Comparison of frequencies and rotational splittings of solar acoustic modes of low angular degree from simultaneous MDI and GOLF observations

During the years 1996 through 1998 the Michelson Doppler Imager (MDI) and t he Global Oscillations at Low Frequency (GOLF) experiments on the Solar and Heliospheric Observatory (SOHO) mission have provided unique and nearly un interrupted sequences of helioseismic observations. This paper describes th e analysis carried out on power spectra from 759 days of calibrated disk-av eraged velocity signals provided by these two experiments. The period inves tigated in this work is from 1996 May 25 to 1998 June 22. We report the res ults of frequency determination of low-degree (l less than or equal to 3) a coustic modes in the frequency range between 1.4 mHz and 3.7 mHz. Rotationa l splittings are also measured for nonradial modes up to 3.0 mHz. The power spectrum estimation of the signals is performed using classical Fourier an alysis and the line-profile parameters of the modes are determined by means of a maximum likelihood method. All parameters have been estimated using b oth symmetrical and asymmetrical line profile-fitting formula. The line asy mmetry parameter of all modes with frequency higher than 2.0 mHz is systema tically negative and independent of l. This result is consistent with the f act that both MDI and GOLF data sets investigated in this paper are predomi nantly velocity signals, in agreement with previous results. A comparison o f the results between the symmetric and asymmetric fits shows that there is a systematic shift in the frequencies for modes above 2.0 mHz. Below this frequency, the line width of the modes is very small and the time base of t he data does not provide enough statistics to reveal an asymmetry. In gener al, the results show that frequency and rotational splitting values obtaine d from both the MDI and GOLF signals are in excellent agreement, and no sig nificant differences exist between the two data sets within the accuracy of the measurements. Our results are consistent with a uniform rotation of th e solar core at the rate of about 435 nHz and show only very small deviatio ns of the core structure from the standard solar model.