Needles in haystacks: how to use contemporaneous data in the search for low-frequency modes of osollation of the sun

We show how to take advantage of contemporaneous data from two different instruments in the search for low-frequency modes of oscillation of the Sun. Contemporaneous data allow searches to be made for prominent, sharp concentrations of power which are coincident in frequency. Crucial to determining objective measures of the joint probability of the random occurrence of such features, which are potential candidates for modes, is a good understanding of the characteristics of the background noise. In this paper we show how to make proper allowance, in the calculation of the probability, for noise that is common to data from different instruments. This common noise is solar in origin, and comes from the solar granulation. Its presence makes calculation of the probability a non-trivial problem. We demonstrate application of the technique in searches for low-frequency p modes. The data we searched comprised 3071 d of contemporaneous Sun-as-a-star Doppler velocity observations made by the ground-based Birmingham Solar-Oscillations Network (BiSON), and the GOLF instrument onboard the ESA/NASA SOHO spacecraft.