scholarly journals Convective blueshifts in the solar atmosphere

2019 ◽  
Vol 622 ◽  
pp. A34 ◽  
Author(s):  
F. Stief ◽  
J. Löhner-Böttcher ◽  
W. Schmidt ◽  
T. Steinmetz ◽  
R. Holzwarth
Keyword(s):  
Solar Atmosphere ◽  
Frequency Comb ◽  
Solar Spectrum ◽  
Solar Limb ◽  
Laser Frequency ◽  
Disk Center ◽  
Spectral Lines ◽  
Significant Discrepancy ◽  
Convective Motions ◽  
The Absolute

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.

scholarly journals Convective blueshifts in the solar atmosphere

2019 ◽  
Vol 624 ◽  
pp. A57 ◽  
Author(s):  
J. Löhner-Böttcher ◽  
W. Schmidt ◽  
R. Schlichenmaier ◽  
T. Steinmetz ◽  
R. Holzwarth
Keyword(s):  
Solar Atmosphere ◽  
Solar Disk ◽  
Frequency Comb ◽  
Solar Limb ◽  
3D Models ◽  
Disk Center ◽  
Spectral Lines ◽  
Visible Range ◽  
Quiet Sun ◽  
Doppler Shifts

Context. Convective motions in the solar atmosphere cause spectral lines to become asymmetric and shifted in wavelength. For photospheric lines, this differential Doppler shift varies from the solar disk center to the limb. Aims. Precise and comprehensive observations of the convective blueshift and its center-to-limb variation improve our understanding of the atmospheric hydrodynamics and ensuing line formation, and provide the basis to refine 3D models of the solar atmosphere. Methods. We performed systematical spectroscopic measurements of the convective blueshift of the quiet Sun with the Laser Absolute Reference Spectrograph (LARS) at the German Vacuum Tower Telescope. The spatial scanning of the solar disk covered 11 heliocentric positions each along four radial (meridional and equatorial) axes. The high-resolution spectra of 26 photospheric to chromospheric lines in the visible range were calibrated with a laser frequency comb to absolute wavelengths at the 1 m s−1 accuracy. Applying ephemeris and reference corrections, the bisector analysis provided line asymmetries and Doppler shifts with an uncertainty of only few m s−1. To allow for a comparison with other observations, we convolved the results to lower spectral resolutions. Results. All spectral line bisectors exhibit a systematic center-to-limb variation. Typically, a blueshifted “C”-shaped curve at disk center transforms into a less blueshifted “\”-shape toward the solar limb. The comparison of all lines reveals the systematic dependence of the convective blueshift on the line depth. The blueshift of the line minima describe a linear decrease with increasing line depths. The slope of the center-to-limb variation develops a reversal point at heliocentric positions between μ = 0.7 and 0.85, seen as the effect of horizontal granular flows in the mid photosphere. Line minima formed in the upper photosphere to chromosphere exhibit hardly any blueshift or even a slight redshift. Synthetic models yield considerable deviations from the observed center-to-limb variation. Conclusions. The obtained Doppler shifts of the quiet Sun can serve as an absolute reference for other observations, the relative calibration of Dopplergrams, and the necessary refinement of atmospheric models. Based on this, the development of high-precision models of stellar surface convection will advance the detection of (potentially habitable) exoplanets by radial velocity measurements.

scholarly journals Convective blueshifts in the solar atmosphere

2018 ◽  
Vol 611 ◽  
pp. A4 ◽  
Author(s):  
J. Löhner-Böttcher ◽  
W. Schmidt ◽  
F. Stief ◽  
T. Steinmetz ◽  
R. Holzwarth
Keyword(s):  
Frequency Comb ◽  
Accurate Determination ◽  
Solar Spectrum ◽  
Solar Limb ◽  
Disk Center ◽  
Spectral Lines ◽  
Solar Photosphere ◽  
Solar Convection ◽  
Line Core ◽  
The Impact

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. Aim. 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.

