In a Corotating Interaction Region (CIR) the stream interface is identified as a relatively sharp density drop, temperature rise, and flow shear in the solar wind, and is now generally believed to mark the boundary between solar wind which was originally slow when it left the Sun and solar wind which was originally fast. This paper summarises some important facts and open questions about the origin and nature of the boundary between fast and slow solar wind near the Sun, the evolution of stream

more »

... interfaces with heliocentric distance in the inner heliosphere, and their relationship.