Helioseismology has become a very successful diagnosis of the equation of state of the plasma of the solar interior. Although the gas in the solar interior is only weakly coupled and weakly degenerate, the great observational accuracy of the helioseismological measurements puts strong constraints on the nonideal part of the equation of state. The helioseismic verification of major nonideal effects in the equation of state of solar matter has become well established. The dominant contribution is the Coulomb pressure, conventionally described in the Debye-Hückel approximation. However, in the last years, the increased precision of the helioseismic diagnosis has brought significant observational progress beyond the Debye-Hückel approximation. The helioseismic detection of a signature of relativistic electrons was a striking example. Very recently, effects of the excited states of the atoms and ions of heavy elements were discovered, which have a promising potential both for statistical mechanics and solar physics, in particular, the helioseismic determination of the heavy-element abundance.

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 and the Reynolds stresses are negligible. The sound-speed profile is governed principally by the specific entropy, the chemical composition and the equation of state, and it is therefore essentially independent of the uncertainties in the radiative opacities.