German Vacuum Tower Telescope 
 from in situ and laboratory measurements

Beck, C.; Schlichenmaier, R.; Collados, M.; Rubio, L. R. Bellot; Kentischer, T.

doi:10.1051/0004-6361:20052935

Cited by 76 publications

(67 citation statements)

References 11 publications

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“…The average uncertainties in the calculated magnetic field strength and inclination angle given by SIR are ±50 G and ±10 deg, respectively. The values agree with a previous error estimate in Beck (2006) derived from a direct analysis of the profile shape of the 1.56 μm lines (Table 3.2 on p. 47; δB ∼ 50 G and δθ ∼ 5 deg).…”

Section: Appendix A: Examples Of Inverted Spectrasupporting

confidence: 91%

“…We show in addition maps of the ratio of linear to circular polarization, L/V. The ratio was calculated for an average of a few spectral pixels (∼10 pm) near the location of the highest V signal of 1564.8 nm (see Beck 2006). The spatial pattern in these maps is strikingly similar to that found by LI08: the polarization signal shows "voids" of some granules extent (about 10 diameter in L tot , 3-5 in V tot ) that are free of any clear signal; the linear polarization signal appears in the shape of small "blobs" around the edges of the voids.…”

Section: Discussionmentioning

confidence: 99%

“…An accurate determination of magnetic field properties from the weak polarization signals prevalent in the quiet Sun in VIS lines is thus strongly dependent on the quality of the observations, the signal-to-noise ratio (Bellot Rubio & Collados 2003;Rezaei et al 2007), and the spatial resolution (Khomenko et al 2005b). IR spectra at 1.56 μm allow us to disentangle most of the ambiguities because the field strength can be inferred from the splitting of the polarization, or in some cases, even the intensity components of the spectral line for magnetic fields above around 400 G (Beck 2006;Beck et al 2007). It is thus unsurprising that studies before HINODE using solely VIS lines (Sánchez Almeida & Lites 2000;Domínguez Cerdeña et al 2003;Socas-Navarro & Sánchez Almeida 2003; or IR lines (Lin 1995;Khomenko et al 2003; Article published by EDP Sciences Martínez González et al 2006a;Domínguez Cerdeña et al 2006a) did not agree especially about the intrinsic field strength: the VIS observations indicated a significant fraction of strong kG fields, whereas the IR was dominated by weaker hG fields.…”

Section: Introductionmentioning

confidence: 99%

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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: Appendix A: Examples Of Inverted Spectrasupporting

confidence: 91%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Beck¹,

Rezaei²

2009

A&A

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“…35. The spectral sampling was 1.1 pm and the integration time per slit position was 6 s. The polarimetric calibration was performed using the VTT telescope model and the near-IR instrumental calibration unit (Beck et al 2005a). The residual crosstalk was corrected with the statistical method of Schlichenmaier & Collados (2002).…”

Section: Infrared Spectropolarimetric Datamentioning

confidence: 99%

Multiwavelength spectropolarimetric observations of an Ellerman bomb

Rezaei

Beck

2015

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Context. Ellerman bombs (EBs) are enhanced emission in the wings of the Hα line in the solar spectrum. Aims. We study the structure of an EB in the photosphere and chromosphere.Methods. We analyze simultaneous observations of four chromospheric lines (Hα, Ca ii H, Ca ii IR 854 nm, and He i 1083 nm) as well as two photospheric lines (Fe i 630 and Si i 1082.7 nm) along with high-cadence 160 and 170 nm ultraviolet (UV) continuum filtergrams. Full Stokes data from the Helioseismic and Magnetic Imager (HMI) are used to trace the temporal evolution of the magnetic structure.Results. We identify the EB by excess emission in the wings of the Hα line, a brightening in the UV continuum, and large emission peaks in the core of the two Ca ii lines. The EB shows a blueshift in all chromospheric lines, while no shifts are observed in the photospheric lines. The blueshift in the chromospheric layer causes very asymmetric emission peaks in the Ca ii H line. The photospheric Si i spectral line shows a shallower line depth at the location of the EB. The UV continuum maps show that the EB was substantially brighter than its surroundings for about 30 min. The continuum contrast of the EB from 170 nm to 1080 nm shows a power-law dependency on the wavelength. The temperature enhancement amounts to 130 K in the low photosphere and 400 K at the temperature minimum level. This temperature excess is also seen in an LTE inversion of the Ca ii spectra. The total thermal and radiative energy content of the EB is about 10 20 J and 10 18 J in the photosphere and chromosphere, respectively. The HMI data hints at a photospheric magnetic flux cancellation as the driver of the EB. Conclusions. Ellerman bombs release the energy in a height range of several pressure scale heights around the temperature minimum such that they affect both the photosphere and the lower chromosphere.

