BiSON Data Show Change in Solar Structure with Magnetic Activity

Abstract

We find evidence for an activity-dependent change in solar structure at the second helium ionization zone (~0.98 Rsolar). We use low-degree p-mode frequencies obtained from 13 years of observations by the Birmingham Solar Oscillations Network (BiSON) to track changes in the amplitude of the rapid-variation signature imposed by the bump in the adiabatic exponent, Γ1, at the He II ionization zone. This is the first indication of structural change detected using only low-degree data and confirms the recent results of high-degree and local helioseismic studies.