STUDIES OF GRANULAR VELOCITIES .1. GRANULAR DOPPLER SHIFTS AND CONVECTIVE MOTION

The two available methods for determining the rms amplitude of the granular convective velocity field, namely the interpretation of line profiles, and direct measurements of velocity fluctuations in highly resolved spectra, give values (∼ 2 km/sec, and ∼ 0.4 km/sec, resp.) which are apparently inconsistent both in magnitude and in their dependence upon optical depth. We give both theoretical and observational evidence for the working hypothesis, that the best resolved spectra mainly show velocity fluctuations due to the oscillation of the solar atmosphere, whereas the contribution of the granular velocity field is greatly reduced because of atmospheric seeing and can be found only as a weak superposition to the oscillatory velocity field. Realistic assumptions for the typical size of the granulation (2″.5) and for the 'seeing parameter' (1″), together with a simplified model of the granular velocity field, lead to correction factors of 30 to 40 between the true and observed amplitudes of the granular velocities. © 1969 D. Reidel Publishing Company.