Space-time diagrams across the jet spire at different heights. These represent the temporal evolution at a horizontal line along the x-direction placed within the jet spire. Panels (a)–(c) show the evolution at heights of 18, 14, and 10 Mm above the surface. The sloped red and blue lines in panels (b) and (c) indicate apparent speeds of 116 km s⁻¹ (red) and 36 km s⁻¹ (blue). See Sect. 3.1 for details.