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

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

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 Structure of sunspot light bridges in the chromosphere and transition region

2018 ◽  
Vol 609 ◽  
pp. A73 ◽  
Author(s):  
R. Rezaei
Keyword(s):  
Transition Region ◽  
Line Width ◽  
Solar Limb ◽  
Solar Radius ◽  
Spectral Lines ◽  
Quiet Sun ◽  
Hot Plasma ◽  
Emission Enhancement ◽  
Energy And Momentum ◽  
Far Ultraviolet

Context. Light bridges (LBs) are elongated structures with enhanced intensity embedded in sunspot umbra and pores. Aims. We studied the properties of a sample of 60 LBs observed with the Interface Region Imaging Spectrograph (IRIS). Methods. Using IRIS near- and far-ultraviolet spectra, we measured the line intensity, width, and Doppler shift; followed traces of LBs in the chromosphere and transition region (TR); and compared LB parameters with umbra and quiet Sun. Results. There is a systematic emission enhancement in LBs compared to nearby umbra from the photosphere up to the TR. Light bridges are systematically displaced toward the solar limb at higher layers: the amount of the displacement at one solar radius compares well with the typical height of the chromosphere and TR. The intensity of the LB sample compared to the umbra sample peaks at the middle/upper chromosphere where they are almost permanently bright. Spectral lines emerging from the LBs are broader than the nearby umbra. The systematic redshift of the Si iv line in the LB sample is reduced compared to the quiet Sun sample. We found a significant correlation between the line width of ions arising at temperatures from 3 × 104 to 1.5 × 105 K as there is also a strong spatial correlation among the line and continuum intensities. In addition, the intensity−line width relation holds for all spectral lines in this study. The correlations indicate that the cool and hot plasma in LBs are coupled. Conclusions. Light bridges comprise multi-temperature and multi-disciplinary structures extending up to the TR. Diverse heating sources supply the energy and momentum to different layers, resulting in distinct dynamics in the photosphere, chromosphere, and TR.

scholarly journals Signatures of ubiquitous magnetic reconnection in the lower solar atmosphere

2020 ◽  
Vol 641 ◽  
pp. L5
Author(s):  
Jayant Joshi ◽  
Luc H. M. Rouppe van der Voort ◽  
Jaime de la Cruz Rodríguez
Keyword(s):  
Magnetic Reconnection ◽  
Solar Atmosphere ◽  
Solar Limb ◽  
Small Scale ◽  
Balmer Line ◽  
Solar Chromosphere ◽  
Quiet Sun ◽  
Single Moment ◽  
Order Of Magnitude ◽  
Lower Solar Atmosphere

Ellerman Bomb-like brightenings of the hydrogen Balmer line wings in the quiet Sun, also known as quiet Sun Ellerman bombs (QSEBs), are a signature of the fundamental process of magnetic reconnection at the smallest observable scale in the lower solar atmosphere. We analyze high spatial resolution observations (0.″1) obtained with the Swedish 1-m Solar Telescope to explore signatures of QSEBs in the Hβ line. We find that QSEBs are ubiquitous and uniformly distributed throughout the quiet Sun, predominantly occurring in intergranular lanes. We find up to 120 QSEBs in the field of view for a single moment in time; this is more than an order of magnitude higher than the number of QSEBs found in earlier Hα observations. This suggests that about half a million QSEBs could be present in the lower solar atmosphere at any given time. The QSEB brightenings found in the Hβ line wings also persist in the line core with a temporal delay and spatial offset toward the nearest solar limb. Our results suggest that QSEBs emanate through magnetic reconnection along vertically extended current sheets in the lower solar atmosphere. The apparent omnipresence of small-scale magnetic reconnection may play an important role in the energy balance of the solar chromosphere.

scholarly journals Polarimetric Measurements of the Fine Structure of a Sunspot Penumbra

1993 ◽  
Vol 141 ◽  
pp. 44-47
Author(s):  
J. Hofmann ◽  
F.-L. Deubner ◽  
B. Fleck ◽  
W. Schmidt
Keyword(s):  
Solar Limb ◽  
Disk Center ◽  
Vacuum Tower ◽  
Magnetic Parameters ◽  
Doppler Shifts ◽  
Field Lines ◽  
Analysis Point ◽  
Clear Relation ◽  
Sunspot Penumbra ◽  
Field Inclination

AbstractObservations of a fairly regular sunspot penumbra near disk center have been carried out using a Stokes V polarimeter at the German Vacuum-Tower-Telescope in Izaña, Tenerife. We obtained two-dimensional spectral data by scanning the limb oriented sector of the penumbra in steps of 1” with the slit parallel to the solar limb. From Stokes V and Stokes I spectra of four magnetic sensitive lines (Fe I 5247.1, Cr I 5247.6, Fe I 5250.2, and Fe I 5250.7) obtained with 1024 × 1024 pixel CCDs we have determined Doppler shifts and several magnetic parameters. The results of a correlation analysis point out a clear relation between continuum brightness and magnetic field inclination in the sense of steeper field lines in the bright penumbral filaments. There is also a doubtless connection between intensity and Doppler shift. We did find indications for a correlation between field strength and brightness which is not uniform across the penumbra, though.

