Comparison between QL theory and PSP observations. Left: QL dynamical paths of the temperature anisotropy (A_b) vs. beam-core drift velocity (v_b − c/v_Ac). The black squares are the initial states, and the magnetic wave energy level is color-coded. The final states after saturation gather on the red curve (see text): red (large) dots for runs 1–3 and 13–15, circles for runs 4–6, black triangles for runs 7–9, and green triangles for runs 10–12. Right: Same as the left panel (black symbols are now shown as white symbols), but superimposed on PSP observations. The (normalized) number of events with hammerhead populations are color-coded (adapted from Figure 2, panel (b), in Verniero et al. (2022) ©AAS).