Top: radial solar wind speed varying with Carrington longitude of PSP measured during Encounter 4. The colors represent the time and PSP travels from the light-colored end toward the dark-colored end as indicated by the red arrows. Red circles connected by dashed lines mark several selected structures that were observed at different radial distances. Bottom: radial wind speed as a function of radial distance to the Sun. Each line corresponds to one single structure marked by the connected red circles in the top panel. The structures are numbered as 1, 2, 3, 4, 5, and 6, as annotated in the top panel.