The solar wind is the supersonic outflow into interplanetary space of plasma from the Sun's corona, the region of the solar atmosphere beginning about 4000 km above the Sun's visible surface and extending several solar radii into space. It is composed of approximately equal numbers of ions and electrons; the ion component consists predominantly of protons (95%), with a small amount of doubly ionized helium and trace amounts of heavier ions. Embedded in the outflowing solar wind plasma is a weak magnetic field known as the interplanetary magnetic field (IMF). The solar wind varies--in density, velocity, temperature, and magnetic field properties--with the solar cycle, heliographic latitude, heliocentric distance, and rotational period. It also varies in response to shocks, waves, and turbulence that perturb the interplanetary flow. Average values for solar wind velocity, density, and magnetic field strength at the orbit of the Earth are 468 km per second; density, 8.7 protons per cubic centimeter, and 6.6 nT, respectively.

The high- and low-speed solar wind

During the declining and minimum phases of the solar cycle, the solar wind is dominated by high-speed (500-800 kilometers per second) flows emanating from coronal holes -- regions of low coronal density and temperature where the magnetic field is weak and the field lines are open to interplanetary space. Coronal holes (the dark regions evident on the Sun's disk in the Yohkoh x-ray image on the right) occur both at low latitudes and at the poles; the polar holes are largest at solar minimum, extending equatorward and often merging with low-latitude holes of the same magnetic field polarity.

In addition to the high-speed flow, the solar wind also has a dense low-speed (300 kilometers per second) component associated with the equatorial coronal streamer belt. Near the ecliptic, the high- and low-speed components form alternating streams in the solar wind flow, moving outward into interplanetary space in an Archimedean spiral (because of the Sun's rotation). As the streams travel away from the Sun, the high-speed streams eventually overtake the slow-speed flows and create regions of enhanced density and magnetic field known as co-rotating interaction regions (CIRs). These compressed interstream regions play an important role in solar-terrestrial relations: when CIRs encounter the Earth, they trigger geomagnetic storms that recur with a 27-day periodicity, corresponding to the Sun's rotational period.

In the ascending phase of the solar activity cycle and at solar maximum, the average solar wind speed slows, as the polar coronal holes shrink (and even disappear) and the high-speed flows narrow and weaken. At the same time, the ambient solar wind flow is increasingly perturbed by coronal mass ejections (CMEs). These are explosive eruptions of coronal plasma from closed magnetic field regions that reach a peak occurrence rate at solar maximum. Fast earthward-directed CMEs are responsible for nonrecurrent geomagnetic storms, which occur with the greatest frequency near solar maximum.

Coronal heating and solar wind acceleration

The processes that heat the corona to over one million degrees Kelvin, several hundred times the temperature of the Sun's visible surface, and those that accelerate the solar wind have not been established and represent major unresolved questions in space science. The ultimate source of the energy that heats the corona is the turbulent motion of the hot gases in the Sun's convection zone; the kinetic energy of the convecting gases is then converted into magnetic energy and finally into thermal energy in the corona. However, there is no agreement among solar physicists about how this energy conversion occurs. Similar uncertainty surrounds the question of solar wind acceleration. According to the classical model of solar wind formation, the solar wind is driven by thermal conduction in the Sun's extremely hot corona. However, thermal conduction alone cannot adequately account for the flow speeds observed in the high-speed solar wind, so additional, non-thermal processes must play a role in solar wind acceleration. Several candidate processes have been proposed, but no consensus has been reached.