solar wind

solar wind The continuous outflow of ionized gas from the Sun's corona. The solar wind consists of electrons, protons, and (to a lesser extent) the nuclei of elements such as helium. As the gas expands into interplanetary space it carries with it magnetic field lines that are twisted into a spiral pattern by the Sun's rotation. Three distinct components to the solar wind can be identified: the fast and slow solar wind streams, and the transient wind due to coronal mass ejections (CMEs). The fast wind arises from coronal holes where magnetic field lines in the corona directly open out into space. It has generally very stable properties, with speeds at 1 AU of 700–800 km/s. The slow solar wind arises from regions where closed magnetic field structures occur in the Sun's corona; average speeds at 1 AU are 300–400 km/s. It shows more variability in terms of temperature, density, and element composition than the fast solar wind. CMEs can travel outwards from the Sun at velocities up to 3000 km/s, and have enhanced density and magnetic fields over the background solar wind. At solar minimum, when the Sun's global magnetic has its simplest form, the slow solar wind is concentrated in the ecliptic plane around the heliospheric current sheet. If the current sheet is tilted relative to the ecliptic plane, then the fast wind streams can be detected at Earth on a repeating cycle of 27 days as the Sun rotates. At solar maximum, the structure is more complex, with fast solar wind streams interspersed among the slow solar wind which extends over all latitudes. Both the fast and slow solar wind are accelerated to supersonic speeds within a few solar radii of the Sun's surface. The solar wind interacts with the Earth and other planets with magnetic fields to produce their magnetospheres. The solar wind extends about 100 AU from the Sun. Its boundary is marked by the heliopause.