Observational constraints on models of the solar background spectrum

We discuss the properties of the solar background signal as observed in hig h-quality, l - v power and phase difference spectra of the continuum (C), v elocity Oil, and line intensity (I) fluctuations of the Ni I 6768 Angstrom line. These spectra were generated from high-resolution images acquired by the Michelson Doppler Imager on board SOHO. We confirm that the background signal in the velocity power spectra can be reproduced by a composite model with two quasi-stationary components, descr ibing large-scale and small-scale convective motions, and a periodic compon ent. The line and continuum intensity power spectra require additional quas i-stationary and periodic components. The extra quasi-stationary component dominates the intensity and continuum background signals over the spectral region where the I-V phase difference spectra show essentially constant neg ative phase difference: i.e., below and in between the p-mode ridges (calle d the plateau-interridge regime by Deubner et al.). Since the I-V phase bet ween the p-mode ridges is not random, the solar background beneath the p-mo des must be considered as coherent. We thus speculate that the negative pha se regime may be the manifestation of a correlated background. Such a backg round has been proposed to explain the opposite sense of the asymmetries of the p-mode line profiles in velocity and brightness oscillations.