The solar wind is a continuous outward flow of electrically charged particles from the Sun's upper atmosphere into interplanetary space. It consists primarily of protons and electrons, with smaller quantities of helium nuclei and heavier ions, and it carries the Sun's magnetic field outward with it. The solar wind is not a conventional wind made of neutral air. It is a plasma flow whose particles are strongly influenced by electromagnetic forces.
Near Earth, typical solar wind speeds are several hundred kilometres per second, although much faster streams and transient disturbances can occur. The solar wind creates the heliosphere, a vast region of space in which the Sun's plasma and magnetic field dominate over the surrounding interstellar environment. Its continuous interaction with planets, moons, asteroids and comets makes it one of the fundamental components of the space environment throughout the Solar System.
The solar wind originates in the solar corona, where temperatures are so high that the plasma cannot remain gravitationally confined in the same way as cooler material near the solar surface. The corona is extremely hot and tenuous, and the high thermal energy of its particles contributes to the outward expansion of plasma. Theoretical descriptions of the solar wind began with models showing that a sufficiently hot corona should expand continuously into space.
The modern picture includes thermal pressure, magnetic fields, wave activity, turbulence and other plasma processes. The solar wind therefore emerges from the interaction of high coronal temperature with the Sun's gravity and magnetic field. Its properties are not uniform because different regions of the corona produce different types of solar wind and because solar magnetic activity changes over time.
Two broad forms of solar wind are commonly distinguished. The fast solar wind has speeds that can exceed 700 kilometres per second and is often associated with coronal holes, regions where magnetic field lines extend outward into interplanetary space. The slow solar wind is generally slower and more variable and can arise from more complex magnetic structures near boundaries between open and closed magnetic field regions.
The distinction is not simply a difference in speed because the two flows can differ in composition, temperature, charge states and variability. The solar wind also contains fluctuations and turbulence at many scales. As it travels outward, it expands and becomes less dense, while interactions between streams travelling at different speeds can produce compressed regions and shocks.
The Sun's magnetic field is carried outward by the solar wind, producing the interplanetary magnetic field. Because the Sun rotates while the solar wind flows outward, magnetic field lines become wound into a spiral configuration known as the Parker spiral. The orientation and strength of this magnetic field are important in determining how charged particles propagate through interplanetary space. Solar wind plasma is highly conducting, so magnetic fields and plasma motions are strongly coupled over many scales.
Turbulence develops as the solar wind travels outward, and waves and fluctuations can exchange energy between particles and electromagnetic fields. These processes help determine the temperature, density and particle distributions observed by spacecraft. Studying the solar wind is therefore a major part of heliophysics and plasma physics.
When the solar wind reaches Earth, it encounters the planet's magnetic field and forms a complex interaction region called the magnetosphere. Much of the solar wind is diverted around the magnetosphere, but some particles and energy can enter through processes involving magnetic reconnection and other plasma interactions. The solar wind can compress the dayside magnetosphere and stretch the nightside magnetic field into a long magnetotail. During strong solar wind disturbances, enhanced energy transfer can produce geomagnetic storms and substorms. These processes can generate auroras when energetic particles precipitate into the upper atmosphere near the polar regions. Strong geomagnetic storms can also produce electrical currents in the ground and affect satellites, navigation, radio communication and power systems.
Solar wind disturbances can be produced by solar eruptions, particularly coronal mass ejections and regions of fast solar wind that overtake slower material. A coronal mass ejection can carry billions of tonnes of magnetised plasma into interplanetary space. If its trajectory and magnetic orientation are favourable for strong coupling with Earth's magnetosphere, it can produce a significant geomagnetic storm. Not every coronal mass ejection impacts Earth, and not every impact produces a severe disturbance.
The orientation of the magnetic field carried by the arriving plasma is particularly important. Southward directed magnetic fields can reconnect efficiently with Earth's northward directed dayside magnetic field, allowing greater transfer of energy into the magnetosphere. Space weather forecasting therefore requires observations of both solar eruptions and the evolving interplanetary plasma.
The solar wind also affects other planets and small bodies. Planets with strong magnetic fields, such as Jupiter and Saturn, develop large magnetospheres that interact strongly with the solar wind. Unmagnetised bodies such as Mars interact directly through their ionospheres and induced magnetic structures. Comets develop ion and dust tails partly through interaction with the solar wind and radiation. The solar wind also gradually modifies exposed surfaces through particle bombardment and contributes to space weathering on airless bodies. At the outer boundary of the heliosphere, the solar wind slows dramatically as it encounters the interstellar medium. The resulting termination shock, heliosheath and heliopause form important regions where the Sun's influence transitions into the surrounding galactic environment.
The solar wind is therefore both a continuous background phenomenon and a highly variable space weather driver. Spacecraft measurements have shown that its density, velocity, temperature, magnetic field and composition vary with solar activity and distance from the Sun. Missions operating close to the Sun can measure the plasma before it has undergone as much interaction and turbulence as occurs farther away, helping scientists investigate how the solar wind is accelerated and heated.
Understanding the solar wind is important for fundamental plasma physics, spacecraft operations and forecasting disturbances in near Earth space. It also helps define the physical extent of the heliosphere and the boundary between the solar environment and interstellar space. The essential distinction is that the solar wind is a persistent plasma outflow from the Sun, while individual solar eruptions can produce much larger transient disturbances superimposed upon that background flow.
