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 uninterrupted two-day sequence of Ca II K filtergrams with a 1 nm bandpass. The 60 degrees x 40 degrees held of view contains both quiet and enhanced network and plages. The algorithm uses a threshold-independent method of steepest descent on spatially smoothed and time-averaged images. 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 normalized to a unit mean. It is this invariance that leads us to believe we have determined the intrinsic distribution of cell areas. Extrapolation of the average cell size to zero spatial smoothing yields a characteristic cell diameter of L = 13-18 Mm. This is roughly half the generally quoted supergranular length scale L approximate to 32 Mm as determined with autocorrelation methods. The difference in characteristic cell size reflects the application of a different measurement method: the autocorrelation method as used by Simon & Leighton and others is preferentially weighted towards relatively large cells. We find no significant dependence of cell size on local magnetic flux density.