Interplanetary stream interfaces

At 1 AU there is a distinct boundary (the stream interface) in the interaction region of a stream in the solar wind, characterized by an abrupt (approximately a factor of 2 change in <10 6 km) drop in density, a similar increase in temperature, and a small increase in speed. In some cases this is a tangential discontinuity, in others it is probably evolving into a tangential discontinuity. It is suggested that stream interfaces form in the interplanetary medium as a consequence of the nonlinear evolution of streams generated by an increase in temperature in the solar envelope. This evolution eventually leads to the formation of a reverse shock behind the interface and a forward shock ahead of it. Two instances in which both a stream interface and a reverse shock had developed at 1 AU are presented. Examples of flare‐generated shocks that passed through a stream and were observed near a stream interface are also presented. It is shown that stream interfaces are definitely not the same structures that others have identified as piston boundaries. It is noted that slow shocks, like stream interfaces, always occur ahead of streams and may develop in the interplanetary medium. The importance of small‐scale interplanetary dynamical processes, especially the development of discontinuities, is emphasized.