LAUSR

Time series of the medium-l spherical-harmonic decomposition of SOHO/MDI Doppler images were used to investigate the noise of solar-oscillation mode-coupling measurements and to reveal the coupling signatures of global-scale Rossby-waves and magnetic activity. A theoretical model of mode-coupling noise was developed starting from the assumption that the Doppler oscillation signal obeys Gaussian statistics. The measured coupling noise was found to agree with the model noise at the level of 20 per cent. The noise of mode-coupling measurements obtained from MDI data turns out to be considerably larger than that of hypothetical, ‘ideal’ measurements, which could only be obtained from observations with more coverage of the Sun’s surface than current instruments provide. The noise analysis was carried out for a simple suboptimal mode-coupling estimation procedure which is described in some detail. A more rigorous, maximum-likelihood, approach to mode-coupling measurement, which generalizes a method currently used to extract global oscillation mode information, is also described and its relationship to the simple mode-coupling analysis is discussed.