Sketch of a stream structure in the ecliptic plane, adapted from Hundhausen (1972). The spiral structure is a consequence of solar rotation. The spiral inclination changes as the solar wind velocity changes. When the high-speed stream (in blue) compresses the slower ambient solar wind (in black), a compression region downstream (in red) and a rarefaction region upstream (in pink) are formed. The velocity knee marks the border between the blue and pink regions.