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 Sol system, the composition of this plasma is identical to Sol's corona, 73% hydrogen and 25% helium with the remainder as trace impurities, and is ionized. Near Earth, the velocity of the solar wind varies from 200km/s-889km/s. The average is 450 km/s. From Sol, approximately 800 kg/s of material is lost 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