3He is an intermediate product in the proton-proton chain, and standard models of the Sun predict a large bulge of enhanced 3He abundance near Mr/M o = 0.6 in the contemporary Sun. The relatively low abundance of 3He at the solar surface, which is derived from solar wind observations, poses severe constraints to non-standard solar models.
Direct measurements of the 3He abundance in the solar atmosphere are extremely difficult, whereas indirect measurements, e.g., in the solar wind, have been performed with considerable precision. The interpretation of solar wind observations with respect to solar surface abundances has been greatly improved in recent years. Abundance measurements have been performed under a large variety of solar wind conditions and refined models have been developed for the transport processes in the chromosphere and the transition region and for the processes occurring in the solar corona. From these measurements we estimate the present isotopic number ratio 3He/4He to be (4.1 _+ 1.0) x 10 -4 at the solar surface, corresponding to the weight abundance X3 = (9.0 +_ 2.4) x 10 -5. The zero-age Main-Sequence abundance of 3He (after burning of D) might have been slightly lower (by about 10 to 20%) than the present-day value.
Non-standard solar models involving mild turbulent diffusion (Lebreton and Maeder, 1987) could account for a slow secular increase of the 3He/4He ratio in the solar atmosphere. On the other hand it is difficult to reconcile models with severe mass loss as proposed by with this constraint. The slowing down of the solar rotation during the early Main-Sequence evolution was accompanied by stronger differential rotation probably implying a more effective mixing of the inner parts. Again, the surface abundance of 3He imposes severe limits on the evolution of the distribution of momentum within the early Sun.