Comparison of high-degree solar acoustic frequencies and asymmetry between velocity and intensity data

Abstract

Using the local helioseismic technique of ring diagram we analyze the frequencies of high-degree f- and p-modes derived from both velocity and continuum intensity data observed by Michelson Doppler Imager. Fitting the spectra with asymmetric peak profiles, we find that the asymmetry associated with velocity line profiles is negative for all frequency ranges, agreeing with previous observations, while the asymmetry of the intensity profiles shows a complex and frequency-dependent behavior. We also observe systematic frequency differences between intensity and velocity spectra at the high end of the frequency range, mostly above 4 mHz. We infer that this difference arises from the fitting of the intensity rather than the velocity spectra. We also show that the frequency differences between intensity and velocity do not vary significantly from the disk center to the limb when the spectra are fitted with the asymmetric profile and conclude that only a part of the background is correlated with the intensity oscillations.