Solar Wind

The area between the Sun and the planets, the interplanetary medium, is a turbulent area dominated by a constant stream of hot plasma that billows out from the Sun's corona. This hot plasma is called the solar wind.

The first indication that the Sun might be emitting a "wind" came in the seventeenth century from observations of comet tails. The tails were always seen to point away from the Sun, regardless of whether the comet was approaching the Sun or moving away from it.

Basic Characteristics

The solar wind is composed mostly of protons and electrons but also contains ions of almost every element in the periodic table. The temperature of the corona is so great that the Sun's gravity is unable to hold on to these accelerated and charged particles and they are ejected in a stream of coronal gases at speeds of about 400 kilometers per second (1 million miles per hour). Although the composition of the solar wind is known, the exact mechanism of formation is not known at this time.

The solar wind is not ejected uniformly from the Sun's corona but escapes primarily through holes in the honeycomb-like solar magnetic field. These gaps, located at the Sun's poles, are called coronal holes. In addition, massive disturbances associated with sunspots , called solar flares , can dramatically increase the strength and speed of the solar wind. These events occur during the peak of the Sun's eleven-year sunspot cycle.

The solar wind affects the magnetic fields of all planets in the solar system. The interaction of the solar wind, Earth's magnetic field, and Earth's upper atmosphere causes geomagnetic storms that produce the awe-inspiring Aurora Borealis (northern lights) and Aurora Australis (southern lights).

Undesirable Consequences

Although the solar wind produces beautiful auroras , it can also cause a variety of undesirable consequences. Electrical current surges in power lines; interference in broadcast of satellite radio, television, and telephone signals; and problems with defense communications are all associated with geomagnetic storms. Odd behavior in air and marine navigation instruments have also been observed, and geomagnetic storms are known to alter the atmospheric ozone layer and even increase the speed of pipeline corrosion in Alaska. For this reason, the U.S. government uses satellite measurements of the solar wind and observations of the Sun to predict space weather.

Major solar wind activity is also a very serious concern during spaceflight. Communications can be seriously disrupted. Large solar disturbances heat Earth's upper atmosphere, causing it to expand. This creates increased atmospheric drag on spacecraft in low orbits, shortening their orbital lifetime. Intense solar flare events contain very high levels of radiation. On Earth humans are protected by Earth's magnetosphere , but beyond it astronauts could be subjected to lethal doses of radiation.

There have been a number of scientific missions that have enabled scientists to learn more about the Sun and the solar wind. Such missions have included Voyager, Ulysses, SOHO, Wind, and POLAR. The latest mission, Genesis, was launched in August 2001 and during its two years in orbit it will unfold its collectors and "sunbathe" before returning to Earth with its samples of solar wind particles. Scientists will study these solar wind samples for years to come.

see also Solar Particle Radiation (volume 2); Space Environment, Nature of the (volume 2); Sun (volume 2).

Alison Cridland Schutt

Bibliography

Kaler, James B. Extreme Stars. Cambridge, UK: Cambridge University Press, 2001.

solar wind stream of ionized hydrogen—protons and electrons—with an 8% component of helium ions and trace amounts of heavier ions that radiates outward from the sun at high speeds. The continuous expansion of the solar corona into the surrounding vacuum of space carries away from the sun about 1 million tons of gas per sec; this blows out like a wind through the solar system. During the days of quiet sunspot activity the wind at the sun has an approximate density of 1 billion atoms per cc and a temperature of about 1 million degrees Fahrenheit. During relatively quiet periods, the wind moves outward from the sun at velocities of 220 to 440 mi (350 to 700 km) per sec (averaging about 1 million mph/1.6 million kph). Near the earth it has a density ranging from 3 to 6 atoms per cc, a velocity of 450 mi (700 km) per sec, and a temperature of about 1,300°F (700°C); during periods of greater sunspot activity it shows corresponding increases in density, temperature, and velocity—reaching speeds of 2 million mph (3.2 million kph). The increased velocity is attributed to acceleration caused by magnetic waves spiraling from the sun. The wind is believed to extend out to between 100 and 200 AU (1 AU is the mean distance between the earth and the sun), far beyond Pluto (at 39 AU), where it is dispersed in the interstellar gases. Information from the Voyager space probes about the region known as the heliosheath, where the solar wind is slowed to subsonic speeds and no longer pushes outward, indicates that it is turbulent, marked by a magnetic bubble froth produced by the interaction of the solar wind and the interstellar medium.

Many effects result from the solar wind. The characteristic that a comet tail always points away from the sun is explained by the pressure of the wind pushing it out. The intensity of the cosmic rays in the inner part of the solar system is reduced by the magnetic fields carried on the wind, which tend to deflect the rays, thus providing a shield against that radiation. The interaction of the wind with the earth's magnetic field is responsible in part for such phenomena as auroras and geomagnetic storms.

Bibliography: See J. R. Jokipii and C. P. Sonett, ed., Cosmic Winds and the Heliosphere (1997).

solar wind The continuous outflow of ionized gas from the Sun's corona. The solar wind consists of electrons, protons, and (to a lesser extent) the nuclei of elements such as helium. As the gas expands into interplanetary space it carries with it magnetic field lines that are twisted into a spiral pattern by the Sun's rotation. Three distinct components to the solar wind can be identified: the fast and slow solar wind streams, and the transient wind due to coronal mass ejections (CMEs). The fast wind arises from coronal holes where magnetic field lines in the corona directly open out into space. It has generally very stable properties, with speeds at 1 AU of 700–800 km/s. The slow solar wind arises from regions where closed magnetic field structures occur in the Sun's corona; average speeds at 1 AU are 300–400 km/s. It shows more variability in terms of temperature, density, and element composition than the fast solar wind. CMEs can travel outwards from the Sun at velocities up to 3000 km/s, and have enhanced density and magnetic fields over the background solar wind. At solar minimum, when the Sun's global magnetic has its simplest form, the slow solar wind is concentrated in the ecliptic plane around the heliospheric current sheet. If the current sheet is tilted relative to the ecliptic plane, then the fast wind streams can be detected at Earth on a repeating cycle of 27 days as the Sun rotates. At solar maximum, the structure is more complex, with fast solar wind streams interspersed among the slow solar wind which extends over all latitudes. Both the fast and slow solar wind are accelerated to supersonic speeds within a few solar radii of the Sun's surface. The solar wind interacts with the Earth and other planets with magnetic fields to produce their magnetospheres. The solar wind extends about 100 AU from the Sun. Its boundary is marked by the heliopause.

solar wind General term for the stream of high-energy particles (mainly protons, electrons, and alpha particles) emitted by the Sun. The particles have velocities of hundreds of kilometres per second and ‘wind’ strength is thought to be greatest during periods of maximum solar activity. In the neighbourhood of the Earth the solar wind has velocities in the range of 300–500 km/sec and an average density of 10⁷ ions/m³. See also COSMIC RADIATION.

solar wind Particles, mostly protons and electrons, accelerated by high temperatures of the solar corona to velocities great enough to allow them to escape from the Sun's gravity. The solar wind deflects the tail of the Earth's magnetosphere and the tails of comets away from the Sun.