A determination of high degree mode parameters based on MDI observations

We present the best to date determination of high degree mode parameters obtained from the longest full-disk high-resolution data set available over the 13 years of MDI operations. A ninety day long time series of full-disk two arc-second per pixel resolution dopplergrams were acquired in 2001, thanks to the high rate telemetry provided by the deep space network. These dopplergrams were decomposed using our best estimate of the image scale and the known components of MDI's image distortion. The spherical harmonics decomposition was carried out up to l=1000, and a sine multi-taper power spectrum estimator was used to generate power spectra for all degrees and all azimuthal orders up to l=1000. We used a large number of tapers to reduce the realization noise. Since at high degrees the individual modes blend into ridges, there is no reason to preserve a high spectral resolution. These power spectra were fitted for all degrees and all azimuthal orders, between l=100 and l=1000, and for all orders with substantial amplitude, generating in excess of 6 million individual estimate of frequencies, line-widths amplitudes and asymmetries, corresponding to some 6,000 singlets. Fitting at high degrees generates characteristics of the blended ridges, characteristics that do not correspond to the underlying mode characteristics. We used a sophisticated forward modeling of the mode to ridge blending to recover the best possible estimate of the underlying mode characteristics for the mode frequency, as well as the line-width, amplitude and asymmetry. We describe this modeling, as it has been recently fine tuned, and the iterative process used to refine its input parameters. Finally not only did we generate corrected mode characteristics and their uncertainties, but we computed the sensitivity of the model to its input set to best estimate the precision of the ridge to mode correction itself. This was carried out to assess the magnitude of any residual systematic errors in the final estimates of the mode characteristics.