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The decrease in the rms contrast of time-averaged images with the averagingtime is compared between four datasets: (1) a series of solar granulationimages recorded at La Palma in 1993; (2) a series of artificial granulationimages obtained in numerical simulations by Rieutord et al. (2002); (3) asimilar series computed by Steffen and his colleagues (see Wedemeyer et al.,2004}); (4) a random field with some parameters typical of the granulation,constructed by Rast (2002). In addition, (5) a sequence of images was obtainedfrom real granulation images using a temporal and spatial shuffling procedure,and the contrast of the average of n images from this sequence as a function ofn is analysed. The series (1) of real granulation images exhibits aconsiderably slower contrast decrease than do both the series (3) of simulatedgranulation images and the series (4) of random fields. Starting from somerelatively short averaging times t, the behaviour of the contrast in series (3)and (4) resembles the t^{-1/2} statistical law, while the shuffled series (5)obeys the n^{-1/2} law from n = 2 on. Series (2) demonstrates a peculiarly slowdecline of contrast, which could be attributed to particular properties of theboundary conditions used in the simulations. Comparisons between the analysedcontrast-variation laws indicate quite definitely that the brightness field ofsolar granulation contains a long-lived component, which could be associatedwith locally persistent dark intergranular holes and/or with the presence ofquasi-regular structures. The suggestion that the random field (4) successfullyreproduces the contrast-variation law for the real granulation (Rast, 2002) canbe declined.

Do Long-Lived Features Really Exist in the Solar Photosphere? II. Contrast of Time-Averaged Granulation Images