HELIOSEISMIC MEASUREMENTS OF THE ROSSBY NUMBER IN THE SUN'S NEAR-SURFACE SHEAR LAYER

Through helioseismic measurement of the Sun's subsurface flows, we assess the Rossby number associated with the convective motions. The helioseismic procedure is a new form of the ring-analysis technique that is capable of resolving supergranulation. The extremely fine spatial resolution is achieved by deconvolving the set of p-mode Doppler shifts measured separately within a multitude of densely overlapping analysis regions. We find that just below the photosphere, the Rossby number is large (Ro approximate to 5) and the convective flows are only weakly influenced by rotation. Below a depth of 10 Mm, the flows become rotationally constrained and the concomitant Rossby number is small (Ro approximate to 0.2). These results support the supposition that the near-surface shear layer is a transition region for the degree of rotational impact on convective motions.