Dynamics of solar mesogranulation
Institut für Physik, IGAM, Karl-Franzens Universität Graz, Universitätsplatz 5, 8010 Graz, Austria e-mail: email@example.com
2 Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany
Accepted: 3 June 2005
Using a 45.5-h time series of photospheric flow fields generated from a set of high-resolution continuum images (SOHO/MDI) we analyze the dynamics of solar mesogranule features. The series was prepared applying a local correlation tracking algorithm with a 4.8´´ FWHM window. By computing 1-h running means in time steps of 10 min we generate 267 averaged divergence maps that are segmented to obtain binary maps. A tracking algorithm determines lifetimes and barycenter coordinates of regions of positive divergence defined as mesogranules (MGs). If we analyze features of lifetimes ≥1 h and of areas ≥5 Mm we find a mean drift velocity of 304 m s-1 (with ± variation of 180 m s-1), a mean travel distance of Mm, a mean lifetime of h, and a decay time of 1.6 h for a total of 2022 MGs. The advective motion of MGs within supergranules is seen for 50 to 70% of the long-lived (≥4 h) MGs while the short-lived ones move irregularly. If only the long-lived MGs are further analyzed the drift velocities reduce to 207 m s-1 and the travel distances increase to 4.1 Mm on average, which is an appreciable fraction of the supergranular radius. The results are largely independent of the divergence segmentation level.
Key words: convection / Sun: granulation / Sun: photosphere
© ESO, 2005