Sergio Javier González Manrique and Christoph Kuckein

We compare two methods to compute the Doppler shifts and infer the line-of-sight (LOS) velocities of the largely used photospheric Si I 10827 Å line. This line yields information about the upper photosphere. The first method consisted of computing the height-depend bisectors of the line. The second method required much longer computational time since an inversion code was used. For this purpose we used the Stokes inversion based on response function (SIR) code, which provided height-dependent information, in an optical depth scale, of the LOS velocities. The used data set of this study was observed on 2015 April 17 with the very fast spectroscopic mode of the GREGOR Infrared Spectrograph (GRIS). Small pores and large quiet Sun areas were within the field of view. Two different data sets at different times were exploited for this study. The wavelength positions of the bisectors were computed in 10%-steps of the line depth using linear interpolation in both line wings. The output model from the SIR code covered 55 optical depth positions which range between 1.4 ≥ log τ ≥ -4.0. These LOS velocity maps from SIR at different log τ were compared with those originated from the bisector method at different percentages. The comparison between both methods allowed us to associate the bisector percentages to a specific optical depth. We associated nine LOS velocity bisector maps, in 10% steps of the line depth, starting from the lowest value at 10% and ending at the highest value of 80%. The bisector and SIR velocity maps were compared using the Pearson linear correlation coefficient. Each of the LOS velocity bisector maps was compared and correlated with the SIR LOS velocity maps. High linear correlations in the range of 95-98% were found. Bisector velocities obtained deeper in the line correspond to lower optical depths. The best correlation (97.8%) was achieved for the SIR velocity map at log τ = -2.6, which corresponds to a bisector of 20%. The inferred correspondence between bisector percentage and optical depth can be used to quickly obtain information about the LOS velocity stratification versus optical depth.