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History
Properties In the heliosphere, the composition of the solar wind is identical to the Sun's corona: By mass, 73% ionized hydrogen and 25% doubly 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 routinely measured on Ulysses and ACE, two spacecraft carrying a Solar Wind Ion Composition Spectrometer. Unexpectedly, the solar wind composition shows substantial composition, likely directly reflecting the physics of the underlying corona. The first detailed composition measurements were performed by Geiss on the Moon, which was part of the first Moon-landing. Solar wind was collected using a specially prepared metal-foil and then brought back for analysis. A similar technique was recently pursued using a robotic approach: 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. heliospheric current sheet results from the influence of the Sun's rotating magnetic field on the Plasma (physics)|plasma in the interplanetary medium http://quake.stanford.edu/~wso/gifs/HCS.html 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 magnetic 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 magnetic 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. Effects on the planets Mercury, the nearest planet to the Sun, bears the full brunt of the solar wind. Any atmosphere that this moon-like world may once have had has long been swept away, leaving its surface bathed in radiation. Mars is larger than Mercury and four times further from the Sun, and yet even here it is thought that the solar wind has stripped away up to a third of its original atmosphere, leaving a bale 100 times thinner than our own. Venus, the nearest planet to the Earth, has an atmosphere 100 times thicker than our own. Modern space probes have discovered a comet-like tail that stretches back to the orbit of the Earth (Grünwaldt 1997.) The clouds on Venus are also being eroded by the solar wind. Earth itself is protected from the solar wind by its magnetic field, which deflects charged particles. We only notice the solar wind when it is strong enough to deform this magnetic field, causing phenomena such as geomagnetic storms and the aurora. 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 direct 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 geomagnetic 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. Outer limits 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 Notes | ||||||||||||
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