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

TL;DR

This study compares contrast decay in real and simulated solar granulation images, revealing evidence of long-lived features in the solar photosphere's brightness field through contrast variation analysis.

Contribution

It demonstrates that the brightness field of solar granulation contains long-lived components, contrasting with purely random or short-lived simulated features.

Findings

Real granulation contrast decreases more slowly than simulations.

Long-lived components may be linked to persistent intergranular structures.

Contrast variation laws differ significantly between real and simulated data.

Abstract

The decrease in the rms contrast of time-averaged images with the averaging time is compared between four datasets: (1) a series of solar granulation images recorded at La Palma in 1993; (2) a series of artificial granulation images obtained in numerical simulations by Rieutord et al. (2002); (3) a similar 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 obtained from 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 of n is analysed. The series (1) of real granulation images exhibits a considerably slower contrast decrease than do both the series (3) of simulated granulation images and the series (4) of random fields. Starting from some relatively 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 slow decline of contrast, which could be attributed to particular properties of the boundary conditions used in the simulations. Comparisons between the analysed contrast-variation laws indicate quite definitely that the brightness field of solar granulation contains a long-lived component, which could be associated with locally persistent dark intergranular holes and/or with the presence of quasi-regular structures. The suggestion that the random field (4) successfully reproduces the contrast-variation law for the real granulation (Rast, 2002) can be declined.