Polarimetric Littrow Spectrograph – instrument calibration   and first measurements

Beck, C.; Schmidt, W.; Kentischer, T.; Elmore, D.

doi:10.1051/0004-6361:20052662

Cited by 110 publications

(130 citation statements)

References 12 publications

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“…The good and stable seeing conditions during the observation and the Kiepenheuer-Institut Adaptive Optics System (von der Lühe et al 2003;Berkefeld et al 2010) provided a spatial resolution of about 1 . We combined the Tenerife Infrared Polarimeter (TIP II; Martínez Pillet et al 1999;Collados et al 2007) attached to the Echelle spectrograph, the POlarimetric LIttrow Spectrograph (POLIS; Schmidt et al 2003;Beck et al 2005b), and two additional channels to record the intensity spectra of the Ca ii IR line via the Echelle Beck et al 2007or Kucera et al 2008 for a similar combination using TIP and TESOS).…”

Section: Observationsmentioning

confidence: 99%

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Multiwavelength spectropolarimetric observations of an Ellerman bomb

Rezaei

Beck

2015

A&A

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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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Section: Observationsmentioning

confidence: 99%

“…POLIS used an integration time of about 4.8 s per slit position. Details of the calibration procedure for the calcium profiles are explained in Beck et al (2005b). An additional camera was installed to record Hα spectra.…”

Section: Visible Spectral Datamentioning

confidence: 99%

Multiwavelength spectropolarimetric observations of an Ellerman bomb

Rezaei

Beck

2015

A&A

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“…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 . The efficiencies for measuring the components of the Stokes vector (cf.…”

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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“…Different spatial locations in the scanning direction are thus sampled at different times. The observation data were acquired with the spectropolarimeters POLIS (Beck et al 2005, 396 nm, 630 nm), TIP (Collados et al 2007(Collados et al , 1083(Collados et al nm, 1565, and a spectroscopic setup using the main spectrograph of the German Vacuum Tower Telescope (VTT) for Ca ii IR 866 nm. Simultaneously with the Ca ii IR 866 nm line, Ca ii H intensity spectra were also recorded for the investigation of cross-correlations; these spectra were, however, not used here.…”

Section: Correlation Matrices From Observationsmentioning

confidence: 99%

“…The spectra were calculated on a non-equidistant wavelength grid with a spectral sampling of around 4 pm in the line wing and 1.5 pm near the line core to reduce the computational effort. We interpolated and resampled the spectra to an equidistant sampling of 2 pm for a better match to the POLIS observations (1.9 pm sampling at a spectral resolution of 220.000, Beck et al 2005) and then determined the correlation matrix as before. The bottom left and right panel in Fig.…”

Section: Nlte 1-d Simulationsmentioning

confidence: 99%

Linear wavelength correlation matrices of photospheric and chromospheric spectral lines

Beck¹,

Rammacher²

2010

A&A

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Context. The process that heats the solar chromosphere is a difficult target for observational studies because the assumption of local thermal equilibrium (LTE) is not valid in the upper solar atmosphere, which complicates the analysis of spectra. Aims. We investigate the linear correlation coefficient between the intensities at different wavelengths in photospheric and chromospheric spectral lines because the correlation can be determined directly for any spectra from observations or modeling. Waves which propagate vertically through the stratified solar atmosphere affect different wavelengths at different times when the contribution functions for each wavelength peak in different layers. This leads to a characteristic pattern of (non-)coherence of the intensity at various wavelengths with respect to each other which carries information on the physical processes. Methods. We derived the correlation matrices for several photospheric and chromospheric spectral lines from observations. We separated locations with a significant photospheric polarization signal and thus magnetic fields from those without a polarization signal. For comparison with the observations, we calculated correlation matrices for spectra from simplified LTE modeling approaches, 1-D NLTE simulations, and a 3-D MHD simulation run. We applied the correlation method also to temperature maps at different optical depth layers derived from a LTE inversion of Ca ii H spectra.Results. We find that all photospheric spectral lines show a similar pattern: a pronounced asymmetry of the correlation between line core and red or blue wing. The pattern cannot be reproduced with a simulation of the granulation pattern, but with waves that travel upwards through the formation heights of the lines. The correct asymmetry between red and blue wing only appears when a temperature enhancement occurs simultaneously with a downflow velocity in the wave simulation. All chromospheric spectral lines show a more complex pattern. The 1-D NLTE simulations of monochromatic waves produce a correlation matrix that qualitatively matches the observations near the very core of the Ca ii H line. The photospheric signature is well reproduced in the correlation matrix derived from the 3-D MHD simulation. Conclusions. The correlation matrices of observed photospheric and chromospheric spectral lines are highly structured with characteristic and different patterns in every spectral line. The comparison with matrices derived from simulations and simple modeling suggests that the main driver of the detected patterns are upwards propagating waves. Application of the correlation method to 3-D temperature cubes seems to be a promising tool for a detailed comparison of simulation results and observations in future studies.

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Polarimetric Littrow Spectrograph – instrument calibration and first measurements

Cited by 110 publications

References 12 publications

Multiwavelength spectropolarimetric observations of an Ellerman bomb

Multiwavelength spectropolarimetric observations of an Ellerman bomb

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

Linear wavelength correlation matrices of photospheric and chromospheric spectral lines

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