Phase and Amplitude Difference between Velocity and Intensity Helioseismic Spectra

Abstract

An explanation for the phase and amplitude difference between velocity and intensity oscillations of the Sun is provided. The phase difference along the modal lines in the power spectra was originally observed by Deubner and coworkers in 1989. From a simple adiabatic theory of solar oscillations, one expects this phase difference to be 90° for modes below the acoustic cutoff frequency (bound states) and zero for modes above the acoustic cutoff frequency (scattered states). But, surprisingly, from observations, the bound states show a phase difference that is below 90° along modal lines, and the scattered states also show a nonzero phase difference. We compute the phase difference between the velocity and intensity oscillations using medium angular degree data obtained from the Michelson Doppler Imager instrument on board the Solar and Heliospheric Observatory and confirm Deubner's result. We conclude that the unusual phase characteristics of the solar oscillations can be attributed to the fact that a part of the background is correlated to the source responsible for exciting the waves. The idea of the correlated background also explains why the high-frequency modes above the acoustic cutoff frequency are stronger in intensity than in the velocity power spectrum relative to the uncorrelated background, while at frequencies below the acoustic cutoff the velocity power relative to the uncorrelated background is stronger compared to the intensity. In addition, this explains the relative shift of the maxima in the velocity and intensity high-frequency power spectra.