Seismology of the solar envelope: sound-speed gradient in the convection zone and its diagnosis of the equation of state

We report the results of an asymptotic inversion of solar oscillation data for the gradient of the sound speed in the convection zone. This gradient r eveals derails of;he non-ideal Coulomb interactions between particles, incl uding pressure ionization, A simplified physical model is used to track dow n the effect of various physical assumptions in the sound-speed derivative, The model contains a calibration for the size of the H and He atoms and th e He+ ion. We find that, for the pressure-ionization regions of hydrogen an d helium, such a model matches the data better than any of the currently av ailable parameter-free theories.