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“…The polarimetric calibration of the Göttingen FPI data was carried out in a two-step procedure with a correction for the instrumental polarization induced by the telescope (Beck et al 2005a) and the cross-talk caused by the optics behind the instrumental calibration unit (ICU) of the VTT. The calibration data obtained with the ICU were used to determine the polarimeter response function with the generic approach for the VTT described in, e.g., Beck et al (2005b) or .…”

Section: Polarimetric Calibrationmentioning

confidence: 99%

Application of speckle and (multi-object) multi-frame blind deconvolution techniques on imaging and imaging spectropolarimetric data

Puschmann¹,

Beck²

2011

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Context. Ground-based imaging and imaging spectropolarimetric data are often subjected to post-facto reconstruction techniques to improve the spatial resolution. Aims. We test the effects of reconstruction techniques on two-dimensional data to determine the best approach to improve our data. Methods. We obtained an 1-h time-series of spectropolarimetric data in the Fe i line at 630.25 nm with the Göttingen Fabry-Pérot Interferometer (FPI) that are accompanied by imaging data in the blue continuum at 431.3 nm and Ca ii H at 396.85 nm. We apply both speckle and (multi-object) multi-frame blind deconvolution ((MO)MFBD) techniques. We use the "Göttingen" speckle and speckle deconvolution codes and the MOMFBD code in the implementation of Van Noort et al. (2005). We compare the resulting spatial resolution and investigate the impact of the image reconstruction on spectral characteristics of the Göttingen FPI data. Results. The speckle reconstruction and MFBD perform similar for our imaging data with nearly identical intensity contrasts. MFBD provides a better and more homogeneous spatial resolution at the shortest wavelength when applied to a series of image bursts. The MOMFBD and speckle deconvolution of the intensity spectra lead to similar results, but our choice of settings for the MOMFBD yields an intensity contrast smaller by about 2% at a comparable spatial resolution. None of the reconstruction techniques introduces significant artifacts in the intensity spectra. The speckle deconvolution (MOMFBD) has a rms noise in Stokes V/I of 0.32% (0.20%). The deconvolved spectra thus require a high significance threshold of about 1.0% to separate noise peaks from true signal. A comparison to spectra with a significantly higher signal-to-noise (S/N) ratio and to spectra from a magneto-hydrodynamical simulation reveals that the Göttingen FPI can only detect about 30% of the polarization signal present in quiet Sun areas. The distribution of NCP values for the speckle-deconvolved data matches that of observations with higher S/N better than MOMFBD, but shows seemingly artificially sharp boundaries and unexpected changes of the sign. Conclusions. For our imaging data, both MFBD and speckle reconstruction are equivalent, with a slightly better and more stable performance of MFBD. For the spectropolarimetric data, the higher intensity contrast of the speckle deconvolution is balanced by the smaller amplification of the noise level in the MOMFBD at a comparable spatial resolution. The noise level prevents the detection of weak and diffuse magnetic fields. Future efforts should be directed to improve the S/N of the Göttingen FPI spectra for spectropolarimetric observations to lower the final significance thresholds.

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A polarization model for the German Vacuum Tower Telescope from in situ and laboratory measurements

Cited by 76 publications

References 11 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

Multiwavelength spectropolarimetric observations of an Ellerman bomb

Application of speckle and (multi-object) multi-frame blind deconvolution techniques on imaging and imaging spectropolarimetric data

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