Solar Wind
The solar wind is a continuous outward flow of plasma (ionized gas) from the Sun 's corona, its outer atmosphere.
The Sun's corona extends millions of kilometers above the surface of the Sun (if a ball of gas can be said to have a "surface"). At these extreme altitudes, the gases, which have temperatures of millions of degrees Kelvin, are not contained by the Sun's gravity. The wind, composed primarily of protons and electrons, streams far beyond Earth's orbit and into interstellar space.
The existence of the solar wind had been suspected since the sixteenth century. It was noticed that as a comet neared the Sun, its tail formed. This tail did not always follow behind the comet; often it preceded it. One thing was certain: no matter where the comet was in its orbit, the tail always pointed away from the Sun. Isaac Newton thought the Sun produced a "repulsive" force that caused the tail to form. It is now known that comets have two tails, one formed by radiation pressure acting on the dust grains surrounding the comets, the other formed by the solar wind's interacting with ionized gas.
Modern understanding of the solar wind began with Edward A. Milne (1896-1950). He studied the solar atmosphere and determined the temperatures at various depths. His work showed that particles could be ejected from the Sun at speeds of 621 miles (1,000 km) per second. This established the concept of the so-called solar wind.
Direct observational evidence of the solar wind did not occur until the space age. In 1959, Eugene N. Parker (1927-), whose field of study was the motion of high-energy particles in magnetic fields, predicted that the charged particles emitted by the Sun would follow along its magnetic lines of force. This was shown to be the case by the Venus probe Mariner 2 in 1962. Bruno B. Rossi (1905-) performed rocket experiments above the Earth's atmosphere with the intention of detecting cosmic rays. He also found the solar wind, flowing outward from the Sun in all directions. Measurements of the velocity of the solar wind shows it varies between 185 and 435 miles (300 and 700 km) per second. Occasional "gusts" reaching speeds of 621 miles (1,000 km) per second have been detected.
The cause was a mystery until x-ray detectors aboard rockets discovered coronal holes from which particles streamed, unaffected by the confining loops in the Sun's magnetic field.
The solar wind occasionally has a spectacular interaction with the Earth's magnetic field. As the charged particles approach the Earth, our magnetic field deflects them. However, during periods of intense solar activity, the commensurate increase in the solar wind can cause the ions to break through this magnetic sheath and stream down toward the Earth's north and south magnetic poles. As they enter the atmosphere, they ionized the oxygen and nitrogen in the atmosphere; the recombination of these ionized atoms produces a shimmering display of light in the upper atmosphere. These displays are called aurorae, also known as the Northern and Southern Lights. The Earth is not the only planet so affected; aurorae were also detected in the atmosphere of Jupiter by the Voyager spacecraft. The solar wind streams outward indefinitely, although there is a point, called the heliopause, where it becomes so diffuse and weak that the interstellar medium between the stars becomes the dominant source of matter.
In 1981, the Pioneer 10 spacecraft, the first satellite to leave the solar system, continued to detect the solar wind at a distance of 24 astronomical units or 2.2 billion miles (3.6 billion km) in 1981. Somewhere deep in space, perhaps 100 astronomical units from the Sun (which is well beyond the orbits of Neptune and Pluto), Pioneer will eventually reach the heliopause, at which point most astronomers would agree the spaecraft had formally left the solar system and become a truly interstellar craft.
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