Solar Internal Composition and Nuclear Reaction Rates in the Light of Helioseismology

Acoustic modes are a suitable probe to check the internal solar composition and give constraints on regions of turbulence or mixing in stellar interiors. The recent results obtained with the SOHO satellite involve practically all the acoustic modes and are extremely useful to check theoretical assumptions. If turbulence is favoured to explain lithium burning in the tachocline layers located at the base of the convection zone, central turbulent mixing seems to be strongly rejected by the present observations. Nuclear reaction rates cannot be directly verified by this type of probe but they are indirectly constrained through the behaviour of the sound speed. So, some puzzling problems concerning Maxwellian distribution or dynamical effects in stellar plasma are enlightened by the accuracy of the present seismic data.