Composition variations in fast solar wind streams

Abstract

The Ulysses spacecraft has now completed its first revolution around the Sun on its nearly-polar orbit. Thereby it has traversed the extended high speed streams from the polar coronal holes (south in 1993/94, north in 1995/96) which were well-developed during that time of close to minimal solar activity. It is evident that the fluctuations of both the kinetic and the compositional parameters are much weaker in the high-speed streams than they are in the slow solar wind, leading Bame to use the term “structure-free” for describing it. It was only the extended time periods Ulysses spent in the polar streams that led to the detection of some structure, the microstreams. From remote observations of the Sun it is clear that the high latitude corona is quite unstructured. The most remarkable features are the polar plumes, which are well detectable because of their higher density and brightness. Also, they are characterized by a difference in composition relative to the coronal hole plasma. These features should in principle be observable in interplanetary space, e.g. by the SWICS mass spectrometer, in the form of abundance variations of heavy ions as well as variations in their charge state composition, which serves as a proxy for the coronal temperature at the site where the stream originated. Using the unique data set of SWICS we examine to what extent polar plumes contribute to fast, coronal hole associated wind. We also study the possible connection between microstreams and polar plumes. © 1999 American Institute of Physics.