Dynamics of solar mesogranulation

¹ Institut für Physik, IGAM, Karl-Franzens Universität Graz, Universitätsplatz 5, 8010 Graz, Austria e-mail: martin.leitzinger@aon.at
² Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany

Received: 29 March 2005
Accepted: 3 June 2005

Abstract

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.