Supergranulation Velocity Field from the MDI (SOHO) Data
  Long (up to 100 hours) time series of SOHO/MDI Doppler data are analyzed. The power spectrum of radial velocity time series in the quiet photosphere is observed to have, along with the known 5-min mode, a stable strong mode with a period of about 32 h, which is close to the supergranulation cell lifetime. The spatial distribution of the amplitudes of these oscillations also coincides with the characteristic size scale of supergranulation (~35 Mm). 
L. D. Parfinenko, V. I. Efremov, and A. A. Solovev
astro-ph.SR