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A solar wind is a stream of charged particles (i.e., a plasma) which are ejected from the upper atmosphere of a star. When originating from stars other than the Earth's Sun, it is sometimes called a stellar wind.
It consists mostly of high-energy electrons and protons (about 500 keV) that are able to escape the star's gravity because of their high thermal energy. Many phenomena can be explained by the solar wind, including: geomagnetic storms that knock out power grids on Earth, auroras, why the tail of a comet always points away from the Sun, and the formation of distant stars.
In 1916, Norwegian researcher Kristian Birkeland was probably the first person to successfully predict that in the Solar Wind, "From a physical point of view it is most probable that solar rays are neither exclusively negative nor positive rays, but of both kinds"; in other words, the Solar Wind consists of both negative electrons and positive ions.
Three years later in 1919, Frederick Lindemann also suggested that particles of both polarities, protons as well as electrons, come from the Sun.
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 became 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 pushes the comet's tail away.
Eugene 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 the editor Subrahmanyan Chandrasekhar (who later received the 1983 Nobel Prize in physics).
In January 1959, the first ever direct observations and measurements of strength of the solar wind were made by the Soviet satellite Luna 1. However, the acceleration of the fast wind is still not understood and cannot be fully explained by Parker's theory.
In the late 1990s the Ultraviolet Coronal Spectrometer (UVCS) instrument on board the SOHO spacecraft observed the acceleration region of the fast solar wind emanating from the poles of the Sun, and found that the wind accelerates much faster than can be accounted for by thermodynamic expansion alone. Parker's model predicted that the wind should make the transition to supersonic flow at an altitude of about 4 solar radii from the photosphere; but the transition (or "sonic point") now appears to be much lower, perhaps only 1 solar radius above the photosphere, suggesting that some additional mechanism accelerates the solar wind away from the Sun.
In the solar system, the composition of the solar wind is identical to the Sun's corona: 73% ionized hydrogen and 25% ionized helium with the remainder as trace impurities. These components are present as a plasma, consisting of about 95% singly ionized hydrogen, 4% doubly ionized helium, and less than 0.5% other ions (often called minor ions). The exact composition has been difficult to measure due to large fluctuations. 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, possibly contaminating the solar samples.
Near Earth, the velocity of the solar wind varies from 200 to 889 km/s. The average is 450 km/s. Approximately 1×109 kg/s  of material is lost by the Sun as ejected solar wind, about one-fifth that lost due to fusion, which is equivalent to about 4.5 Tg (4.5×109 kg) of mass converted to energy every second. The total mass loss is equivalent to a lump of Earth-density rock about 125 m across every second, and at that rate the Sun would last for 10 million million (1×1013) years. However, our current understanding of star formation implies that the Sun's solar wind may have been about 1000 times more massive in the distant past, which would seriously affect the history of planetary atmospheres and that of the martian atmosphere in particular.
Since the solar wind is a plasma, it has the characteristics of a plasma, rather than a simple gas. For example, it is highly electrically conductive so that magnetic field lines from the Sun are carried along with the wind. The dynamic pressure of the wind dominates over the magnetic pressure through most of the solar system (or heliosphere), so that the magetic field is pulled into an Archimedean spiral pattern (the Parker spiral) by the combination of the outward motion and the Sun's rotation. Depending on the hemisphere and phase of the solar cycle, the magnetic field spirals inward or outward; the magnetic field follows the same shape of spiral in the northern and southern parts of the heliosphere, but with opposite field direction. These two magnetic domains are separated by a two current sheet (an electric current that is confined to a curved plane). This heliospheric current sheet has a similar shape to a twirled ballerina skirt, and changes in shape through the solar cycle as the Sun's magnetic field reverses about every 11 years.
The plasma in the interplanetary medium is also responsible for the strength of the Sun's magenetic field at the orbit of the Earth being over 100 times greater than originally anticipated. If space were a vacuum, then the Sun's 10-4 tesla magnetic dipole field would reduce with the cube of the distance to about 10-11 tesla. But satellite observations show that it is about 100 times greater at around 10-9 tesla. Magnetohydrodynamic (MHD) theory predicts that the motion of a conducting fluid (e.g. the interplanetary medium) in a magnetic field, induces electric currents which in turn generates magnetic fields, and in this respect it behaves like a MHD dynamo.
Fast and slow solar wind
Outside the plane of the ecliptic the solar wind is steady and rapid, at speeds between 600-800 km/s; this is called the fast solar wind and it is known to emanate from solar coronal holes. In the plane of the ecliptic, near the heliospheric current sheet, the wind is slower, denser, and more variable, with typical speeds between 200 and 600 km/s and daily fluctuations by a factor of two or more. This is called the slow solar wind and its location of origin on the Sun is less well known.
Variability and space weather
The solar wind is responsible for the overall shape of Earth's magnetosphere, and fluctuations in its speed, density, direction, and entrained magnetic field strongly affect Earth's local space environment. For example, the levels of ionizing radiation and radio interference can vary by factors of hundreds to thousands; and the shape and location of the geopause (Earth's bow shock wave in the solar wind) can change by several Earth radii, exposing geosynchronous satellites to the direrct solar wind. These phenomena are collectively called space weather.
Both the fast and slow solar wind can be interrupted by large, fast-moving bursts of plasma called interplanetary coronal mass ejections, or ICMEs. ICMEs are the interplanetary manifestation of solar coronal mass ejections, which are caused by release of magnetic energy at the Sun. ICMEs are often called "solar storms" or "space storms" in the popular media. They are sometimes, but not always, associated with solar flares, which are another manifestation of magnetic energy release at the Sun. ICMEs cause shock waves in the thin plasma of the heliosphere, launching electromagnetic waves and accelerating particles (mostly protons and electrons) to form showers of ionizing radiation) that precede the ICME.
When an ICME impacts the Earth's magnetosphere, it temporarily deforms the Earth's magnetic field, changing the direction of compass needles and inducing large electrical ground currents in Earth itself; this is called a magnetic storm and it is a global phenomenon. ICME impacts can induce magnetic reconnection in Earth's magnetotail (the midnight side of the magnetosphere); this launches protons and electrons downward toward Earth's atmosphere, where they form the aurora.
ICMES are not the only cause of space weather. Different patches on the Sun are known to give rise to slightly different speeds and densities of wind depending on local conditions. In isolation, each of these different wind streams would form a spiral with a slightly different angle, with fast-moving streams moving out more directly and slow-moving streams wrapping more around the Sun. Faster-moving streams tend to overtake slower streams that originate westward of them on the Sun, forming turbulent corotating interaction regions that give rise to wave motions and accelerated particles, and that affect Earth's magnetosphere in the same way as, but more gently than, ICMEs.
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.
- Are the Solar Corpuscular Rays that penetrate the Earth's Atmosphere Negative or Positive Rays? in Videnskapsselskapets Skrifter, I Mat -- Naturv. Klasse No.1, Christiania, 1916.
- Philosophical Magazine, Series 6, Vol. 38, No. 228, December, 1919, 674 (on the Solar Wind)
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