Solar-cycle variation of solar structure near the surface: the adiabatic index

Rabello-Soares M. Cristina, cristina@sun.stanford.edu, Stanford University, United States

Abstract
Using acoustic mode frequencies observed by MDI/SOHO through most of solar cycle 23, we have estimated the sound-speed difference between solar maximum and minimum from 0.53 to 0.992R. The sound-speed variation with solar activity seems to have a two-layer configuration, similar to the one observed below an active region, which consists of a negative layer near the solar surface and a positive one in the layer immediately below the first one (Rabello-Soares, 2012). Here we extend our analysis to the adiabatic index Gamma1 and compare our results with Basu & Mandel (2004). They presented evidence that the depression in the adiabatic index Gamma1 in the second helium ionization zone decreases with increasing solar activity.