Abstract
This paper focuses on the interactions between the fast solar wind from coronal holes and the intervening slower solar wind, leading to the creation of stream interaction regions that corotate with the Sun and may persist for many solar rotations. Stream interaction regions have been observed near 1 AU, in the inner heliosphere (at sim 0.3–1 AU) by the Helios spacecraft, in the outer and distant heliosphere by the Pioneer 10 and 11 and Voyager 1 and 2 spacecraft, and out of the ecliptic by Ulysses, and these observations are reviewed. Stream interaction regions accelerate energetic particles, modulate the intensity of Galactic cosmic rays and generate enhanced geomagnetic activity. The remote detection of interaction regions using interplanetary scintillation and white-light imaging, and MHD modeling of interaction regions will also be discussed.
Highlights
A solar wind stream interaction region (SIR) is formed by the interaction of a stream of high-speed solar wind originating in a “coronal hole” at the Sun with the preceding slower solar wind
Belcher and Davis (1971) concluded that since these fluctuations were present throughout high-speed streams, and were propagating outwards, they were generated at the Sun rather than by the stream interaction, in contrast to the earlier proposal of Coleman (1968) that these fluctuations were turbulence generated by the shear in the solar wind speed across the interaction region
This indicates that the slow solar wind is deflected toward the west ahead of the interface, while the fast solar wind is deflected toward the east following the interface, passing through the radial direction in the vicinity of the stream interface
Summary
A solar wind stream interaction region (SIR) is formed by the interaction of a stream of high-speed solar wind originating in a “coronal hole” at the Sun with the preceding slower solar wind. Since the source coronal holes tend to be long-lived, often persisting for many months, the interaction regions and high-speed streams tend to sweep past an observer at regular intervals of approximately the solar rotation period (∼ 27 days as viewed from Earth). The increases in plasma density N and magnetic field strength B are indicative of compressed plasma in the vicinity of the positive gradient in the solar wind speed (Vw) at the stream leading edge and form the interaction region, which follows an approximately Archimedean-spiral configuration. At the time of the vertical yellow line, both STEREO spacecraft were predicted to be encountering interaction regions associated with the leading edges of two different streams, indicated by the spiral density enhancements sweeping past each spacecraft in the equatorial plane figure. The reader is invited to provide feedback and other material which will help to increase the usefulness of this review
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