An estimation of the acoustic cutoff frequency of the sun based on the properties of the low-degree pseudomodes

The acoustic cutoff frequency (nu(ac)) is an important atmospheric parameter whose estimation has in the past been based on the study of power spectra that have yielded a wide range of values between 5300 and 5700 mu Hz. The discovery of a solar signal well beyond the acoustic cutoff frequency (pseudomodes) might lead one to think that the determination of nu(ac) would be even more complicated because, for example, looking for a sudden drop in the power density signal could no be longer used. Contrary to what might be thought at first sight, the existence of pseudomodes helps to provide a good estimation of nu(ac), because the frequency pattern of pseudomodes is shifted with respect to that of p-modes. In this study a bivariate analysis (coherence and phase shift) between the intensity signals of VIRGO and the velocity signal of GOLF (both instruments on board the SOHO probe) is carried out over the frequency range of p-modes and pseudomodes. The results shows clear evidence that the acoustic cutoff frequency of the Sun is close to the theoretical value of 5300 mu Hz; specifically, a value around 5100 mu Hz is found in this research.