Corotating pressure waves without fast streams in the solar wind

Abstract

Voyager 1 and 2 magnetic field and plasma data are presented which demonstrate the existence of large scale, corotating, nonlinear pressure waves between 2 AU and 4 AU that are not accompanied by fast streams. The pressure waves are presumed to be generated by the interaction between fast corotating streams and slower flows near the sun. For two of the three pressure waves that are discussed, the absence of a relatively fast stream is probably a real, physical effect, viz, a consequence of deceleration of the stream and acceleration of the ambient flow by the associated compression wave. For the third pressure wave, the apparent absence of a stream may be a geometrical effect; it is likely that the stream was at latitudes just above those of the spacecraft, while the associated shocks and compression wave extended over a broader range of latitudes so that they could be observed by the spacecraft. It is suggested that the development of large‐scale, nonlinear pressure waves at the expense of part of the kinetic energy of streams produces a qualitative change in the solar wind in the outer heliosphere. Within a few astronomical units the quasi‐stationary solar wind structure is determined by corotating streams whose structure is determined by the boundary conditions near the sun. Beyond several astronomical units there is a zone in which the solar wind structure is determined by nonlinear pressure waves without streams, in which memory of the source conditions has largely been erased. Far from the sun (≳25 AU), these pressure waves should interact extensively with one another producing a zone which is more homogenous on a large scale yet more disordered on a smaller scale, where a statistical description may be more appropriate than deterministic models. This new view of heliospheric structure should provide a better foundation on which to interpret retrospectively prior observations and to analyze future data with respect to basic physical processes.