|The Large Scale Solar Wind|
Is the solar wind featureless on large scales?
No. As the Sun rotates, the wind in the ecliptic plane(a) will be a mixture of fast flow from coronal holes and slow wind from active regions.
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Clementine star tracker image of the ecliptic plane, with (from right to left) the Moon, lit by Earthshine, the Sun's corona rising over the dark limb of the Moon, and the planets Saturn, Mars, and Mercury.
As the wind expands radially, the flow follows the same spiral pattern, but fast wind "catches up" to slow wind and crashes into it. At the interface between the fast and slow wind (b), a large pressure pulse forms and eventually steepens to create a forward and reverse shock pair. The thin boundaries separating the streams are called co-rotating interaction regions or CIRs.
(b): Cartoon showing the interaction of a fast and a slow stream. The fast stream runs into the slow wind, forming a compression region between the two, which results eventually in the formation of a forward-reverse shock pair.
(c): A numerical simulation of a CIR. A fast stream catches up and collides with a slow stream, forming initially a pressure pulse, which then steepens to form a forward-reverse shock pair.
Voyager has studied the formation and evolution of CIRs (d) throughout the heliosphere.
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Voyager data showing a forward-reverse shock pair. The vertical lines identify the location of the shock pair in the data. The solar wind speed, density, and magnetic field intensity serve to identify the shocks very clearly.
Voyager discovered that groups of CIRs themselves can interact, catching up to one another sometimes, and merging, which leads to merged interaction regions, or MIRs (e). This typically occurs beyond some 10AU. Then complex interactions and structures can persist to great distances in the heliosphere. Burlaga estimates that almost all the equatorial wind will have passed through CIRs by approximately 25AU.
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Five streams merged to form one compound stream. Figure courtesy of Burlaga et al., JGR 91, p13331, 1986.
Even more dramatic large-scale disturbances of the solar wind were discovered by a combination of Voyager and Pioneer observations. Voyager and Pioneer observed almost co-incident massive enhancements in solar wind density, velocity, and magnetic field strength, at opposite sides of the Sun (f). Shortly before these observations, widely distributed disturbances of the Sun had occurred, which drove numerous shock waves into interplanetary space. Their subsequent merging eventually created an enormously extended shock-like structure of global extent, called a global merged interaction region, or GMIR.
(f): Cartoon schematic illustrating the interaction of the solar wind and the interstellar medium at the largest scales. The termination shock, heliopause, and bow shock, which we define and discuss, are labeled. The Voyager and Pioneer spacecraft positions are shown on opposite sides of the Sun.
(g): A simulation of a GMIR propagating through the heliosphere and eventually colliding with the boundaries separating the solar wind and the interstellar medium.
Because the fast and slow winds have distinct sources and are different in composition, the physical processes in the many fast and slow streams formed in the solar wind are often quite different. Studying the composition of the solar wind (h) can therefore provide valuable clues into the corona and associated heating mechanisms.
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Solar wind data of the magnesium to oxygen ratio, Mg/O, showing that high ratios correspond to slow speed streams and low ratios to high speed streams.
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