LARS: An Absolute Reference Spectrograph for solar observations

J., Loehner-Boettcher,; Schmidt, Wolfgang; Doerr, Hans-Peter; Kentischer, T.; Steinmetz, Tilo; Probst, Rafael A.; Holzwarth, Ronald

doi:10.1051/0004-6361/201731164

Cited by 21 publications

(30 citation statements)

References 24 publications

(27 reference statements)

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“…In 2016, a substantial upgrade of the laser frequency comb was performed which enabled the stable operation of LARS as a turn-key instrument. The current state of the instrument in its science-ready configuration was described by Löhner-Böttcher et al (2017). For the sake of clarity, we briefly explain the setup of LARS here, and highlight its main subsystems.…”

Section: Lars Instrumentmentioning

confidence: 99%

“…We used the data pipeline developed by to process the spectroscopic LARS data. An overview of the current software and included calibration steps therein was given by Löhner-Böttcher et al (2017). In the following, we briefly describe the steps which have been executed to obtain the final comb-calibrated data.…”

Section: Data Calibrationmentioning

confidence: 99%

“…The key advance compared to other comparable or former studies comes from the instrument being used for the measurements. The Laser Absolute Reference Spectrograph (LARS; Löhner-Böttcher et al 2017) allows for spectroscopic observations of the Sun with an unprecedented spectral quality. The combination of a highresolution echelle spectrograph (λ/∆λ > 700 000) with a laser frequency comb for the absolute wavelength calibration enables the measurement of spectral line profiles with a spectral accuracy of 1 m s −1 (Steinmetz et al 2008).…”

Section: Introductionmentioning

confidence: 99%

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Convective blueshifts in the solar atmosphere

Löhner-Böttcher

Schmidt

Stief

et al. 2018

A&A

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Context. The solar convection manifests as granulation and intergranulation at the solar surface. In the photosphere, convective motions induce differential Doppler shifts to spectral lines. The observed convective blueshift varies across the solar disk. Aims. We focus on the impact of solar convection on the atmosphere and aim to resolve its velocity stratification in the photosphere. Methods. We performed high-resolution spectroscopic observations of the solar spectrum in the 6302 Å range with the Laser Absolute Reference Spectrograph at the Vacuum Tower Telescope. A laser frequency comb enabled the calibration of the spectra to an absolute wavelength scale with an accuracy of 1 m s −1 . We systematically scanned the quiet Sun from the disk center to the limb at ten selected heliocentric positions. The analysis included 99 time sequences of up to 20 min in length. By means of ephemeris and reference corrections, we translated wavelength shifts into absolute line-of-sight velocities. A bisector analysis on the line profiles yielded the shapes and convective shifts of seven photospheric lines. Results. At the disk center, the bisector profiles of the iron lines feature a pronounced C-shape with maximum convective blueshifts of up to −450 m s −1 in the spectral line wings. Toward the solar limb, the bisectors change into a "\"-shape with a saturation in the line core at a redshift of +100 m s −1 . The center-to-limb variation of the line core velocities shows a slight increase in blueshift when departing the disk center for larger heliocentric angles. This increase in blueshift is more pronounced for the magnetically less active meridian than for the equator. Toward the solar limb, the blueshift decreases and can turn into a redshift. In general, weaker lines exhibit stronger blueshifts. Conclusions. Best spectroscopic measurements enabled the accurate determination of absolute convective shifts in the solar photosphere. We convolved the results to lower spectral resolution to permit a comparison with observations from other instruments.

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Section: Lars Instrumentmentioning

confidence: 99%

Section: Data Calibrationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Convective blueshifts in the solar atmosphere

Löhner-Böttcher

Schmidt

Stief

et al. 2018

A&A

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“…Speckle reconstructions with the Kiepenheuer-Institute Speckle Interferometry Package (KISIP, Wöger & von der Lühe 2008) enabled observations at a spatial resolution approaching the diffraction limit of the telescope. For more information on the instrument specifications and the data calibration, we refer to the detailed descriptions by Doerr (2015) and Löhner-Böttcher et al (2017). Figure 1 shows the final context image of NOAA 12146 observed on August 24 2014 at 8:41 UTC.…”

