29 - Towards a helioseismic calibration of the equation of state in the solar convective envelope

Summary

Abstract

We report the results of a nonlinear inversion of solar oscillation data that enable us to detect nonideal Coulomb interactions between particles, including pressure ionization, in the solar convection zone.

Introduction

Precise measurements of solar oscillation frequencies provide data for accurate inversions for the sound speed in the solar interior. Except in the very outer layers, the stratification of the convection zone is almost adiabatic. There, the sound-speed profile is governed principally by the specific entropy, the chemical composition and the equation of state, being essentially independent of the uncertainties in the radiative opacities. The inversions thus reveal, via tiny effects on the adiabatic compressibility of the solar plasma, physical processes that influence slightly the equation of state. We have carried out a nonlinear inversion based on a recent accurate asymptotic description of intermediate- and high-degree solar p modes (Brodsky & Vorontsov 1993; Gough & Vorontsov 1993), using the observational data of Libbrecht, Woodard & Kaufman (1990).

The equations of state (EOS) used in the analysis

In the reference models, we use the following equations of state. We are mostly brief, with the exception of the pressure-ionization model used in the helioseismic calibration.

• Saha EOS: a free-energy-minimization type realization for a mixture of reacting ideal gases, with ground-state-only partition functions of the bound species. Note that by assuming only ground states we are using the term ‘Saha’ in a rather restricted sense.

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