Solar wind

The solar wind is a continuous stream of charged particles that flows outward from the Sun’s outer atmosphere, the corona. Composed primarily of electrons, protons, and alpha particles, with trace amounts of heavier ions such as carbon, nitrogen, oxygen, magnesium, silicon, sulfur, and iron, the solar wind carries both matter and magnetic fields throughout interplanetary space. Its behaviour underpins many heliophysical phenomena, including aurorae, comet-tail alignment, and disturbances in Earth’s magnetosphere.

Composition and Physical Properties

The solar wind is a plasma consisting mainly of electrons and positively charged ions. Although protons constitute most of the ion component, a small fraction consists of alpha particles (helium nuclei) and heavier atomic nuclei. These heavy ions appear in characteristic charge states that reflect coronal temperatures of several million kelvin. The interplanetary magnetic field is embedded within the flow, shaping and being shaped by its dynamics. As the plasma expands, it reaches speeds of several hundred to over a thousand kilometres per second, forming a supersonic outflow beyond a few solar radii from the surface.
The boundary at which the corona transitions to the solar wind is called the Alfvén surface. Beyond this point, magnetic tension no longer restrains the plasma, and the wind propagates freely. At great distances, the wind eventually slows to subsonic speeds at the termination shock before encountering the interstellar medium at the heliopause.

Solar Wind Variability and Structure

The speed, density, and temperature of the solar wind vary according to heliographic latitude and the solar-activity cycle. During solar minimum, observations reveal a structured pattern: slow wind near the solar equator and fast wind at higher latitudes emanating from coronal holes. The alternating red and blue signatures in heliospheric magnetic field measurements indicate outward and inward polarities.
Slow wind typically travels at 300–500 km/s and originates in complex coronal regions rich in closed magnetic loops. Fast wind, travelling at 600–800 km/s, emerges from open-field regions near the solar poles. Both types of wind show temporal variability connected to the Sun’s magnetic-cycle evolution.

Early Theoretical Development

Nineteenth-century studies of solar phenomena laid the groundwork for the concept of a solar outflow. In 1859 Richard Carrington and Richard Hodgson observed the first solar flare, followed by a geomagnetic storm on Earth, hinting at a link between solar eruptions and terrestrial magnetic effects. Later, Kristian Birkeland’s work on auroral activity and his experiments with magnetised globes suggested that charged particles from the Sun influenced Earth’s magnetic environment. His insights foreshadowed the concept of a continuous flow of both positive and negative particles.
In the mid-twentieth century, researchers sought to explain the high temperature of the corona, which spectroscopy showed to be around one million degrees Celsius. Sydney Chapman’s calculations demonstrated that a corona at such temperatures could not be static and must expand outward. In parallel, Ludwig Biermann’s study of comet tails, which consistently pointed away from the Sun, suggested a persistent outward force acting on cometary material.
Eugene Parker unified these insights in 1957–58 by developing a hydrodynamic model showing that the corona must expand into interplanetary space as a supersonic flow. He termed this outflow the solar wind. Parker showed that gravity’s diminishing influence combined with thermal and magnetic effects produces a transition from subsonic to supersonic flow analogous to the behaviour of a de Laval nozzle. Although initially met with scepticism, Parker’s ideas proved correct and transformed heliophysics.

Observational Confirmation

The first direct measurements of the solar wind were made by the Soviet Luna 1 spacecraft in 1959 using ion traps. Subsequent missions, including Luna 2, Luna 3, and Venera 1, confirmed its existence. In 1962, Mariner 2 provided the first American measurements, characterising the density, temperature, and speed of the wind.
As observational capabilities improved, numerical modelling followed. In 1971, Pneuman and Kopp produced the first magnetohydrodynamic simulation of the coronal wind, showing how solar magnetic fields influence the flow.
The launch of the Ulysses mission in 1990 enabled the first high-latitude studies, revealing the structured two-speed solar wind during solar minimum. Later, the Ultraviolet Coronagraph Spectrometer on the Solar and Heliospheric Observatory detected rapid acceleration of the fast solar wind close to the Sun, at heights as low as one solar radius above the photosphere. This finding indicated that additional mechanisms—beyond thermal expansion—accelerate the fast wind.

Spacecraft Discoveries and Solar Wind Dynamics

From 1999, the Advanced Composition Explorer and WIND spacecraft recorded a dramatic 98 per cent drop in solar wind density, which expanded Earth’s magnetosphere to several times its usual size and produced an unusual polar auroral event. In 2006, the STEREO mission captured stereoscopic images of the solar wind, revealing large-scale turbulent structures through Thomson scattering.
In 2010, Voyager 1 observed that the solar wind’s outward velocity had slowed to zero at its distant location, indicating its arrival at the stagnation region preceding the interstellar medium.
Recent missions, including the Parker Solar Probe, continue to refine understanding of solar wind acceleration, turbulence, and magnetic structure. Some aspects, such as the rapid acceleration of fast wind and the precise mechanisms behind coronal heating, remain active areas of research.

Effects and Related Phenomena

The solar wind shapes the heliosphere and drives space-weather effects throughout the planetary system. Its interactions with Earth’s magnetosphere generate aurorae, geomagnetic storms, and variations in radiation belts. Comet tails are pushed away from the Sun by solar-wind pressure, revealing the direction of the flow. At large scales, the wind’s embedded magnetic field carves out a protective cavity around the Solar System, shielding it from some interstellar particles.