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A solar wind is a stream of particles (mostly high-energy protons ~ 500 keV) which are ejected from the upper atmosphere of a star (in the case of a star other than the Earth's Sun, it may be called a stellar wind instead).
In 1958, Eugene Parker discovered that a stiff wind blows incessantly from the Sun, filling local interplanetary space with ionized gas (plasma). Before the discovery scientists regarded space as a true vacuum. The discovery forever changed how scientists perceive space and helped explain many phenomena, from geomagnetic storms that knock out power grids on Earth to the formation of distant stars.
Around the 1930s, scientists had determined that the temperature of the solar corona must be a million degrees Celsius because of the way it stood out into space (as seen during total eclipses). Some very clever spectroscopic detective work confirmed this extraordinary temperature. In the mid-1950s the British mathematician Sydney Chapman calculated the properties of a gas at such a temperature and determined it was such a superb conductor of heat that it must extend way out into space, beyond the orbit of Earth. Also in the 1950s, a German scientist named Ludwig Biermann got interested in the fact that no matter whether a comet is headed towards or away from the Sun, its tail always points away from the Sun. Biermann postulated that this happens because the Sun emits a steady stream of particles that push the comet tail away.
Parker realised that the heat flowing from the Sun in Chapman's model and the comet tail blowing away from the Sun in Biermann's hypothesis had to be the result of the same phenomenon. Parker showed that even though the Sun's corona is strongly attracted by solar gravity, it is such a good conductor of heat that it is still very hot at large distances. Since gravity weakens as distance from the Sun increases, the outer coronal atmosphere escapes into interstellar space.
Opposition to Parker's hypothesis on the solar wind was strong. The paper he submitted to the Astrophysical Journal in 1958 was rejected by two reviewers. It was saved by then editor Subrahmanyan Chandrasekhar (who later received the 1983 Nobel Prize in physics).
In the 1960s the hypothesis was confirmed through direct satellite observations of the solar wind, which also made it possible to explain magnetic storms, auroras, and other solar-terrestrial phenomena.
In the solar system, the composition of the solar wind is identical to the Sun's corona, 73% hydrogen and 25% helium with the remainder as trace impurities. The exact composition has not yet been measured. A sample return mission, Genesis, returned to Earth in 2004 and is undergoing analysis, but it was damaged by crash-landing when its parachute failed to deploy on re-entry to Earth's atmosphere.
Near Earth, the velocity of the solar wind varies from 200-889 km/s. The average is 450 km/s. Approximately 800 kg/s of material is lost by the Sun as ejected solar wind, a negligible amount compared to the Sun's light output, which is equivalent to about 4.5 Tg (4.5×109 kg) of mass converted to energy every second.
Since solar wind is a plasma, it carries with it the Sun's magnetic field. Out to a distance of approximately 160 Gm (100,000,000 miles), the sun's rotation sweeps the solar wind into a spiral pattern by dragging its magnetic field lines with it, but beyond that distance solar wind moves outwards without much additional influence directly from the sun. Unusually energetic outbursts of solar wind caused by solar flares and other such solar weather phenomena are known as "solar storms" and can subject space probes and satellites to strong doses of radiation. Solar wind particles trapped in Earth's magnetic field tend to collect within the Van Allen radiation belts and can cause the polar aurora, when they impact with Earth's atmosphere near the poles. Other planets with magnetic fields similar to Earth's also have their own auroras.
The solar wind blows a "bubble" in the interstellar medium (the rarefied hydrogen and helium gas that permeates the galaxy). The point where the solar wind's strength is no longer great enough to push back the interstellar medium is known as the heliopause, and is often considered to be the outer "border" of the solar system. The distance to the heliopause is not precisely known, and probably varies widely depending on the current velocity of the solar wind and the local density of the interstellar medium, but it is known to lie far outside the orbit of Pluto.
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