MOTIONS OF THE PHOTOSPHERE
In 1960 it was discovered that there was a vertical oscillatory motion in and above the granulation. It has an average period of almost exactly 5 minutes and velocities of about 0.5 kilometer per second. Thus the layers above the convection zone are moving up and down with respect to the mean position of the photosphere
and low chromosphere. The typical excursion is on the order of 50 to 100 kilometers. The motion seems to be organized over a few thousand kilometers, and has been reported to cover areas as large as 50,000 kilometers, with roughly two-thirds of the solar surface experiencing oscillations at any given moment. It appears that the 5-minute oscillation may be one extreme in a whole range of solar pulsation modes, with a 160-minute vibration of the whole sun as the other extreme.
Coexisting with the solar granules and the 5-minute oscillations of the solar photosphere is a completely different type of motion detected by Doppler studies of the full disk of the sun. These motions, shown in Figure 11.7, are called supergranulation cells because of their resemblance to convective motions and the fact that they are typically an order of magnitude larger than granules (about 30,000 kilometers in diameter). The supergranules show no pattern of bright centers and dark boundaries because the temperature differences are apparently not great enough. Supergranules have a lifetime of about 1 day compared to the several-minute
lifetimes of the granules. At what depth in the sun the supergranulation system begins is unclear. Beyond the name the granules and supergranules may have little in common.
Some solar astronomers believe that an even larger pattern of giant convective cells exists with sizes equal to a substantial fraction of the solar radius. The observational evidence for these giant cells is still weak, and the search continues.
