Widespread Occurrence of Trenching Patterns in the Granulation Field: Evidence for Roll Convection?

Abstract

Time-averaged series of granulation images are analysed using COLIBRI, a purpose-adapted version of a code originally developed to detect straight or curvilinear features in aerospace images. The image-processing algorithm utilises a nonparametric statistical criterion that identifies a straight-line segment as a linear feature (lineament) if the photospheric brightness at a certain distance from this line on both sides is stochastically lower or higher than at the line itself. Curvilinear features can be detected as chains of lineaments, using a modified criterion. Once the input parameters used by the algorithm are properly adjusted, the algorithm highlights “ridges” and “trenches” in the relief of the brightness field, drawing white and dark lanes. The most remarkable property of the trenching patterns is a nearly universally present parallelism of ridges and trenches. Since the material upflows are brighter than the downflows, the alternating, parallel light and dark lanes should reflect the presence of roll convection in the subphotospheric layers. If the numerous images processed by us are representative, the patterns revealed suggest a widespread occurrence of roll convection in the outer solar convection zone. In particular, the roll systems could form the fine structure of larger scale, supergranular and/or mesogranular convection flows. Granules appear to be overheated blobs of material that could develop into convection rolls owing to instabilities of roll motion.