TURBULENT POWER SPECTRA OF SOLAR GRANULATION

High resolution ''Multichannel Subtractive Double Pass'' spectrograms and white light photographs have been used to compute power spectra of the solar granulation. The 5 min oscillations have been carefully filtered, and the influence of the variation of the mean size of granules at the mesogranulation scale has been taken into account. It results that both the power spectra of velocity and intensity fluctuations, displayed in a log P - log k scale, are characteristic of a turbulent atmosphere: they present a discontinuity at 3 '' = 2000 km (which corresponds to the size of a large granules) and an energy which decreases according to the Kolmogorov - 5/3 power law in the granulation range; another discontinuity, at 1.4'' = 1000 km (which corresponds approximately to a Peclet number Pe = 1, for which thermal diffusion and advection are of equal importance) is present in the intensity power spectrum, which decreases with a - 17/3 power law, in the range of sizes smaller than 1.4''. These results indicate that granules are turbulent eddies. They have some convective characters (like the correlation between velocities and brightness) because they are formed in a strongly superadiabatic atmosphere.