Velocity profile, wave velocities, and acceleration forces within HSSs at various distances from the Sun. Top: sketch of the formation of the velocity profile across the HSS cross section close to the Sun, which propagates in a channel within a slow solar wind environment. Velocities are delineated as arrows. Bottom left: acoustic, Alfvénic, and magneto-acoustic velocity in an HSS environment defined by Eqs. (1) and (3) versus its distance from the Sun. Bottom right: Acceleration of a plasma parcel in an HSS due to gravitation and pressure gradients as a function of distance from the Sun.