  Supergranulation on the surface of the Sun is an organized cellular flow
pattern with a characteristic scale of 30 Mm. It is superficially similar to
the well understood granulation that operates at the 1.5 Mm natural scale of
convection, which has led to the conventional view that supergranulation has
its origin in the convective motion of cells of gas, though this does not
explain the observation that the supergranulation pattern appears to move
faster around the Sun than the bulk of its surface. A wave origin has been
proposed for supergranulation that may explain the superrotation, but it has
never had much support. Here we report that the supergranulation pattern has
oscillatory components with periods of 5-10 days, for which the best
explanation is a spectrum of traveling waves. We show that there is excess
power in the prograde and equatorward directions, which explains the
observation of superrotation.
