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Helioseismology provides a unique tool for investigating chemical composition of the interior of the Sun. There are two basic ways of obtaining the information. The first one is to calibrate theoretical solar models by comparing either the observed oscillation frequencies with the eigenfrequencies of the models, or primary seismic parameters (e.g. the sound speed, the density and the adiabatic exponent) inverted from the observed frequencies with the corresponding parameters of the solar models. The second approach is to measure abundances by direct (`secondary') inversions of the frequencies, incorporating additional equations of the stellar structure into the helioseismic inverse problem. The additional equation to estimate composition of the convection zone is the equation of state which relates variations of the adiabatic exponent in zones of ionization of elements to their abundances. In the radiative interior where the most abundant elements are almost totally ionized, the energy equations together with equations of the energy generation rate and the opacity are used to relate primary seismic parameters with abundances.
Inversions of the current solar data give a value for the helium abundance in the convection zone, which is significantly lower than the value in standard solar models, thus suggesting that gravitational settling of helium takes place in the Sun. This is consistent with the inversion results in the radiative interior, where evidence is found for large-scale material redistribution. There is also indication of variations of the helium abundance in the energy-generating core, which are not described by standard solar evolution theory.
It is anticipated that the new data from the GONG and SOI will substantially improve accuracy of the measurements of the helium abundance.
