Triple correlations in local helioseismology

A central step in time-distance local helioseismology techniques is to obtain travel times of packets of wave signals between points or sets of points on the visible surface. Standard ways of determining group or phase travel times involve cross-correlating the signal between locations at the solar photosphere and determining the shift of the envelope of this cross correlation function, or a zero crossing, using a standard wavelet or a reference wave packet. Here a novel method is described which makes use of triple correlations, i.e. cross-correlating signal between three locations. By using an average triple correlation as reference, differential travel times can be extracted in a straightforward manner.