|The Large Scale Solar Wind|
Does the solar wind vary in time?
Sunspots(a,b) have been known to vary temporally since the days of Johannes Kepler(c) and Tycho Brahe(d). Their frequency varies with an 11-year period, but during the Maunder minimum (1645-1715), named after the Englishman who reported the phenomenon to the Royal Astronomical Society in 1890, there were very few sunspots, whereas today we have a very active period. The 11-year reversal of sunspot group polarities implies a 22-year magnetic solar oscillation, as we mentioned previously.
(a): Close-up of a sunspot, showing the dark umbra and lighter penumbra
(b): SOHO image of a group of sunspots
(c): Johannes Kepler (1571-1630) described how the planets move, by using simple mathematics. Kepler used Brahe's (to whom he was an assistant) data to show that planets move in ellipses around the Sun, that planets move proportionally faster in their orbits when they are nearer the Sun , and that more distant planets take proportionally longer to orbit the Sun.
(d): Tycho Brahe (1546-1601) was the most accurate astronomical observer of his time.
The behavior of the solar wind depends on the magnetic activity. During solar minimum(e), the "Quiet Sun" has large, long-lived polar coronal holes that produce a very steady, high-speed wind, which extends over most of the heliosphere. It travels at seven to eight hundred km/s over the poles and 400 km/s in the ecliptic. The wind has possibly higher dynamic pressure over the poles than in the ecliptic plane.
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The Ulysses spacecraft made a latitudinal survey of the solar wind during both solar minimum and solar maximum. The wind velocity and magnetic field strength and polarity are superimposed on an image of the sun during solar minimum. The fast solar wind (~800 km/s) emerges from the coronal holes at the poles and exhibits only small variations. In the ecliptic, the solar magnetic field tends to remain closed, and any emerging wind is primarily slow.
The interplanetary magnetic field displays different behavior over the poles, where there are low frequency, large amplitude Alfvén waves. As the solar cycle(f) progresses to solar maximum, the polar coronal holes shrink and there is a tangled mix of active regions, with closed fields, and isolated coronal holes.
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YOHKOH images of the Sun from 1991-1995 in four-month increments, illustrating the changing character of the Sun from solar minimum to solar maximum
The solar maximum(g,h) wind contains fewer streams, more slow wind, and there are no longer two classes of wind-instead the wind over the poles and in the ecliptic is very similar.
(g): Sketch of the inferred solar magnetic field during solar maximum. Observe the highly complex structure and the absence of any large coronal holes.
(h): The Ulysses spacecraft made a latitudinal survey of the solar wind during both solar minimum and solar maximum. The wind velocity and magnetic field strength and polarity are superimposed on an image of the sun during solar maximum. The wind speed, although subject to large fluctuations, is approximately 400 km/s at all latitudes.
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