Heating fronts in the simulated spicules propagate with the Alfvén speed. The top shows a map of the Alfvén speed at t = 660 s (i.e., the same simulation snapshot as in Panels (B) and (F) in Figures 3 and 4) which has values of between 150 and 450 km s−1 in the spicule. The white contour shows locations with temperatures of 104 K. The black contour shows the region over which we sum (at each height) to calculate the space–time plots in the bottom row. The bottom row shows, as a function of time, the Si IV 1402 Å emissivity, Joule heating rate and electrical current along the length of the spicule as measured from the photosphere (left vertical axis, for equivalent height, see right vertical axis). The heating front in the right spicule dominates the signal around t = 640 s, while the heating front in the left spicule (at greater heights) occurs around t = 680 s. We see a clear correlation between the rapid Si IV 1402 Å increase in both “jets” and Alfvénic propagation of electrical current and associated dissipation.