scholarly journals Absolute velocity measurements in sunspot umbrae

2018 ◽  
Vol 617 ◽  
pp. A19 ◽  
Author(s):  
J. Löhner-Böttcher ◽  
W. Schmidt ◽  
R. Schlichenmaier ◽  
H.-P. Doerr ◽  
T. Steinmetz ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Acoustic Waves ◽  
Frequency Comb ◽  
Convective Motion ◽  
Sunspot Umbra ◽  
Quiet Sun ◽  
Doppler Shifts ◽  
The Magnetic Field

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. We aim to clarify the amount of convective motion in the darkest part of umbrae, by directly measuring 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 I5713.9 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 averaged each sequence to reduce the superimposed wave signal. Compared to convective blueshifts of up to −350 m s−1in the quiet Sun, sunspot umbrae yield 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 I5713.9 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.

scholarly journals On the dependence of magnetic line ratios on time and spatial scales

2008 ◽  
Vol 4 (S259) ◽  
pp. 231-232
Author(s):  
Mikhail L. Demidov
Keyword(s):  
Magnetic Fields ◽  
Spatial Resolution ◽  
Solar Atmosphere ◽  
Solar Disk ◽  
Large Scale ◽  
Spatial Scales ◽  
Spectral Lines ◽  
Magnetic Line ◽  
Made In

AbstractComparison of magnetic fields measurements made in different spectral lines and observatories is an important tool for diagnostics of magnetohydrodynamic conditions in the solar atmosphere. But there is a deficit of information about the dependence of results on detailed position on the solar disk, spatial resolution and time. In this study these issues are discussed in application to the solar large-scale and Sun-as-a-star magnetic fields observations.

scholarly journals The Quiet Sun at cm- and mm-Wavelengths

1980 ◽  
Vol 86 ◽  
pp. 25-39 ◽  
Author(s):  
E. Fürst
Keyword(s):  
Solar Atmosphere ◽  
Solar Radio ◽  
Theoretical Estimate ◽  
Sunspot Cycle ◽  
Active Regions ◽  
Spectral Lines ◽  
Radio Radiation ◽  
Quiet Sun ◽  
Solar Radio Radiation ◽  
Active Centres

Soon after the first detection of radio emission from the sun two components of the solar radio radiation were identified: The emission related to active centres on the disk and the radiation of the undisturbed, static solar atmosphere, in which the active regions are embedded. The undisturbed component is observed to vary only slightly during the solar sunspot cycle, it is called the emission of the quiet sun. A theoretical estimate of this component was first given by Martyn (1946) and subsequently developed in more detail by many other authors. The basic observations were performed with poor angular resolution. Still at present most experimental data are taken with angular resolutions of about 1 to 4 arc min, too low to discriminate between the different solar atmospheric fine structures, clearly seen in various spectral lines. The quiet component of the solar radio radiation therefore represents the average emission of an inhomogenous solar atmosphere.

scholarly journals Ion-atom radiative processes in the solar atmosphere: quiet Sun and sunspots

2014 ◽  
Vol 54 (7) ◽  
pp. 1264-1271 ◽  
Author(s):  
V.A. Srećković ◽  
A.A. Mihajlov ◽  
Lj.M. Ignjatović ◽  
M.S. Dimitrijević
Keyword(s):  
Solar Atmosphere ◽  
Quiet Sun ◽  
Radiative Processes

scholarly journals Chromospheric diagnosis with forward scattering polarization

2014 ◽  
Vol 10 (S305) ◽  
pp. 146-153
Author(s):  
E. S. Carlin
Keyword(s):  
Radiative Transfer ◽  
Solar Disk ◽  
Disk Center ◽  
Forward Scattering ◽  
Dynamic Effects ◽  
Polarity Inversion ◽  
Physical Context ◽  
Hanle Effect ◽  
Definition Of ◽  
Spatial Maps

AbstractIs it physically feasible to perform the chromospheric diagnosis using spatial maps of scattering polarization at the solar disk center? To investigate it we synthesized polarization maps (in 8542 Å) resulting from MHD solar models and NLTE radiative transfer calculations that consider Hanle effect and vertical macroscopic motions. After explaining the physical context of forward scattering and presenting our results, we arrive at the definition of Hanle polarity inversion lines. We show how such features can give support for a clearer chromospheric diagnosis in which the magnetic and dynamic effects in the scattering polarization could be disentangled.

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