Solar wind

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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 solar system, the composition of this plasma is identical to the Sun's corona, 73% hydrogen and 25% helium with the remainder as trace impurities, and is ionized. 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 reentry 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.

Since solar wind is a plasma, it carries with it the Sun's magnetic field. Out to a distance of approximately 160,000,000 km (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 Aurora borealis and the Aurora australis, 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.

See also: magnetopause, magnetosphere, ionosphere, shock wave ms:Angin suria da:Solvind de:Sonnenwind es:Viento solar fr:Vent solaire it:Vento solare he:רוח השמש la:Ventus solaris nl:Zonnewind ja:太陽風 fi:Aurinkotuuli sv:Solvind th:ลมสุริยะ zh:太阳风