Velocity fields of individual supergranules

Abstract

The velocity field of the Ca+ K network is measured at different heliocentric angles using a 100 × 100 photodiode array and a set of slit positions separated by 2.5 arc sec. In order to eliminate the five minute oscillation the integration time was chosen to 10 min. It is found that the correlation between the network cells and the supergranular velocity cells amounts to 85% on the average. Supergranules near the disk centre typically show a central upflow of $$\bar V_{max} \approx 70 m s^{ - 1} $$ corresponding to a massflow of ≈ 1015 m3 s−1. At the boundary of the network a downflow appears to exist only statistically. The maximum velocity of the horizontal component is about 400 m s−1; a layer of one scale height thickness is sufficient to account for the horizontal flow to balance the observed vertical mass flow at cell centre. The majority of the cells show a small scale structure of their horizontal velocity fields. Some indications for the influence of Coriolis forces have been found.