Measuring the Sun's Eigenfrequencies from Velocity and Intensity Helioseismic Spectra: Asymmetrical Line Profile-fitting Formula

Abstract

Solar eigenfrequencies are generally determined by fitting a Lorentzian to the spectral lines in the power spectrum. This assumes that the spectral line is symmetric. Recent observations from the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory have indicated that the power spectra of p-modes show varying amounts of asymmetry. Line asymmetry is an intrinsic property of solar oscillations and depends on the properties of the excitation source and the background noise correlated with the oscillations. Neglecting asymmetry leads to systematic errors in the determination of frequencies and thus affects the results of inversions. In this Letter, we use a simple physical model to derive a new fitting formula that incorporates the effects of asymmetry. It is then tested on artificial and real solar MDI data. A comparison of the results of a symmetric fit with those of an asymmetric one shows that there is a systematic shift in the eigenfrequencies. Our formula will yield more accurate estimates of the solar eigenfrequencies, which is important for improving the accuracy of helioseismic inversions.