Improvements in global mode analysis

As with any data analysis, the standard MDI medium-l analysis pipeline is based on several approximations. Physical effects such as line asymmetry, horizontal displacement at the solar surface, and distortion of eigenfunctions have been ignored, as well as cubic distortion in the optics and instrumental errors in the plate scale and orientation of the CCD. Furthermore, we see several systematic errors in the results of the analysis, most notably an annual variation in f-mode frequencies and a bump in the normalized residuals of the a-coefficients around 3.4 mHz, which may relate to polar jets in the inversions. We have reprocessed several years of data applying the above corrections, and made improvements in the pipeline algorithm itself by recomputing the locations of bad data points and using updated routines for detrending and gapfilling. Along the way the pipeline has been almost entirely automated. Here we discuss the resulting changes in mode parameters and their effect on the magnitude of systematic errors.