SEISMOLOGICAL MEASUREMENT OF SOLAR HELIUM ABUNDANCE

The internal structure and evolution of the Sun depends on its chemical composition, particularly the helium abundance. In addition, the helium abundance in the solar envelope is thought to represent the protosolar value, making it a datum of cosmological significance. Spectroscopic measurements of the helium abundance are uncertain, and the most reliable estimates until now have come from the calibration of solar evolutionary models. The frequencies of solar acoustic oscillations are sensitive, however, to the behaviour of the speed of sound in the Sun's helium ionization zone, which allows a helioseismological determination of the helium abundance. Sound-speed inversion of helioseismological data can be used for this purpose1-3, but precise frequency measurements of high-degree oscillation modes are needed. Here we describe a new approach based on an analysis of the phase shift of acoustic waves4,5 of intermediate-degree modes. From the accurate intermediate-mode data now available6, we obtain a helium mass fraction Y = 0.25 +/- 0.01 in the solar convection zone, significantly smaller than the value Y = 0.27-0.29 predicted by recent solar evolutionary models7-9. The discrepancy indicates either that initial helium abundance was reduced in the envelope by downward diffusion9 or that the protosolar value was lower than currently accepted.