A Mode Fitting Methodology Optimized for Very Long Time Series

I describe and present the results of a newly developed fitting methodology optimized for very long time series. The development of this new methodology was motivated by the fact that we now have more than half a decade of nearly uninterrupted observations by GONG and MDI, with fill factors as high as 89.8% and 82.2% respectively. It was recently prompted by the availability of a 2088-day-long time series of spherical harmonic coefficients produced by the MDI team. The fitting procedure uses an optimal sine-multi-taper spectral estimator -- whith the number of tapers based on the mode linewidth, the complete leakage matrix (i.e., horizontal as well as vertical components), and an asymmetric mode profile to fit simultaneously all the azimuthal orders with individually parametrized profiles. This method was applied to 2088-day-long time series of MDI and GONG observations, as well as 728-day-long subsets, and for spherical harmonic degrees between 1 and 25. The values resulting from these fits are inter-compared (MDI versus GONG) and compared to equivalent estimates from the MDI team and the GONG project. I also compare the results from fitting the 728-day-long subsets to the result of the 2088-day-long time series. This comparison shows the well known change of frequencies with solar activity -- and how it scales with a nearly constant pattern in frequency and m/l. This comparison also shows some changes in the mode linewidth and the constancy of the mode asymmetry.