On the Geometrical Properties of the Chromospheric Network

A sequence of Ca K images obtained in a period of minimum solar activity , from July to November 1996, at the Rome Observatory with the PSPT (Precision Solar Photometric Telescope) prototype instrument have been used to analyze the geometrical properties of cells identiied by the chromospheric network. In particular , we used 256 256 sub-arrays of the calibrated full disk PSPT images. These sub-arrays, centered on the solar disk, are reduced to two-levels (binary) images by means of a suitable threshold after an FFT highpass ltering. A medial axis transform , better known as skeleton, combined to a cellular automaton, is applied to the two-levels images, in order to derive the cell boundaries. The regions corresponding to the cells are then lled by a growing algorithm. In this way we can derive a set of output parameters describing the cells geometry. The size distribution of the iden-tiied cells shows a continuos increase toward the smaller scales, rather than a small dispersion around a characteristic scale. Nevertheless the analysis of the intercell distances and of the area distribution pointed out a characteristic scale (square root of the area) of 24 Mm. To describe the cells irregularity and to probe the nature of solar turbulence we apply a Mandelbrot fractal analysis to such irregularly shaped features. Examining the cells perimeter-area relationship we found the existence of a `critical' area at which a change in the geometrical properties occurs. This area corresponds to the scale of 24 Mm. The estimated fractal dimension for cells with area greater than thècritical' is 1.36. This value, close to that predicted for isobars in the Kolmogorov 3-D turbulent theory, does not exclude a turbulent origin for such cells. The analysis seems to point out a common origin for solar granulation and supergranulation.