THE HEIGHT DEPENDENCE OF INTENSITY AND VELOCITY STRUCTURES IN THE SOLAR PHOTOSPHERE

Results about a statistical analysis of the solar granulation, obtained by analyzing a series of narrow band (20 mangstrom FWHM) images in the 6162.18 angstrom Ca I photospheric line, are presented. The observations have been performed at the Vacuum Solar Tower of the National Solar Observatory at Sac. Peak (NM-USA) in 1988, using a Fabry-Perot interferometer and a Universal Birefringent Filter mounted in tandem. We computed coherence, phase and power spectra of intensity and velocity fields in a 27'' x 27'' quiet region at the disk center. Energy spectra, plotted in the usual log-log coordinates, clearly show a linear shape for wavenumbers between 3 and 10 Mm-1 The exponent is - 17/3: it does not significatively vary within the considered photospheric layers and largely differs from both the theoretical value and the results of previous 1-D observations. This result indicates that in the photosphere we are in presence of a redistribution of the convective energy through a cascade from larger granules to smaller ones, although the size distribution does not follow the Kolmogorov law. The physical processes involved in the granulation have been investigated by studying the height dependence of coherence and phase spectra of Velocity-Velocity (V-V) and Velocity-Intensity (V-I) fields. We find that the photosphere is divided in two regions: the velocity structures existing in the lower layers (first region) are convective and extend up to about 170 km. The decay of these granular motions generates well correlated velocity structures in the second region (height range 170-400 km), at spatial frequencies 5-10 Mm-1. In this region, moreover, the coherence moderately increases with height, while the phase is stable around +/- 180-degrees. This means that velocity and intensity fields are predominantly anticorrelated, as expected for gravity waves.