Solar Oscillations and the Equation of State

Abstract

With suitable inversion techniques of solar five-minute oscillation frequencies, the local sound speed c(r) inside the Sun (r being the distance from the center) has become an observable quantity (Christensen-Dals-gaard et al., 1985). Since c² = γ p/ ρ, where γ = (∂ lnp/ ∂ ln ρ)_S, one expects modulations of the sound speed from the lowering of y in the zones of partial ionization of hydrogen and helium. Gough (1984) proposed to use this modulation to determine the solar helium abundance. Däppen and Gough (1984,1986) assessed the potential of this method with the help of theoretical solar models and numerically computed solar oscillation frequencies. They concluded that the quantitative influence of the He II ionization zone on the sound speed (or more precisely on the derivative of sound speed with respect to depth) is sufficiently large to serve as a calibration of the helium abundance.