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| The Sun radiation and the solar wind |
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Physical processes on the Earth require an energy source to drive them. Not surprisingly, our main energy source is the Sun. After all, the Sun powers the food we eat, the coal and gasoline we burn; none would exist without the energy provided by sunlight.
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The diameter of the Sun is about 400 times greater than that of the moon. By coincidence, the Sun is also about 400 times farther from the Earth than is the Moon. Compare the angle subtended by the moon. Note that this angle leads to a solar eclipse.
In the first half of the twentieth century, many believed that auroras and geomagnetic storms were caused by electrons coming from the Sun. The Norwegian scientist Kristian Birkeland proposed that as early as 1896. Today we call this streaming, ionized plasma from the Sun, the solar wind. The early models only included beams of solar particles during disturbed periods. However, in 1957, Hannes Alfvén postulated that these particle-beams were magnetized and that the solar flow draped that magnetic field over the planet, forming a long magnetic tail downstream in the anti-solar direction. L. Biermann’s study of comet tails correctly interpreted their direction as due to an interaction with solar plasma with a velocity of about 450 km/s. |
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The term solar wind was introduced in 1958 by Professor E. Parker. In 1962 he showed that the density of the magnetized plasma, in order to be consistent with the geomagnetic records, should seldom exceed 30 particles per cubic centimetre (p/cm3).
That was the dawn of the space age, as confirmation was not long in coming. Instruments on the Soviet Lunar Spacecraft, developed by Konstantin I. Gringauz, provided the first measurements of the solar wind in1962. Electrons from the Sun travel in the company of an equal number of ions, or else the beam would disrupt.
The plasma of the corona is far too hot for the Sun's gravity to hold it captive. Instead, it constantly expands away from the Sun and is blown off as the solar wind, filling the solar system and reaching Earth, and far beyond, past Pluto's orbit. |
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The eruption which Carrington saw is now called a solar flare, and using special light filters, flares of all sizes can be regularly monitored; only very rarely can some be seen in unfiltered light. Solar flares are violent magnetic storms on the Sun that spew out enormous quantities of charged particles. They do occur near sunspots, and are definitely associated with magnetic storms. The association was better understood after the discovery of coronal mass ejections or CME’s, huge bubbles of hot gas, blown away from the Sun. Their first extensive observations were made in 1973 from the space station Skylab. Maximum auroral and geomagnetic activity occurs after CMEs.
The 11-year sunspot cycle was discovered by a German pharmacist. Heinrich Schwabe. The glare of the Sun would make it hard to see such a planet (except during a total eclipse), so Schwabe tried to catch it while passing in front of the Sun's disk, the way Venus did during its recent travel. |
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Solar wind properties: Interplanetary space, not long ago believed to be empty of matter, is filled with a matter of solar origin, streaming past the Earth and the planets at a velocity exceeding the thermal velocity. This streaming plasma is the connecting link between solar activity and geomagnetic disturbances, such as auroras and magnetic storms.
The solar plasma also carries with the ejected matter a magnetic field called the Interplanetary Magnetic Field (IMF). Several of the early speculations were confirmed when interplanetary space became accessible to in situ measurements, but even under highly idealized assumptions the theory of the solar wind is quite complicated. |
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