Optimal masks for low-degree solar acoustic modes

We suggest a solution to an important problem in observational helioseismol ogy of the separation of lines of solar acoustic (p) modes of low angular d egree in oscillation power spectra by constructing optimal masks for Dopple r images of the Sun. Accurate measurements of oscillation frequencies of lo w-degree modes are essential for the determination of the structure and rot ation of the solar core. However, these measurements for a particular mode are often affected by leakage of other p-modes arising when the Doppler ima ges are projected on to spherical harmonic masks. The leakage results in ov erlapping peaks corresponding to different oscillation modes in the power s pectra. In this Letter, we present a method for calculating optimal masks f or a given (target) mode by minimizing the signals of other modes appearing in its vicinity. We apply this method to time series of 2 yr obtained from the Michelson Doppler Imager instrument on board the Solar and Heliospheri c Observatory space mission and demonstrate its ability to reduce efficient ly the mode leakage.