Ring Diagram Analysis of Near‐Surface Flows in the Sun

Ring diagram analysis of solar oscillation power spectra obtained from MDIdata is carried out to study the velocity fields in the outer part of the solarconvection zone. The three dimensional power spectra are fitted to a modelwhich has a Lorentzian profile in frequency and which includes the advection ofthe wave front by horizontal flows, to obtain the two components of thesub-surface flows as a function of the horizontal wave number and radial orderof the oscillation modes. This information is then inverted using OLA and RLSmethods to infer the variation in horizontal flow velocity with depth. Theaverage rotation velocity at different latitudes obtained by this techniqueagrees reasonably with helioseismic estimates made using frequency splittingdata. The shear layer just below the solar surface appears to consist of twoparts with the outer part up to a depth of 4 Mm, where the velocity gradientdoes not show any reversal up to a latitude of 60 degrees. In the deeper partthe velocity gradient shows reversal in sign around a latitude of 55 degrees.The zonal flow velocities inferred in the outermost layers appears to besimilar to those obtained by other measurements. A meridional flow from equatorpolewards is found. It has a maximum amplitude of about 30 m/s near the surfaceand the amplitude is nearly constant in the outer shear layer.Comment: aastex, 12 figures, to appear in Ap.