Structure of the near-surface layers of the sun: asphericity and time variation

Abstract

We present results on the structure of the near-surface layers of the Sun obtained by inverting frequencies of high-degree solar modes from "ring diagrams." We have results for eight epochs between 1996 June and 2003 October. The frequencies for each epoch were obtained from ring diagrams constructed from Michelson Doppler Imager (MDI) Dopplergrams spanning complete Carrington rotations. We find that there is a substantial latitudinal variation of both sound speed and the adiabatic index Γ₁ in the outer 2% of the Sun. We find that both the sound-speed and Γ₁ profiles change with changes in the level of solar activity. In addition, we also study differences between the northern and southern hemispheres of the Sun and find a small asymmetry that appears to reflect the difference in magnetic activity between the two hemispheres.