|ESA Science & Technology||05-Jul-2005 13:21:28|
The existence of the solar wind was first put forward by Ludwig Biermann in 1951, who suggested that comet tails are always directed away from the Sun because they are pushed by a continuous flow of charged particles streaming out from the Sun. It was already known that the Sun's atmosphere, or corona, is very hot - a million degrees or more - and in 1957 Sydney Chapman showed that it should extend far out into space. One year later, Eugene Parker, the "father" of solar wind science, proposed that this hot corona not only extends far into space, but escapes the Sun's gravity as a "supersonic" wind, dragging the solar magnetic field with it as it expands. His ideas were greeted with scepticism initially, but supporting evidence soon followed from space probes like Mariner 2.
This stream of outflowing particles, however, is far from uniform or constant: it comes in two varieties, "fast" (about 750 km/s) and "slow" (350 to 400 km/s), which vary with solar latitude and level of solar activity. During periods of low activity, the fast wind emanates from the poles and the slow wind from near the equator. Ulysses found that they meet at a clearly defined boundary. Somewhat counter-intuitively, the fast wind comes from relatively cool gaps in the corona, called coronal "holes" and the slow wind from hotter coronal "streamers". During periods of low activity, holes tend to develop around the poles and streamers around the equator. But during high activity, the pattern of holes and streamers becomes more jumbled.
Three instruments on Ulysses measure the magnetized solar wind:
SWOOPS (Solar Wind Observations Over the Poles of the Sun)
SWICS (Solar Wind Ion Composition Spectrometer)
URAP (Unified Radio and Plasma Wave experiment)
MAG (Vector Helium and Flux Gate MAGnetometers)
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