Measuring the sun's eigenfrequencies from velocity and intensity helioseismic spectra: Asymmetrical line profile-fitting formula

Solar eigenfrequencies are generally determined by fitting a Lorentzian to the spectral lines in the power spectrum. This assumes that the spectral li ne is symmetric. Recent observations from the Michelson Doppler Imager (MDI ) on board the Solar and Heliospheric Observatory have indicated that the p ower spectra of p-modes show varying amounts of asymmetry. Line asymmetry i s an intrinsic property of solar oscillations and depends on the properties of the excitation source and the background noise correlated with the osci llations. Neglecting asymmetry leads to systematic errors in the determinat ion of frequencies and thus affects the results of inversions. In this Lett er, we use a simple physical model to derive a new fitting formula that inc orporates the effects of asymmetry. It is then tested on artificial and rea l 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 eigenfr equencies. Our formula will yield more accurate estimates of the solar eige nfrequencies, which is important for improving the accuracy of helioseismic inversions.