Speckle measurements of the centre-to-limb variation of the solar granulation

The Vacuum Tower Telescope (VTT) at the Observatorio del Teide/Tenerife was used to perform speckle observations of photospheric granulation from disc centre to limb at λ = 550 ± 5 nm. Images were reconstructed with the spectral ratio technique (von der Lühe 1984) and the speckle masking method (Weigelt 1977, Weigelt & Wirnitzer 1983, Lohmann et al. 1983). The granular rms contrast relative to the local average intensity decreases monotonically from 13.5±1.0% at disc centre to 8–9 % at cosθ = 0.1. The granular images as well as the power spectra of the intensity fluctuations show a fading of the coarse granular pattern towards the limb while smaller structures become more pronounced off the disc centre. We identify these as the bright worm-like structures appearing sometimes at the granular borders (de Boer et al. 1992). This finding supports the result from simulations of granular dynamics by Steffen et al. (1994). There the bright structures are locations of low pressure between the centres of granules and intergranular spaces with penetration of hot gas into the lower photosphere.