Helioseismic Imaging of Supergranulation Throughout the Sun’s Near-Surface Shear Layer

Abstract

We present measurements of the Sun's sub-surface convective flows and provide evidence that the pattern of supergranulation is driven at the surface. The pattern subsequently descends slowly throughout the near-surface shear layer in a manner that is inconsistent with a 3D cellular structure. The flow measurements are obtained through the application of a new helioseismic technique based on traditional ring analysis. We measure the flow field over the course of eleven days and perform a correlation analysis between all possible pairs of depths and temporal separations. In congruence with previous studies, we find that the supergranulation pattern remains coherent at the surface for slightly less than two days and the instantaneous surface pattern is imprinted to a depth of 7 Mm. However, these correlation times and depths are deceptive. When we admit a potential time lag in the correlation, we find that peak correlation in the convective flows descends at a rate of 10-40 m s −1 (or equivalently 1-3 Mm per day). Furthermore, the correlation extends throughout all depths of the near-surface shear layer. This pattern-propagation rate is well matched by estimates of the speed of downflows obtained through the anelastic approximation. Direct integration of the measured speed indicates that the supergranulation pattern that first appears at the surface eventually reaches the bottom of the near-surface shear layer a month later. Thus, the downflows have a Rossby radius of deformation equal to the depth of the shear layer and we suggest that this equality may not be coincidental.

Gravity darkening in late-type stars

“…The absence of stronger latitudinal variations of the mean solar photospheric intensity could then be explained by the fact that convective flows are probably not rotationally constrained anymore in the nearsurface shear layer that spans the outermost 35 Mm of the Sun (Greer et al 2016a,b). Greer et al (2016a) suggest weak rotational constraint in the outer layers above r ≈ 0.96r o , while we find for the thin shell model displayed in Fig. 2d that the transition Ro = 1 occurs at r ≈ 0.9r o -a value which is slightly lower than the one predicted from observations, but we may have deeper transitions in numerical models given the much lower density stratification of the convective zone.…”

Evolution and wave-like properties of the average solar supergranule

“…This is consistent with the findings by Shine et al (2000) who observed several individual supergranules forming in LCT divergence maps obtained from Michelson Doppler Imager (Scherrer et al 1995) intensity images. The sign reversal of the horizontal divergence after about two days was also observed by Greer et al (2016) using helioseismic ring-diagram analysis (see Figs. 7 and 8a therein).…”

The Sun’s supergranulation

“…Typically, vertical variations can be evaluated down to 10-15 Mm below the surface, but the accuracy of measurements deeper than 10 Mm is still debated. A comparison between the tracking and helioseismic reconstructions of large-scale solar surface flows, showing good agreement between the two, can be found for instance inŠvanda et al (2007) and Greer et al (2016).…”