Time difference, distance, average propagation speed, and orientation of the brightening in the Hβ line core with respect to the brightening in the wing. The measurement method is illustrated in Fig. 5. Positive values of the propagation speed d/Δt can be interpreted as upward propagation from lower to higher altitude. The vertical dashed red line in panel d indicates the direction toward the nearest limb (36°). The total number of measurements is 396.