Solar granulation properties derived from three different time series

Abstract

We need automatic pattern recognition algorithms to extract a large statistical sample of granules from high spatial and temporal resolution images series of photospheric fields. In this paper, we present the new Two-level Structure Tracking (TST) algorithm, based on a two-level representation of granulation, which allows us to monitor the characteristics of identified photospheric structures during their duration. TST is also able to retrieve horizontal velocity maps from measured granule displacements. Direct comparison of the results from different works describing granular evolution is often not possible as discrepancies stemming from the use of different procedures or from different data cannot be distinguished. Here, three different solar granulation broadband time series, acquired at THEMIS in July 1999, at NSO-DST in October 1996, and at SVST in June 1995, with different spatial and temporal resolutions, are coherently analyzed via the same TST procedure, allowing direct comparison of the results. Among the obtained results, we confirm the dynamical heterogeneity of photospheric small scale structures pointed out by granular lifetime histograms, characterized by a stretched exponential function. Furthermore, by monitoring the breaking-up of a granule and the movements of its neighbours and of its fragments, we found evidence of how an exploding granule may produce a divergence signal on spatial and temporal mesogranular scales.