Application of coherent structure tracking to solar Doppler maps to determine horizontal velocity fields at the sun's surface

Abstract

Context. Coherent structure tracking (CST) is a technique for determining solar surface horizontal flows at high spatial and temporal resolution by tracking the proper motion of granules. This technique has traditionally been applied to solar intensity images in the continuum, which clearly depict granular patterns. However, solar granulation is also visible in Dopplergrams. Aims. We show that CST can be applied to solar Dopplergrams to derive solar surface horizontal velocity fields with the same level of confidence as those determined by CST on intensity images. Methods. We applied CST to continuum intensity images and Dopplergrams obtained from SDO/HMI and from a numerical simulation of granulation. We compared the resulting solar surface horizontal velocity fields and their derivatives (namely, the horizontal divergence and the vertical component of the vorticity) for different time windows. Results. Pearson's linear global correlation coefficient (GCC) between horizontal velocity fields determined from CST on Doppler and on intensity images of a relatively less active Sun is about 73% for a 30-minute time average, while the corresponding local correlation coefficient (LCC) near the disk center is about 80%. For the divergence of the horizontal velocity field, we obtain a GCC of 72% and a near-disk-center LCC of 84%. The curl of the horizontal velocity field is noisier and exhibits somewhat reduced GCC and LCC. These coefficients increase with an increasing time window. We observe a similar trend for Spearman's and Kendall's rank-order correlation coefficients, although their values are somewhat smaller. The different correlation coefficients slightly decrease for a magnetically more active Sun with sunspots or emerging pores in plage regions. We obtain a high correlation between the horizontal flows derived by applying CST to intensity and vertical velocity maps from a numerical simulation.