  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 June 1996 and October 2003.
The frequencies for each epoch were obtained from ring diagrams constructed
from MDI Dopplergrams spanning complete Carrington rotations. We find that
there is a substantial latitudinal variation of both sound speed and the
adiabatic index Gamma_1 in the outer 2% of the Sun. We find that both the
sound-speed and Gamma_1 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.
