INTERPLANETARY MEDIUM

thinly scattered matter that exists between the planets and other bodies of the solar system, as well as the forces (e.g., magnetic and electric) that pervade this region of space. The material components of the interplanetary medium consist of neutral hydrogen, plasma gas of electrically charged particles (chiefly protons and electrons produced by the ionization of hydrogen released by the Sun), cosmic rays, and dust particles. Extremely small amounts of neutral hydrogen have been detected throughout much of interplanetary space; e.g., at the orbit of the Earth the concentration of nonionized hydrogen is about 0.01 atom per cubic centimetre. Some of the neutral hydrogen that enters the solar system from interstellar space is reduced in quantity by photoionization and charge exchange with the solar wind plasma. The solar wind is a flow of completely ionized gas with a density of about 1,000,000 protons and 1,000,000 electrons per cubic centimetre that continuously expands outward through the solar system from the Sun's corona. This outflow of plasma transports the magnetic fields of force present at the surface of the Sun radially away from it and is responsible for deflecting both the tail of the Earth's magnetosphere and the tails of comets away from the Sun. Most of the cosmic rays in the solar system originate beyond its boundaries, presumably in exploding stars called supernovae. Some are emitted by the Sun during solar flares. All such primary cosmic rays consist of atomic nucleiprincipally the nuclei of hydrogen atoms (protons) and substantial numbers of nuclei of such heavier elements as lithium, boron, neon, and sulfurand some electrons. Cosmic rays are distinguished from the particles of the solar wind by their extremely high speeds (i.e., they travel near the speed of light) and their kinetic energies, which measure millions of electron volts as opposed to the 800 electron volts of solar wind particles. Relatively small amounts of dust exist in the solar system, most of which appear to be orbiting the Sun in planes near the zone of planetary orbits. Much of the dust is thought to have been produced by collisions between asteroids. The dust grains vary considerably in density, ranging from 2 to 8 grams per cubic centimetre. Some originating from comets have a lower density. Particles that measure one micrometre or less in size are quickly blown out of the solar system by radiation pressure from the Sun. About 16,000 tons of interplanetary dust particles of larger size are estimated to enter the Earth's atmosphere each year. The magnetic field lines of the Sun are carried outward into space by the solar wind, but the lines of force remain attached to the surface of the Sun. Because of its rotation, the lines assume a spiral structure. Closely associated with the interplanetary magnetic field are electric forces that act to attract or repel charged particles. The intensity of the electric field is directly proportional to the intensity of the magnetic field and to the sine of the angle between the direction of the solar wind and the direction of the magnetic field. Besides the Sun, the planets Earth, Mercury, Jupiter, Saturn, Uranus, and Neptune are known to have a measurable magnetic field.