Solar Wind

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.

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).

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solar wind

Flux of particles, chiefly protons, electrons, and helium nuclei, accelerated by the hot solar corona's high temperatures to speeds high enough to allow them to escape the Sun. Solar flares increase its intensity. The solar wind deflects planets' magnetospheres and the ion tails of comets away from the Sun. The uninterrupted portion of the solar wind continues to travel to a distance of about 110–170 astronomical units, where it cools and eventually diffuses into interstellar space. See also heliopause.

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solar wind [′sō·lər ′wind]

(geophysics)

The supersonic flow of gas, composed of ionized hydrogen and helium, which continuously flows from the sun out through the solar system with velocities of 180 to 600 miles (300 to 1000 kilometers) per second; it carries magnetic fields from the sun.

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Solar Wind 

a continuous radial flow of the plasma of the solar corona into interplanetary space. The formation of the solar wind is due to the energy flux entering the corona from deeper layers of the sun. The energy is apparently carried by magnetohy-drodynamic waves and weak shock waves (seePLASMA and ). To maintain the solar wind, the energy carried by the waves and by thermal conduction must also be transmitted to the outer layers of the corona. The continuous heating of the corona, which has a temperature of 1.5–2 million degrees, is not balanced by the energy loss to radiation, since the density of the corona is low. The excess energy is carried away by the particles of the solar wind.

In essence the solar wind is the continuously expanding solar corona. The pressure of the heated gas causes a steady hydrody-namic outflow of the corona with a gradually increasing speed. At the base of the corona (—10,000 km from the surface of the sun), the particles have a radial velocity of the order of hundreds of meters per second; at a distance of a few radii from the sun, the speed reaches the speed of sound in the plasma (100–150 km/sec); and at a distance of 1 astronomical unit (at the earth’s orbit), the speed of the protons in the plasma is 300–750 km/sec. Near the earth’s orbit the solar-wind plasma temperature, as determined from the thermal component of the particle velocities (from the difference between the particle velocities and the average flow velocity), is ~10⁴°K in quiet periods of the sun and can be as high as 4 × 10⁵°K in active periods. The solar wind contains the same particles as the solar corona, that is, primarily protons, electrons, and helium nuclei (from 2 to 20 percent).

Depending on the state of solar activity, the proton flux near the earth’s orbit ranges from 5 x 10⁷ to 5 x 10⁸ protons/cm²-sec, and the proton density ranges from a few particles to several tens of particles per cubic centimeter. It has been established by means of interplanetary space probes that, up to Jupiter’s orbit, the particle flux density in the solar wind is inversely proportional to the square of the distance from the sun. The energy carried into interplanetary space each second by the solar-wind particles is estimated at 10²⁷-10²⁹ ergs; it may be noted that the energy of the sun’s electromagnetic radiation is ~4 × 10³³ ergs/sec. The amount of mass lost by the sun to the solar wind in a year is ~2 × 10~¹⁴of the mass of the sun.

The solar wind carries away loops of lines of force of the solar magnetic field, since the lines of force are “frozen in” the outflowing coronal plasma (seeMAGNETOHYDRODYNAMICS). The combination of the sun’s rotation with the radial motion of the solar-wind particles gives the lines of force a spiral shape. At the earth’s orbit the magnetic field strength of the solar wind ranges from 2.5 × 10^–6 to 4 × 1(H oersted. In the plane of the ecliptic the field has a sector structure; in each sector the field is directed either away from or toward the sun (Figure 1). In 1963 and 1964, a period of low solar activity, four sectors were observed that lasted 1½ years. As the activity increased, the field structure became more dynamic, and the number of sectors increased.

Figure 1. Sector structure of the interplanetary magnetic field, as found by the American satellite IMP I

The magnetic field carried by the solar wind partially sweeps galactic cosmic rays out of the space about the sun and thereby causes a change in their intensity on earth. The study of variations in cosmic rays permits investigation of the solar wind at great distances from earth and, what is especially important, outside the plane of the ecliptic. It apparently will be possible to discover many properties of the solar wind far from the sun by investigating the interaction of the solar-wind plasma with the plasma of comets, which thus are used, as it were, as space probes. The size of the region, or cavity, occupied by the solar wind is not yet known exactly; by means of space probes the solar wind has been traced out to Jupiter’s orbit. At the boundaries of this region, the dynamic pressure of the solar wind should be balanced by the pressure of the interstellar gas, galactic magnetic field, and galactic cosmic rays.

Figure 2. Localization of the geomagnetic field by the solar wind: (1) lines of force of the solar magnetic field, (2) shock wave, (3) mag-netosphere of the earth, (4) boundary of the magnetosphere, (5) orbit of the earth, (6) particle trajectory

The collision of the supersonic solar-plasma flow with the geomagnetic field gives rise to a standing shock wave upstream from the earth’s magnetosphere (Figure 2). The solar wind flows around the magnetosphere and confines it to a certain region (seeEARTH). The geomagnetic field is compressed sunward by the particle flux in the solar wind; here, the boundary of the magnetosphere is at a distance of ~ 10 earth radii. On the other side, the geomagnetic field is elongated to form a tail extending away from the sun for hundreds of earth radii. In the layer between the shock front and the magnetosphere, there is no quasi-regular interplanetary magnetic field, and the particles move in complicated trajectories; some of the particles may be trapped in the earth’s radiation belts. Variations in the intensity of the solar wind are the principal cause of disturbances in the geomagnetic field (seeVARIATIONS, MAGNETIC), of magnetic storms, of auroras, of heating of the earth’s upper atmosphere, and of a number of biophysical and biochemical phenomena.

Since the sun is a more or less ordinary star, it is natural to assume that other stars also have an outflow of matter like the solar wind. Stellar winds more powerful than the solar wind have been found, for example, for hot stars with a surface temperature of ~30,000°-50,000°K.

The term “solar wind” was proposed in 1958 by the American physicist E. Parker, who worked out the foundations of the hy-drodynamic theory of the solar wind.

REFERENCES

Parker, E. Dinamicheskie protsessy v mezhplanetnoi srede. Moscow, 1965. (Translated from English.)
Solnechnyi veter. Moscow, 1968. (Translated from English.)
Hundhausen, A. Rasshirenie korony i solnechnyi veter. Moscow, 1976. (Translated from English.)

M. A. LIVSHITS and S. B. PIKELNER