Solar wind

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A stream of particles that is continually ejected from the surface of the Sun. The composition of the solar wind is identical to the sun's outer atmosphere, 73% hydrogen and 25% helium with the remainder as trace impurities, and is ionized. The velocity of solar wind varies, ranging between 450,000 miles per hour and 2,000,000 miles per hour depending on current solar activity. Approximately 3000 tons of material is lost from the sun every hour as solar wind.

Since solar wind is a plasma, it carries with it the sun's magnetic field. Out to a distance of approximately 100 million 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 the Earth's magnetic field give rise to the Aurora borealis and the Aurora australis when they impact with the 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 rareified 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 precicely 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 magnetosphere, and ionosphere.