Solar supergranulation as a wavelike phenomenon

Abstract

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.