Slow- and fast-wave evolution in a vertical cutting plane through the 3D null spine. Spatial maps of slow- (left column) and fast- (right column) wave amplitudes (in terms of their associated wave energy fluxes, i.e., and ) in the y = 0 plane at various time instants (time evolves from top to bottom, as indicated at the top of each panel). The yellow curves represent the Alfvén-acoustic equipartition layers, and the turquoise line displays the bottom of the transition layer at z = 2 Mm. The maps are oversaturated to highlight the dynamics taking place close to these crucial layers.