Cross correlations of the oscillation at different locations along the loop. The five curves show the cross correlation of the loop apex with five positions along the loop (apex, two positions midway from apex to loop, and two near the footpoint at both sides). The solid line shows the self-correlation at the loop apex. The range of the time lag shown here is about four times the oscillation period of 52.5 s at the loop apex. The red diamonds indicate the peaks of the cross-correlation functions. See Sect. 3.4.