The primary inversion of the accurately measured frequencies of solar oscillations determines the mechanical properties of the Sun , i.e. , the sound speed and density as a function of solar radius .
In order to infer the temperature and hydrogen abundance profiles inside the Sun , it becomes necessary to use , in addition , the equations of thermal equilibrium along with the input physics , such as opacities , equation of state and nuclear reaction rates .
It then becomes possible to estimate the effects of uncertainties in opacity and nuclear reaction rates on the inferred thermal and composition profiles .
The seismically determined temperature and hydrogen abundance profiles turn out to be close to those in a standard evolutionary solar model that includes the diffusion of helium and heavy elements below the convection zone .
The most significant departure occurs just below the base of the convection zone where the inferred hydrogen abundance profile is smoother than that in a standard diffusion model .
The region just beneath the solar convection zone appears to be mixed , a process which could account for the observed low lithium abundance in the solar envelope .
With a reasonable allowance for errors in opacities , the helioseismically estimated cross-section for pp nuclear reaction rate turns out to be ( 4.15 \pm 0.25 ) \times 10 ^ { -25 } MeV barns .