Motions in the Sun at the Photospheric Level: VII. Vertical Distribution of the Equatorial Velocity Field

Abstract

In an attempt to determine the vertical distribution of the velocity field from its centre-limb variation, 274 line-of-sight velocities obtained in an earlier paper (from Fe line Doppler displacements) are analysed. They are divided into 3 groups, whose successive positions are separated by 11°·0 of heliographic longitude, and a relation is sought between the velocity dispersion displayed by a group and its position on the disk. No definitive result is obtained. A positive result is, however, derived from a comparison of velocity measurements on strong (the two Na D) lines and on weak (one Ni and one Ti) lines having, respectively, average Rowland intensities 25 and 2·5 The Doppler shifts are found from microphotometer tracings at 40 positions on each of 4 of the spectra earlier used for the Fe line measurements. At each of the 40 positions the four measurements are combined, thereby reducing the error in the velocity at any point to |$\pm0\cdot 05(8)\pm0\cdot 03(7)\,\text{km}\,\text{sec}^{-1}$| for the Ni–Ti sequence and to |$\pm0\cdot 06(0)\pm0\cdot 03(8)\,\text{km}\,\text{sec}^{-1}$| for the Na sequence. After correction for effects of measurement error, the mean velocity amplitude is |$\pm0\cdot 16(2)\,\text{km}\,\text{sec}^{-1}$| for the Ni–Ti sequence, compared with |$\pm0\cdot 12(0)\,\text{km}\,\text{sec}^{-1}$| for the Na. A statistical discussion shows that the velocity sequences are similar, but that the difference in mean amplitude is significant. From a simple model of line formation and taking account of redistribution of light by the apparatus function of the spectroscope, the weak and strong line velocities are shown to refer to approximate optical depths 0·42 and 0·32, respectively, as measured in the continuum. The amplitude of the velocity field, therefore, diminishes extremely rapidly with increasing height in the photosphere.