Adapted from G. G. Howes et al., 
Schematic of the experiment performed by Howes et al. . Counterpropagating Alfvén waves, which travel parallel to an applied magnetic field, are generated from a loop antenna and an arbitrary spatial waveform (ASW) antenna. The nonlinear interaction of the two waves results in a third wave which has frequency and wave number equal to the sum of the frequency and wave number of the parent waves.