scholarly journals Infrared Helioseismology: Detection of the chromospheric mode

1988 ◽  
Vol 123 ◽  
pp. 425-428
Author(s):  
Drake Deming ◽  
David A. Glenar ◽  
Hans Ulrich Käufl ◽  
Fred Espenak
Keyword(s):  
Solar Atmosphere ◽  
Solar Spectrum ◽  
Disk Center ◽  
Solar Chromosphere ◽  
Line Profiles ◽  
Pure Rotation ◽  
Rotation Line ◽  
Show Evidence ◽  
Laser Heterodyne ◽  
Heterodyne Technique

We have observed solar oscillations using a new instrumental technique in a relatively unexplored region of the solar spectrum. We obtained a 2-day sequence of line profiles, at 30 second intervals, for a pure rotation line of OH at 11.065 μm, using a laser heterodyne spectrometer to view a 2 arc-sec portion of the quiet Sun at disk center. The continuous opacity of the solar atmosphere increases with wavelength longward of 1.6 μm, so 11 μm lines are formed in the upper photosphere, near h = 250 km. In this region the OH rotational transitions have δJ=1 collisional rates which are two orders of magnitude larger than their radiative rates. Hence the OH lines have source functions which are equal to the Planck function, and the high spectral purity provided by the laser heterodyne technique makes their line profiles especially appropriate for investigating the dynamics of the solar atmosphere. We have recently reported (Deming et al. 1986) that oscillations in this OH line show evidence of a resonance due to a cavity in the solar chromosphere.

scholarly journals Researches in spectrum analysis in connexion with the spectrum of the sun

1878 ◽  
Vol 27 (185-189) ◽  
pp. 279-284
Keyword(s):  
Solar Atmosphere ◽  
Spectrum Analysis ◽  
Solar Spectrum ◽  
Spectral Lines ◽  
The Sun

In a map of the solar spectrum, containing a greatly increased number of lines, such as that upon which I am now engaged, it is possible to seek under favourable conditions the coincidence or non-coincidence of lines due to elements hitherto undetected in the solar atmosphere, on account of their existing in quantities insufficient to give very marked spectral lines. A search has accordingly been made, on the principles laid down in previous communications, for those elements which contain in their spectra long and well-characterised lines in the photographic region.

scholarly journals Astro-comb: revolutionizing precision spectroscopy in astrophysics

2008 ◽  
Vol 4 (S253) ◽  
pp. 499-501
Author(s):  
Claire E. Cramer ◽  
Chih-Hao Li ◽  
Andrew J. Benedick ◽  
Alexander G. Glenday ◽  
Franz X. Kärtner ◽  
...  
Keyword(s):  
Doppler Shift ◽  
Extrasolar Planets ◽  
Frequency Comb ◽  
Laser Frequency ◽  
Spectral Lines ◽  
Resolving Power ◽  
Long Term Stability ◽  
Fabry Perot ◽  
Line Spacing ◽  
Wide Line

AbstractSearches for extrasolar planets using the periodic Doppler shift of stellar spectral lines have recently achieved a precision better than 60cm/s. To find a 1-Earth mass planet in an Earth-like orbit, a precision of 5cm/s is necessary. The combination of a laser frequency comb with a Fabry-Perot filtering cavity has been suggested as a promising approach to achieve such Doppler shift resolution via improved spectrograph wavelength calibration. Here we report the fabrication of such a filtered laser comb with up to 40 GHz (~1 Angstrom) line spacing, generated from a 1 GHz repetition-rate source, without compromising long-term stability, reproducibility or spectral resolution. This wide-line-spacing comb (astro-comb) is well matched to the resolving power of high-resolution astrophysical spectrographs. The astrocomb should allow a precision as high as 1cm/s in astronomical readial velocity measurements.

scholarly journals Solar Irradiance Variability Due to Solar Flares Observed in Lyman-Alpha Emission

