On the brightness and velocity structure of solar granulation

Institut für Geophysik, Astrophysik und Meteorologie, Universitätsplatz 5, 8010 Graz, Austria

Corresponding author: jkh@igam.uni-graz.at

Received: 22 April 2002
Accepted: 14 June 2002

Abstract

A 45 min time series of two-dimensional spectra has been obtained with the Vacuum Tower Telescope at the Observatorio del Teide in Izaña, Tenerife. Scans over the non-magnetic  Å line of a quiet granular field at disk center were taken simultaneously with a time series of broad band images. From the spectra intensity and velocity maps have been calculated at different line-depths. From the white light images granular shapes have been computed by means of an automatic image segmentation algorithm. A statistical analysis of the intensity and velocity distribution in the detected granular shapes has been carried out. Intensities and velocities are well correlated at low photospheric levels. In the higher photosphere the intensity pattern dissolves whereas the velocities show almost no variation within the probed height interval. The intensity excess of small granules dissolves at lower heights than that of larger ones. The intensity and velocity distribution within the granules depends on the granular size. In smaller structures the maximum intensities and velocities are located close to the granular barycenters whereas for larger granules the maxima are shifted towards the granular boundaries. The width of the transition zone between granules and intergranular lanes is independent on the granular intensity and is constant at approximately . The time evolution of the granular pattern shows a clear dependence of the lifetime of structures on the spatial wavenumber. The e-folding times of the temporal coherences decrease according to a power law with an exponent of which is incompatible with the Kolmogorov energy spectrum of homogeneous and isotropic turbulence and might be taken as a hint against the overall turbulent character of granular motions.