HAO Colloquium - David Hathaway, Stanford University

Hydrodynamic Properties of the Sun’s Giant Cell Flows

The existence of giant solar convection cells has been suggested by both theory and observation for over 50 years. Previously, Hathaway, Upton, & Colegrove (2013) tracked the motions of supergranules in SDO/HMI Doppler velocity images and found large (giant cell) flows that persisted for months and had many dynamical properties expected from the effects of the Sun’s rotation.

Several substantial improvements have been made to the Local Correlation Tracking method used in that study. These improvements have resulted in substantial improvements in the giant cell flow measurements. The improved flow maps (generated for the full Sun at daily intervals) indicate two different flow regimes – a low latitude regime within about 30 to 40 degrees of the equator and a high latitude regime. The tangent cylinder associated with the demarcation between these regimes passes near the base of the Sun’s convection zone. The flows in both regimes are clockwise around centers of divergence in the north and counter-clockwise in the south.

In the high latitude regime, equatorward flows are correlated with prograde flows - giving an equatorward transport of angular momentum toward the equator like that needed to maintain the Sun’s rapid equatorial rotation. This momentum flux is not evident in the low latitude regime. The high latitude features exhibit differential rotation and poleward meridional flow characteristic of the near surface layers. The low latitude cells exhibit a nearly rigid rotation profile at a slower rate than the Carrington rate – slower than that found at any depth within the equatorial convection zone. A spherical harmonic + time analysis of the vorticity pattern produced by these flows indicates interesting wave-like characteristics of the cells in the low latitude regime.

Date and time: 
Wednesday, January 23, 2019 - 2:00pm to 3:00pm