Same as Fig. 13, but now showing the frequency power spectrum versus height. Peaks in the PS can be found at 5–10 mHz which correspond to the box-modes frequencies (see Fig. 11). In the vertical flow speed, the box-modes are able to survive into the low corona by travelling along the network of magnetic funnel structures. A similar, but less pronounced, enhancement in the PS is visible in the horizontal flow speed, which moves to progressively higher frequencies with height.