Properties of the Sun’s Giant Cellular Flows and their Implications for the Sun’s Activity Cycle

Abstract:

Measurements of the large cellular flows on the Sun were made by correlation tracking of supergranules seen in Dopplergrams obtained by HMI on the NASA SDO satellite. Measurements were averaged over 34 days to produce daily maps of the vector velocities. Flows at all latitudes are in the form of vortices with helicity left-handed in the north and right-handed in the south. There are key distinctions between the low latitude and high latitudes cells. The high latitude cells have long extensions that spiral inward toward the poles, lifetimes of several months, differential rotation, poleward drift at speeds approaching 2 m s−1, and a strong equatorward momentum flux. The low latitude cells have roughly circular shapes, lifetimes of about one month, rotate rigidly, do not drift in latitude, and do not exhibit any momentum flux. Spherical harmonic analysis confirms that the flows are dominated by the toroidal flow component with RMS velocities of about 12 m s−1 at wavenumber ℓ = 10 (similar in strength to that of the meridional flow near the surface). Fourier transforms in time also indicate two components - an m=±ℓ feature representing the low latitude component and an m=±1 feature representing the high latitude component. The dispersion relation for the low latitude component is well represented by that derived for Rossby waves. The high latitude component has a constant temporal frequency for all ℓ indicating advection by differential rotation at rates representative of the base of the convection zone. The poleward motions of these features indicate that the meridional flow at the base of the convection zone is poleward. These giant cellular flows facilitate surface flux transport by transporting flux elements along their boundaries. The evidence for poleward meridional flow at the base of the convection zone conforms with other measurements of the Sun’s internal meridional flow but denies a key component (equatorward flow at the base of the convection zone) of flux transport dynamos.