Inversions for Average Supergranular Flows Using Finite-frequency Kernels

I analyse the maps recording the travel-time shifts caused by averaged plasma anomalies under an "average supergranule", constructed by means of statistical averaging over 5582 individual supergranules with large divergence signals detected in two months of HMI Dopplergrams. By utilising a three-dimensional validated time-distance inversion code, I measure the peak vertical velocity of 117+/-2 m/s in depths around 1.2 Mm in the centre of the supergranule and root-mean-square vertical velocity of 21 m/s over the area of the supergranule. A discrepancy between this measurement and the measured surface vertical velocity (a few m/s) can be explained by the existence of the large-amplitude vertical flow under the surface of supergranules with large divergence signals, recently suggested by Duvall & Hanasoge (2012).