Absolute convective velocities in the solar atmosphere

The energy from the solar interior is transported by convection to the solar surface. When observing the lowest layer of the solar atmosphere with a high-resolution solar telescope the granular structure of the hot up-flowing gas cells becomes recognizable. Recently, scientists from the Kiepenheuer Institut for Solar Physics have performed unprecedented spectroscopic observations with the Vacuum Tower Telescope (VTT) on Tenerife to systematically investigate the large-scale convective motion in the photosphere. The measurement of accurate absolute velocities was enabled by the scientific instrument LARS (Laser Absolute Reference Spectrograph) which employs a state-of-the-art Laser Frequency Comb as a calibration ruler for the solar spectrum.

Center-to-limb variation of the convective blue-shift (of FeI 617.3nm) observed with LARS and HMI. The single measurements describe the Doppler velocities averaged for each 20-min LARS-observation starting from disc center (µ=1.0) to the solar limb (µ→0) along the four axes. The error bars represent the standard deviation. The black thick line displays the mean curve. For comparison, the average evolution along the north-south-axis in HMI-Dopplergrams is shown in turquoise. In the upper left of the figure: the logo of LARS.

To determine the velocities in the photosphere the neutral iron line at 617.3nm was selected for the study. Based on the shape and shift of the spectral line, the height-dependent velocity evolution was analyzed by means of a bisector analysis. At disk center, the upward velocity (blue-shift) in the lower photosphere was still 300m/s. Roughly 200km higher up, in the middle photosphere, the velocities reach only 150m/s.

To qualitatively investigate the so-called "convective blue-shift", its change from the solar disk center toward the limb (center-to-limb variation) was measured systematically. Ten positions on the solar disk were selected at heliocentric parameters µ=cos(θ) between 1.0 (disk center) and 0.3 (near the solar limb) for all four axes (north, south, east, west). Employing a Laser Frequency Comb, the LARS-measurements were calibrated to absolute velocities reaching an accuracy down to 1m/s.

The results of the single observations are displayed as measurement points in the alongside figure. Each point represents the barycenter of Doppler velocities in the lower 80% of the spectral profile. The results at disk center lie so close together that only their average is plotted. The black solid line shows the average trend. Despite the obvious scatter which is caused by the solar activity (essentially by the solar p-modes and supergranulation), a trend is clearly recognizable. Between µ=1.0 and µ=0.6, the convective velocity is basically stable between -200m/s und -300m/s (the negative sign indicates a blue-shift). Toward the solar limb (smaller µ), the velocity decreases to 0m/s. However slightly shifted, this trend is confirmed by the analysis of HMI Dopplermaps. The reason for the decrease in velocity toward the limb is the observation of a higher atmospheric layer in which the vertical convective motion has decreased to a minimum.

Further information in the Bachelor thesis of Franziska Stief (PDF, german).