Study of supergranules

Results of a detailed study on supergranule lifetime and velocity fields are presented. The correlation between the observed downdraft velocity and the network magnetic flux elements on the quiet sun is shown. After excluding areas with magnetic flux density 25 G or greater, it is found that the upper limit of the supergranule vertical speed is 0.1 km/s for both downdraft and updraft, and the rms speed is 0.03 km/s. By observing the evolution of individual supergranules, it is found that the average lifetime of supergranules might be 50 hours or greater. Different ways of formation and decay of supergranular cells are described. New cells usually form in an area containing no preexisting supergranule velocity fields. Cells may disappear in two ways: fragmentation and fading away.