RESULTS FROM THE MDI MEDIUM-L PROGRAM

The medium-/ program of the Michelson Doppler Imager instrument on board SOHO provides continuous observations of oscillation modes of angular degree. I. from 0 to _ 300. The data for the program are partly processed on board because only about 3% of MDI observations can be transmitted continuously to the ground. The on-board data processing, 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 SOl Stanford Support Center to determine the mean multiplet frequencies and splitting coeflicients. The initial results show that the noise in the medium-/oscillation power spectrum is substantially lower than in ground-based measurements. This enables us to detect lower amplitude modes and, thus. to extend the range of measured mode fiequencies. This is important for inferring the Sun's internal structure anti rotation. The MDI observations also reveal the asymmetry of oscillation spectral lines. The line asymmetries agree with the theory of mode excitation by acoustic sources localized in the upper convective boundary layer. The sound-speed profile inferred from the mean fiequencies gives evidence lot a sharp w_riation at the edge of the energy-generating core. The results also coniirm the pJvvious 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 predicted by