Solar Physics ◽  
2021 ◽  
Vol 296 (3) ◽  
Author(s):  
Ryan O. Milligan
Keyword(s):  
Solar Flares ◽  
Solar Spectrum ◽  
Neutral Hydrogen ◽  
Solar Limb ◽  
High Background ◽  
Superposed Epoch Analysis ◽  
Lyman Alpha ◽  
Alpha Emission ◽  
Epoch Analysis ◽  
Very High

AbstractAs the Lyman-alpha (Ly$\upalpha $ α ) line of neutral hydrogen is the brightest emission line in the solar spectrum, detecting increases in irradiance due to solar flares at this wavelength can be challenging due to the very high background. Previous studies that have focused on the largest flares have shown that even these extreme cases generate enhancements in Ly$\upalpha $ α of only a few percent above the background. In this study, a superposed-epoch analysis was performed on ≈8500 flares greater than B1 class to determine the contribution that they make to changes in the solar EUV irradiance. Using the peak of the 1 – 8 Å X-ray emission as a fiducial time, the corresponding time series of 3123 B- and 4972 C-class flares observed in Ly$\upalpha $ α emission by the EUV Sensor on the Geostationary Operational Environmental Satellite 15 (GOES-15) were averaged to reduce background fluctuations and improve the flare signal. The summation of these weaker events showed that they produced a 0.1 – 0.3% enhancement to the solar Ly$\upalpha $ α irradiance on average. For comparison, the same technique was applied to 453 M- and 31 X-class flares, which resulted in a 1 – 4% increase in Ly$\upalpha $ α emission. Flares were also averaged with respect to their heliographic angle to investigate any potential center-to-limb variation. For each GOES class, the relative enhancement in Ly$\upalpha $ α at the flare peak was found to diminish for flares that occurred closer to the solar limb due to the opacity of the line and/or foreshortening of the footpoints. One modest event included in the study, a C6.6 flare, exhibited an unusually high increase in Ly$\upalpha $ α of 7% that may have been attributed to a failed filament eruption. Increases of this magnitude have hitherto only been associated with a small number of X-class flares.

scholarly journals Controlling and Phase‐Locking a THz Quantum Cascade Laser Frequency Comb by Small Optical Frequency Tuning

2021 ◽  
pp. 2000417
Author(s):  
Luigi Consolino ◽  
Annamaria Campa ◽  
Michele De Regis ◽  
Francesco Cappelli ◽  
Giacomo Scalari ◽  
...  
Keyword(s):  
Quantum Cascade Laser ◽  
Frequency Tuning ◽  
Frequency Comb ◽  
Phase Locking ◽  
Laser Frequency ◽  
Optical Frequency ◽  
Quantum Cascade

Line profile analysis of an astronomical spectrograph with a laser frequency comb

2014 ◽  
Vol 14 (8) ◽  
pp. 1037-1045 ◽  
Author(s):  
Fei Zhao ◽  
Gang Zhao ◽  
Gaspare Lo Curto ◽  
Hui-Juan Wang ◽  
Yu-Juan Liu ◽  
...  
Keyword(s):  
Line Profile ◽  
Profile Analysis ◽  
Frequency Comb ◽  
Laser Frequency ◽  
Line Profile Analysis

scholarly journals Traceability of laser frequency/wavelength calibration through the frequency comb at Inmetro

2016 ◽  
Vol 733 ◽  
pp. 012058 ◽  
Author(s):  
I L M Silva ◽  
I B Couceiro ◽  
M A C Torres ◽  
P A Costa ◽  
H P H Grieneisen
Keyword(s):  
Frequency Comb ◽  
Laser Frequency ◽  
Wavelength Calibration

scholarly journals Self referenced Yb-fiber-laser frequency comb using a dispersion micromanaged tapered holey fiber

2007 ◽  
Vol 15 (19) ◽  
pp. 12161 ◽  
Author(s):  
Parama Pal ◽  
Wayne H. Knox ◽  
Ingmar Hartl ◽  
Martin E. Fermann
Keyword(s):  
Fiber Laser ◽  
Frequency Comb ◽  
Laser Frequency ◽  
Holey Fiber
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