On the nature of the current GOLF p-mode signal

The GOLF experiment on board SOHO is designed to measure global oscillations of the disk-integrated sunlight with high sensitivity and with long term stability. The GOLF data is thus ideally suited for both the study of the p-mode spectrum and to address the search for gravity modes. Due to their occasional malfunction the project decided to stop the rotating polarizing elements at an optimum place in order to maintain a precise sequence of measurements with the highest possible duty cycle. This action means that subsequently GOLF only measures two monochromatic intensities Ib and I − b on the blue wing of the sodium doublet. In this work we investigate the nature of these signals separately and in combinations. Our method is to study the temporal relative phase relations between the low degree (` ≤ 3) p-mode signals derived from data sets obtained from simultaneous observations, both from other SOHO instruments (GOLF and SOI) and from Mark– I, the Tenerife station of the ground-based BiSON network. It is found that these signals are “almost” pure velocity signals. A simple model indicates that a contamination of a pure intensity-like signal of 14% amplitude would fully explain the true nature of the current GOLF signal. Moreover, it is found that the ratios, defined for other instruments (Mark–I, SOI and, by extension BiSON and IRIS), also have exactly the same nature as the GOLF ratio.