Observations from Earth The existence of particles flowing outward from the
Sun to the
Earth was first suggested by British astronomer
Richard C. Carrington. In 1859, Carrington and
Richard Hodgson independently made the first observations of what would later be called a
solar flare. This is a sudden, localised increase in brightness on the solar disc, which is now known to often occur in conjunction with an episodic ejection of material and magnetic flux from the Sun's atmosphere, known as a
coronal mass ejection. The following day, a
powerful geomagnetic storm was observed, and Carrington suspected that there might be a connection; the
geomagnetic storm is now attributed to the arrival of the coronal mass ejection in near-Earth space and its subsequent interaction with the Earth's
magnetosphere. Irish academic
George FitzGerald later suggested that matter was being regularly accelerated away from the Sun, reaching the Earth after several days. s were created in a
terrella, a magnetised anode globe in an evacuated chamber. In 1910, British astrophysicist
Arthur Eddington essentially suggested the existence of the solar wind, without naming it, in a footnote to an article on
Comet Morehouse. His geomagnetic surveys showed that auroral activity was almost uninterrupted. As these displays and other geomagnetic activity were being produced by particles from the Sun, he concluded that the Earth was being continually bombarded by "rays of electric corpuscles emitted by the Sun". Three years later, in 1919, British physicist
Frederick Lindemann also suggested that the Sun ejects particles of both polarities: protons as well as electrons. Around the 1930s, scientists had concluded that the temperature of the
solar corona must be a million degrees
Celsius because of the way it extended into space (as seen during a total
solar eclipse). Later
spectroscopic work confirmed this extraordinary temperature to be the case. In the mid-1950s, British mathematician
Sydney Chapman calculated the properties of a gas at such a temperature and determined that the corona being such a superb conductor of heat, it must extend way out into space, beyond the orbit of Earth. Also in the 1950s, German astronomer
Ludwig Biermann became interested in the fact that the tail of a
comet always points away from the Sun, regardless of the direction in which the comet is travelling. Biermann postulated that this happens because the Sun emits a steady stream of particles that pushes the comet's tail away. German astronomer
Paul Ahnert is credited (by Wilfried Schröder) as being the first to relate solar wind to the direction of a comet's tail based on observations of the comet Whipple–Fedke (1942g).
Theoretical prediction In 1956, Biermann came to the University of Chicago, where he discussed his results with the astrophysicist
Eugene Parker. Parker also discussed the
solar corona with mathematician Sydney Chapman, who mentioned that "the corona is so hot that it should extend clear to the orbit of the Earth". Parker then conjectured that "the corona and solar corpuscular radiation must be the same thing": Parker himself said that the math needed for the solar wind discovery was just "four lines of algebra". The math needed to discover the solar wind was, per Parker just "four lines of algebra". Parker proposed that although the Sun's corona is strongly attracted by solar gravity, it is such a good conductor of heat that it is still very hot at large distances from the Sun. As solar gravity weakens with increasing distance from the Sun, the
hydrodynamic effect is identical to a
de Laval nozzle, inciting a transition from
subsonic to supersonic flow. When Parker wrote hydrodynamic equations for an
isothermal, extended coronal atmosphere, \left[\frac{v^2}{v_m^2} - \ln\left(\frac{v^2}{v_m^2}\right)\right] = 4\ln\left(\frac{r}{a}\right) + \left(\frac{v_{\text{esc}}^2}{v_m^2}\right)\left(\frac{a}{r}\right) - 4\ln\left(\frac{v_{\text{esc}}^2}{v_m^2}\right) - 3 + \ln 256 One solution to this equation was immediately recognizable as a solar wind. His theoretical modeling was not immediately accepted by the astronomical community: when Parker submitted the results to
The Astrophysical Journal in 1958, A colleague at the University of Chicago,
Joseph W. Chamberlain, Chamberlain's subsonic solution was called the "solar breeze", and Italian plasma physicist
Marco Velli later showed that "the breeze solution is unstable" to low frequency perturbations.
Observations from space s, active regions, and
coronal streamers Parker's theoretical predictions were confirmed by satellite observations; it is called to be "a unique example in astrophysics, due to its immediate and brief confirmation by observations". In January 1959, the
Soviet spacecraft
Luna 1 first directly observed the solar wind and measured its strength, using hemispherical ion traps. The discovery, made by , was verified by
Luna 2,
Luna 3, and the more distant measurements of
Venera 1. Three years later, a similar measurement was performed by American geophysicist
Marcia Neugebauer and collaborators using the
Mariner 2 spacecraft. Mariner 2 data revealed two types of solar wind, a low- and a high-speed components. In 1990, the
Ulysses probe was launched to study the solar wind from high solar latitudes. All prior observations had been made at or near the Solar System's
ecliptic plane. In the late 1990s, the Ultraviolet Coronal Spectrometer (UVCS) instrument on board the
SOHO spacecraft observed the acceleration region of the fast solar wind emanating from the poles of the Sun and found that the wind accelerates much faster than can be accounted for by thermodynamic expansion alone. Parker's model predicted that the wind should make the transition to supersonic flow at an altitude of about four
solar radii (approx. 3,000,000 km) from the
photosphere (surface); but the transition (or "sonic point") now appears to be much lower, perhaps only one solar radius (approx. 700,000 km) above the photosphere, suggesting that some additional mechanism accelerates the solar wind away from the Sun. The acceleration of the fast wind is still not understood and cannot be fully explained by Parker's theory. However, the gravitational and electromagnetic explanation for this acceleration is detailed in an earlier paper by 1970
Nobel laureate in Physics,
Hannes Alfvén. From May 10 to May 12, 1999, NASA's
Advanced Composition Explorer (ACE) and
WIND spacecraft observed a 98% decrease of solar wind density. This allowed energetic electrons from the Sun to flow to Earth in narrow beams known as "
strahl", which caused a highly unusual "polar rain" event, in which a visible
aurora appeared over the North Pole. In addition, Earth's magnetosphere increased to between 5 and 6 times its normal size. The
STEREO mission was launched in 2006 to study coronal mass ejections and the solar corona, using
stereoscopy from two widely separated imaging systems. Each STEREO spacecraft carried two heliospheric imagers: highly sensitive wide-field cameras capable of imaging the solar wind itself, via
Thomson scattering of sunlight off of free electrons. Movies from STEREO revealed the solar wind near the ecliptic, as a large-scale turbulent flow. On December 13, 2010,
Voyager 1 determined that the velocity of the solar wind, at its location from Earth had slowed to zero. "We have gotten to the point where the wind from the Sun, which until now has always had an outward motion, is no longer moving outward; it is only moving sideways so that it can end up going down the tail of the heliosphere, which is a comet-shaped-like object", said Voyager project scientist Edward Stone. In 2018, NASA launched the
Parker Solar Probe, named in honor of American astrophysicist Eugene Parker, on a mission to study the structure and dynamics of the solar corona, in an attempt to understand the mechanisms that cause particles to be heated and accelerated as solar wind. During its seven-year mission, the probe will make twenty-four orbits of the Sun, passing further into the corona with each orbit's
perihelion, ultimately passing within 0.04
astronomical units of the Sun's surface. It is the first NASA spacecraft named for a living person, and Parker, at age 91, was on hand to observe the launch. ==Acceleration mechanism==