Accordingly, the downflow in the intergranular lanes causes a redshift toward longer wavelengths. This negative Doppler shift is called a blueshift, by convention. Observing the spatially resolved granulation at the disk center, the upflowing granular plasma shifts spectral lines to shorter wavelengths. The plasma distributes horizontally toward the intergranular lanes and finally sinks back into the convection zone. The upflowing hot gas within the bright granular cell rises up into the photosphere in which it cools down with increasing atmospheric height. The well-known pattern of solar granulation is caused by convective overshoot from the convection zone reaching into the visible photosphere. Key words: convection / Sun: atmosphere / Sun: activity / methods: observational / techniques: spectroscopic / line: profiles LARS measurements can be taken as a reference for the correction of systematic errors in the synoptic HMI Dopplergrams. The center-to-limb variation of HMI suffers from an additional blueshift for μ < 0.9 that is incompatible with our results. The accurate observation of the absolute convective blueshift with LARS allows the identification of systematic discrepancy with Doppler velocities measured by HMI. Thirdly, the line-of-sight effect of solar activity, including p-mode oscillations and supergranular flows, leads to a scatter of up to ☑00 m s −1 at intermediate heliocentric positions.Ĭonclusions. Secondly, we obtained a significant decrease of convective blueshift toward the solar limb. 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). The analysis of the time- and bisector-averaged line shifts yields three distinct results. When approaching the solar limb, the bisector transforms into a “\”-shape. Being a signature for convective motions, the bisector curve of Fe I 6173.3 Å describes a “C”-shape at disk center. We applied a bisector analysis on the spectral lines to reveal the changes of convective blueshift and line asymmetry at different heliocentric positions. 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. The solar disk was scanned along the meridian and the equator, from the disk center toward the limb. We performed systematic observations of the quiet Sun with the Laser Absolute Reference Spectrograph (LARS) at the German Vacuum Tower Telescope. 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. This Doppler shift to shorter wavelengths is commonly known as convective blueshift.Īims. Granular convective motions reach into the lower solar atmosphere, typically causing photospheric spectral lines to exhibit a differential line shift. Menlo Systems GmbH, Am Klopferspitz 19, 82152 Martinsried, GermanyĬontext. 6, 79104 Freiburg, GermanyĮ-mail: für Quantenoptik, Hans-Kopfermann-Strasse 1, 85748 Garching, Germany Kiepenheuer-Institut für Sonnenphysik, Schöneckstr.
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