Section: Spatial Context Informationmentioning

confidence: 99%

Absolute velocity measurements in sunspot umbrae

Löhner-Böttcher¹,

Schmidt²,

Schlichenmaier³

et al. 2018

A&A

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Context. In sunspot umbrae, convection is largely suppressed by the strong magnetic field. Previous measurements reported on negligible convective flows in umbral cores. Based on this, numerous studies have taken the umbra as zero reference to calculate Doppler velocities of the ambient active region. Aims. To clarify the amount of convective motion in the darkest part of umbrae, we directly measured Doppler velocities with an unprecedented accuracy and precision. Methods. We performed spectroscopic observations of sunspot umbrae with the Laser Absolute Reference Spectrograph (LARS) at the German Vacuum Tower Telescope. A laser frequency comb enabled the calibration of the high-resolution spectrograph and absolute wavelength positions for 13 observation sequences. A thorough spectral calibration, including the measurement of the reference wavelength, yielded Doppler shifts of the spectral line Ti i 5713.9 Å with an uncertainty of around 5 m s −1 . A bisector analysis gave the depth-dependent line asymmetry. Results. The measured Doppler shifts are a composition of umbral convection and magneto-acoustic waves. For the analysis of convective shifts, we temporally average each sequence to reduce the superimposed wave signal. Compared to convective blueshifts of up to −350 m s −1 in the quiet Sun, sunspot umbrae yield a strongly reduced convective blueshifts around −30 m s −1 . We find that the velocity in a sunspot umbra correlates significantly with the magnetic field strength, but also with the umbral temperature defining the depth of the Ti i 5713.9 Å line. The vertical upward motion decreases with increasing field strength. Extrapolating the linear approximation to zero magnetic field reproduces the measured quiet Sun blueshift. In the same manner, we find that the convective blueshift decreases as a function of increasing line depth. Conclusions. Simply taking the sunspot umbra as a zero velocity reference for the calculation of photospheric Dopplergrams can imply a systematic velocity error reaching 100 m s −1 , or more. Setting up a relationship between vertical velocities and magnetic field strength provides a remedy for solar spectropolarimetry. We propose a novel approach of substantially increasing the accuracy of the Doppler velocities of a sunspot region by including the magnetic field information to define the umbral reference velocity.

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“…The series concentrates on the determination of absolute convective blueshifts of the most frequently used spectral lines in the visible range of the solar spectrum. The observations were performed with the Laser Absolute Reference Spectrograph (LARS; Doerr 2015;Löhner-Böttcher et al 2017). In order to obtain absolute wavelengths and Doppler velocities with m s −1 accuracy, the solar spectra are calibrated with a laser frequency comb.…”

Section: Introductionmentioning

confidence: 99%

Convective blueshifts in the solar atmosphere

Stief

Löhner-Böttcher

Schmidt

et al. 2019

A&A

Self Cite

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Context. Granular convective motions reach into the lower solar atmosphere, typically causing photospheric spectral lines to exhibit a differential line shift. This Doppler shift to shorter wavelengths is commonly known as convective blueshift. Aims. Spectroscopic high-accuracy measurements provide us with a refined determination of the absolute convective blueshift and its atmospheric distribution from disk center to the solar limb. Methods. We performed systematic observations of the quiet Sun with the Laser Absolute Reference Spectrograph (LARS) at the German Vacuum Tower Telescope. The solar disk was scanned along the meridian and the equator, from the disk center toward the limb. The solar spectrum around 6173 Å was calibrated with a laser frequency comb on an absolute wavelength scale with an accuracy of a few meters per second. We applied a bisector analysis on the spectral lines to reveal the changes of convective blueshift and line asymmetry at different heliocentric positions. Results. Being a signature for convective motions, the bisector curve of Fe i 6173.3 Å describes a "C"-shape at disk center. When approaching the solar limb, the bisector transforms into a "\"-shape. The analysis of the time-and bisector-averaged line shifts yields three distinct results. Firstly, the center-to-limb variation of Doppler velocities measured with LARS reveals a significant discrepancy (up to 200 m s −1 ) to the full-disk Dopplergrams of the Helioseismic and Magnetic Imager (HMI). Secondly, we obtained a significant decrease of convective blueshift toward the solar limb. Thirdly, the line-of-sight effect of solar activity, including p-mode oscillations and supergranular flows, leads to a scatter of up to ±100 m s −1 at intermediate heliocentric positions. Conclusions. The accurate observation of the absolute convective blueshift with LARS allows the identification of systematic discrepancy with Doppler velocities measured by HMI. The center-to-limb variation of HMI suffers from an additional blueshift for µ < 0.9 that is incompatible with our results. LARS measurements can be taken as a reference for the correction of systematic errors in the synoptic HMI Dopplergrams.Article published by EDP Sciences A34, page 1 of 8 A&A 622, A34 (2019)

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LARS: An Absolute Reference Spectrograph for solar observations

Cited by 21 publications

References 24 publications

Convective blueshifts in the solar atmosphere

Convective blueshifts in the solar atmosphere

Absolute velocity measurements in sunspot umbrae

Convective blueshifts in the solar atmosphere

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