STRUCTURE AND ROTATION OF THE SOLAR INTERIOR - INITIAL RESULTS FROM THE MDI MEDIUM-L PROGRAM

The medium-l program of the Michelson Doppler Imager instrument on boa rd SOHO provides continuous observations of oscillation modes of angul ar degree, l, from 0 to similar to 300. The data for the program are p artly processed on board because only about 3% of MDI observations can be transmitted continuously to the ground. The on-board data processi ng, the main component of which is Gaussian-weighted binning, has been optimized to reduce the negative influence of spatial aliasing of the high-degree oscillation modes. The data processing is completed in a data analysis pipeline at the SOI Stanford Support Center to determine the mean multiplet frequencies and splitting coefficients. The initia l results show that the noise in the medium-l oscillation power spectr um is substantially lower than in ground-based measurements. This enab les us to detect lower amplitude modes and, thus, to extend the range of measured mode frequencies. This is important for inferring the Sun' s internal structure and rotation. The MDI observations also reveal th e asymmetry of oscillation spectral lines. The line asymmetries agree with the theory of mode excitation by acoustic sources localized in th e upper convective boundary layer. The sound-speed profile inferred fr om the mean frequencies gives evidence for a sharp variation at the ed ge of the energy-generating core. The results also confirm the previou s finding by the GONG (Gough et al., 1996) that, in a thin layer just beneath the convection zone, helium appears to be less abundant than p redicted by theory. Inverting the multiplet frequency splittings from MDI, we detect significant rotational shear in this thin layer. This l ayer is likely to be the place where the solar dynamo operates. In ord er to understand how the Sun works, it is extremely important to obser ve the evolution of this transition layer throughout the 11-year activ ity cycle.