Supergranulation supports waves

Supergranulation on the surface of the Sun is a pattern of horizontal outflows with a distinct scale of 30 Mm and an apparent lifetime of 1 day, outlined by a network of small magnetic features. The dynamics of the supergranulation is poorly understood and there is as yet no explanation for the observation that the supergranular pattern appears to rotate faster than the magnetic features. In this paper we show that supergranulation undergoes oscillations and supports waves with periods of 6-9 days. The nature of supergranulation appears to be travelling-wave convection. Waves are predominantly prograde, which explains the apparent superrotation of the pattern. We also show that supergranular flows have a net kinetic helicity, which is negative in the northern hemisphere.