The Vertical Component of the Supergranular Motion

Abstract

Supergranules are observed at the solar photosphere as a cellular horizontal flow pattern with flow diverging from cell centers and converging on cell boundaries. Clark & Johnson calculated that mass conservation leads to an expected vertical flow of only 10 m s^-1, which has been difficult to observe. In the present work, Doppler images near the disk center from Michelson Doppler Imager are averaged about locations of cell centers to obtain the necessary signal-to-noise ratio to see the vertical flow. It is found that, for an average over 1100 cell centers, there is a 10 m s^-1 upflow at cell center and a 5 m s^-1 downflow at the cell boundaries, confirming the previous estimate. The rms vertical flow is 4 m s^-1, smaller than Giovanelli's upper limit of 10 m s^-1.