Context map of a south polar coronal hole from AIA/SDO 171 Å passband. The continuous white line box in off-limb region indicates the selected plume region for which a time–distance intensity map (see Fig. 3) and a frequency–distance Fourier power map (see Fig. 6) are obtained. Different small boxes indicate the individual spatial locations chosen for detailed analysis. To make the far off-limb structures visible, a radial enhancement filter is applied on the image.