The structure of the near-surface layers of the Sun: asphericity
and time variation

Sarbani Basu
Richard S. Bogart
H. M. Antia

We present results on the structure of the near-surface layers of the Sun.
These results were obtained by inverting frequencies of high-degree solar
modes obtained by using "ring diagrams". We have results for seven epochs
from June 1996 to March 2002. The frequencies of each epoch were obtained
from ring diagrams constructed from MDI Dopplergrams spanning one complete
Carrington rotation. 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. In addition, it appears that both the sound-speed and
Gamma_1 profiles change with change in the level of solar activity.