Centre of mass x displacement and corresponding spectra for two different loops. Left panels: magnified (×20) x displacement of the loop centre of mass per height (projected on z) from slits placed along the coronal part of the loop. Right panels: fourier power spectra of the displacement taken from the slits on the left. The top and middle panels correspond to the original and detrended displacement signals for a loop with a coronal magnetic field B_z ∼ 30 G. The bottom panels correspond to the detrended displacement signal for a loop with B_z ∼ 20 G.