Solar wind

The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between. The composition of the solar wind plasma also includes a mixture of particle species found in the solar plasma: trace amounts of heavy ions and atomic nuclei of elements such as carbon, nitrogen, oxygen, neon, magnesium, silicon, sulfur, and iron. There are also rarer traces of some other nuclei and isotopes such as phosphorus, titanium, chromium, and nickel's isotopes ⁵⁸Ni, ⁶⁰Ni, and ⁶²Ni. Superimposed with the solar-wind plasma is the interplanetary magnetic field. The solar wind varies in density, temperature and speed over time and over solar latitude and longitude. Its particles can escape the Sun's gravity because of their high energy resulting from the high temperature of the corona, which in turn is a result of the coronal magnetic field. The boundary separating the corona from the solar wind is called the Alfvén surface.
At a distance of more than a few solar radii from the Sun, the solar wind reaches speeds of and is supersonic, meaning it moves faster than the speed of fast magnetosonic waves. The flow of the solar wind is no longer supersonic at the termination shock. Other related phenomena include the aurora, comet tails that always point away from the Sun, and geomagnetic storms that can change the direction of magnetic field lines.

History

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.
File:Birkeland-anode-globe-fig259.jpg|thumb|Laboratory simulation of the magnetosphere's influence on the solar wind; these aurora-like Birkeland currents 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. Eddington's proposition was never fully embraced, even though he had also made a similar suggestion at a Royal Institution address the previous year, in which he had postulated that the ejected material consisted of electrons, whereas in his study of Comet Morehouse he had supposed them to be ions.
The idea that the ejected material consisted of both ions and electrons was first suggested by Norwegian scientist Kristian Birkeland. 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". He proposed in 1916 that, "From a physical point of view it is most probable that solar rays are neither exclusively negative nor positive rays, but of both kinds"; in other words, the solar wind consists of both negative electrons and positive ions. 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. 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 as being the first to relate solar wind to the direction of a comet's tail based on observations of the comet Whipple–Fedke.

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, the plasma flow velocity integrated to a closed form:
One solution to this equation was immediately recognizable as a solar wind.
Parker's theory of supersonic solar wind also predicted the shape of the solar magnetic field in the outer Solar System. Parker argued that a million-degree corona cannot remain static: pressure forces must drive a radially expanding flow that accelerates from subsonic near the Sun to supersonic beyond a critical point. He further noted that solar rotation winds outward-advected magnetic field lines into a spiral pattern in the ecliptic, now called the Parker spiral.
His theoretical modeling was not immediately accepted by the astronomical community: when Parker submitted the results to The Astrophysical Journal in 1958, two reviewers recommended its rejection. One reviewer commented on the paper: "Well I would suggest that Parker go to the library and read up on the subject before he tries to write a paper about it, because this is utter nonsense." The editor of the journal and Parker's colleague at the University of Chicago, future Nobel prize-winner Subrahmanyan Chandrasekhar, finding no obvious errors in the paper, overruled the reviewers and published the paper, even though he disagreed with Parker's theory.
A colleague at the University of Chicago, Joseph W. Chamberlain, who published a paper in 1960 showing that the plasma flow velocity equation also admitted a solution with an exponential decay in flow velocity away from the sun. 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

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.
The first numerical simulation of the solar wind in the solar corona, including closed and open field lines, was performed by Pneuman and Kopp in 1971. The magnetohydrodynamics equations in steady state were solved iteratively starting with an initial dipolar configuration.
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 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 from the photosphere ; but the transition now appears to be much lower, perhaps only one solar radius 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 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.
file:Solar wind at Voyager 1.png|thumb|Plot showing a dramatic decrease in the rate of solar wind particle detection by Voyager 1
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.