The solar wind is a continuous outward flow of ionized gas ( plasma) from the corona of the Sun, which extends beyond the Earth's orbit and into interstellar space. Measurements from space probes since 1962 — the date of its first direct detection — show that its velocity at the Earth's orbit varies between 300 and 700 km/sec (185 and 435 mi/sec).

The charged particles of the solar wind consist mainly of protons and electrons, with 5% by number of helium nuclei, along with the nuclei of hydrogen and oxygen atoms. It is theorized that they are pushed along by magnetic waves generated within the Sun's corona. The total densities of the stream range between 1 and 10 particles/cm3. The magnetic field carried with the wind has a mean intensity at the Earth's orbit of about 5 X 10 -5 gauss, compared with a maximum magnetic field for the Earth of 0.3 gauss. Probe studies in the ecliptic plane (the line of sight from the Earth to the Sun) show that the mean outflow velocity increases with distance, at least to approximately the Earth's orbit. Relatively little is known about the wind velocity, density, and time dependence out of the ecliptic plane except indirectly, from study of its influence on comet tails. This fact makes it difficult to derive an accurate figure for the rate of total mass loss from the Sun, although the loss is estimated at approximately 10 -13 solar masses per year. This rate is very low compared to that measured in stellar winds of some hot young stars, where loss rates of 10 -6 solar masses per year have been inferred from observations. In August 2001, NASA launched a robotic spacecraft named Genesis to conduct a three-year study of the solar wind, collecting minute samples to be returned to Earth for analysis. The samples are to be measured to gain a precise knowledge of the isotopic composition of the plasma.

For interactions of the solar wind with the Earth's magnetic field, see Earth, geomagnetic field of; magnetosphere; and SOHO

Peter Foukal

Bibliography: Akasofu, S.-I., and Kamide, Y., eds., The Solar Wind and the Earth (1987); Burch, J. L., Solar System Plasmas in Space and Time (1994); Gombosi, Tamas I., Physics of the Space Environment (1998); Jokipii, J. R., et al., eds., Cosmic Winds and the Heliosphere (1997); Tu, C.-Y., and Marsch, E., MHD Structures, Waves and Turbulence in the Solar Wind (1995).