Accurate measurements of SOI/MDI high-degree frequencies and frequency splittings

We present accurate measurements of high-degree p-mode frequencies and frequency splittings obtained from the Full-Disk Program of the Michelson Doppler Imager (MDI) experiment onboard the Solar and Heliospheric Observatory (SOHO). The frequencies and frequency splittings are computed from unaveraged zonal, tesseral, and sectoral power spectra using a new fitting method of Reiter et al. (2002) based upon a maximum-likelihood fitting approach. In this method, both the spectral power distribution and contributions of the various observational and instrumental effects to the spatial leakage matrices are modelled accurately. We demonstrate that one of the most long-standing problems in high-degree helioseismology, viz. the jumps in the frequency splitting coefficients, can be solved by taking into account the distortion of the leakage matrix by the solar differential rotation. The results of inversion of the initial frequency set determined using this new method in the range of angular degree ℓ = 45 - 300 show a substantially better resolution of the subsurface layers compared to the previous studies with ℓ below 220.