Solar wind is a continuous flow of particles from the sun. It results chiefly from the expansion of gases in the corona, the outermost atmosphere of the sun. The corona’s high temperature, which averages about 4,000,000 °F (2,200,000 °C), heats the gases and causes them to expand. Many of the gas atoms collide as they are heated. As a result, they lose their electrons, which have a negative electric charge. The atoms thereby become ions (electrically charged atoms) with a positive charge. The electrons and ions—mostly hydrogen ions—make up the solar wind.
The velocity of the solar wind ranges from 155 to 625 miles (250 to 1,000 kilometers) per second. The solar wind has a density of about 82 ions per cubic inch (5 ions per cubic centimeter). It is responsible for a variety of occurrences in the solar system. For example, the magnetosphere, a region of strong magnetic forces surrounding Earth, is pushed into a teardrop shape by the solar wind as it streams past the earth. The magnetosphere prevents particles of the solar wind from reaching the surface of Earth. The blowing of the solar wind against a comet produces an ion tail, which is one of the various types of tails that comets have. Ion tails are long and straight and consist of ionized material that the solar wind has blown off the comet.
In 1959, the Soviet Luna 2 spacecraft confirmed the existence of the solar wind and made the first measurements of its properties. Several American spacecraft have also had equipment that studied this wind. The Apollo 11 and Apollo 12 astronauts placed large metal screens called foils on the moon to collect solar wind particles. The moon has no magnetosphere, and so the particles reach its surface. The astronauts brought the foils back to the earth for analysis. In 1984, the Active Magnetospheric Particle Tracer Explorer (AMPTE) space mission produced an artificial comet as part of a series of experiments designed to study the solar wind and magnetosphere. The mission was conducted by the United Kingdom, the United States, and West Germany.
Studies of stars other than the sun show that gases also stream away from them. As a result, astronomers believe many stars produce winds, called stellar winds, that resemble the solar wind.
See also Heliosphere; Magnetic storm.
