Dynamical evolution of interplanetary magnetic fields and flows between 0.3 AU and 8.5 AU: Entrainment

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Abstract
The radial evolution of interplanetary flows and associated magnetic fields between 0.3 AU and 8.5 AU was analyzed using data from Helios 1 and Voyager 1, respectively. During a 70‐day interval in 1980 Voyager 1 observed two streams which appeared to be recurrent and which had little fine structure. The corresponding flows observed by Helios 1 were much more complex, showing numerous small streams, transient flows and shocks as well as a few large corotating streams. It is suggested that in moving to 8 AU the largest corotating streams swept up the slower flows (transient and/or corotating streams) and shocks into a relatively thin region in which they coalesced to form a single large‐amplitude compression wave. We refer to this combined process of sweeping and coalescence as "entrainment". The resulting large‐amplitude compression wave is different from that formed by the steepening of a corotating stream from a coronal hole, because different flows from distinct sources, with possibly different composition and magnetic polarity, are brought together to form a single new structure. As a result of entrainment, memory of the sources and flow configurations near the sun is lost. Small‐scale features are erased as the flows move outward and energy is transferred from small scales to large scales by entrainment. Thus in the outer solar system the structure of the solar wind may be dominated by large scale pressure waves (compressions followed by rarefactions) separated by several AU. Beyond several AU most of the compression waves are no longer driven by streams, and the compression waves expand freely. At large distances (≳ 25 AU) they will have interacted extensively with one another producing yet another state of the solar wind, with fewer large‐scale non‐uniformities and more small‐scale non‐uniformities.

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