Ring diagram analysis of near-surface flows in the Sun

Abstract

Ring diagram analysis of solar oscillation power spectra obtained from MDI data is carried out to study the velocity fields in the outer part of the solar convection zone. The three dimensional power spectra are fitted to a model which has a Lorentzian profile in frequency and which includes the advection of the wave front by horizontal flows, to obtain the two components of the sub-surface flows as a function of the horizontal wave number and radial order of the oscillation modes. This information is then inverted using OLA and RLS methods to infer the variation in horizontal flow velocity with depth. The average rotation velocity at different latitudes obtained by this technique agrees reasonably with helioseismic estimates made using frequency splitting data. The shear layer just below the solar surface appears to consist of two parts with the outer part up to a depth of 4 Mm, where the velocity gradient does not show any reversal up to a latitude of 60 degrees. In the deeper part the velocity gradient shows reversal in sign around a latitude of 55 degrees. The zonal flow velocities inferred in the outermost layers appears to be similar to those obtained by other measurements. A meridional flow from equator polewards is found. It has a maximum amplitude of about 30 m/s near the surface and the amplitude is nearly constant in the outer shear layer.