THE DISTRIBUTION OF CELL SIZES OF THE SOLAR CHROMOSPHERIC NETWORK

This paper studies the cellular pattern of the supergranular network. We present an algorithm to draw a surface-filling cell pattern on an u ninterrupted two-day sequence of Ca II K filtergrams with a 1 nm bandp ass. The 60 degrees x 40 degrees held of view contains both quiet and enhanced network and plages. The algorithm uses a threshold-independen t method of steepest descent on spatially smoothed and time-averaged i mages. We determine the distribution function of cell areas and find a broad, asymmetric spectrum of areas. The distribution is found to be invariant for different spatial smoothings if the cell areas are norma lized to a unit mean. It is this invariance that leads us to believe w e have determined the intrinsic distribution of cell areas. Extrapolat ion of the average cell size to zero spatial smoothing yields a charac teristic cell diameter of L = 13-18 Mm. This is roughly half the gener ally quoted supergranular length scale L approximate to 32 Mm as deter mined with autocorrelation methods. The difference in characteristic c ell size reflects the application of a different measurement method: t he autocorrelation method as used by Simon & Leighton and others is pr eferentially weighted towards relatively large cells. We find no signi ficant dependence of cell size on local magnetic flux density.