Noise reduction in helioseismic power spectra by non-orthogonal wavelets

We present a method to reduce noise in helioseismic power spectra using a n on-orthogonal wavelet transform based on quadratic spline functions. The qu ality of our method is tested by applying it to artificially generated time -series approximating solar p-modes. The mode frequencies and line widths o btained from least-squares fits to the smoothed spectra are compared with t he corresponding parameters deduced from maximum likelihood fits to the ori ginal spectra. The results from both approaches are very similar and sugges t that there is no major bias in either of these rather independent approac hes. As a practical example we denoise parts of the power spectrum obtained from the two first years of operation of the GOLF instrument onboard SOHO.