Detection of Zonal Shear Flows beneath the Sun's Surface from [CLC][ITAL]f[/ITAL][/CLC]-Mode Frequency Splitting

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Abstract
We report on the first successful measure of the zonal variations of the Sun's differential rotation (so-called torsional oscillations) by helioseismology. Using new helioseismic data from the Michelson Doppler Imager on board SOHO, we have detected zonal flow bands with velocity variation of 5 m s-1 at a depth of 2-9 Mm beneath the surface. The subsurface flow is inferred from rotational splitting of frequencies of the fundamental mode of solar oscillations in the range of angular degree l from 120 to 250, using a 144 day uninterrupted time series of the Sun's Doppler velocities. The structure of the subsurface shear flow resembles the pattern of the torsional oscillations observed on the surface. Comparing with previous surface measurements, we found evidence for migration of the flow bands towards the equator.