TheInfoList

The solar wind is a stream of
charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through Spa ...
s released from the upper atmosphere of the Sun, called the
corona Corona (from the Latin for 'crown') most commonly refers to: * Stellar corona, the outer atmosphere of the Sun or another star * Coronavirus, a group of RNA viruses ** Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a coronavirus r ...
. This
plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood plasma ...
mostly consists of
electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has ma ...

s,
proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

s and
alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Proton ...
s with
kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion (physics), motion. It is defined as the work (physics), work needed to accelerate a body of a given mass from rest to its stated velocity. Having gaine ...
between . The composition of the solar wind plasma also includes a mixture of materials found in the solar plasma: trace amounts of heavy ions and atomic nuclei such as C, N, O, Ne, Mg, Si, S, and Fe. There are also rarer traces of some other nuclei and isotopes such as P, Ti, Cr, 54Fe and 56Fe, and 58Ni, 60Ni, and 62Ni. Superposed with the solar-wind plasma is the
interplanetary magnetic field The interplanetary magnetic field (IMF), now more commonly referred to as the heliospheric magnetic field (HMF), is the component of the solar magnetic field that is dragged out from the solar corona by the solar wind flow to fill the Solar Syst ...

. The solar wind varies in
density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its per unit . The symbol most often used for density is ''ρ'' (the lower case Greek letter ), although the Latin letter ''D'' can also ...

,
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

and
speed In everyday use and in kinematics, the speed (commonly referred to as ''v'') of an object is the magnitude (mathematics), magnitude of the rate of change of its Position (vector), position with time or the magnitude of the change of its posit ...

over time and over solar latitude and longitude. Its particles can escape the Sun's
gravity Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

because of their high energy resulting from the high temperature of the corona, which in turn is a result of the coronal magnetic field. At a distance of more than a few
solar radii Solar radius is a unit of distance used to express the size of stars in astronomy relative to the Sun. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the Optical_depth_(astrophysics), optical depth eq ...
from the Sun, the solar wind reaches speeds of 250–750 km/ s and is supersonic, meaning it moves faster than the speed of the fast
magnetosonic wave A magnetosonic wave (also called magnetoacoustic wave) is a linear Magnetohydrodynamics, magnetohydrodynamic (MHD) wave that is driven by both pressure (thermal and magnetic) and magnetic tension. There are two types of magnetosonic waves, the ''fas ...
. The flow of the solar wind is no longer supersonic at the termination shock. Other related phenomena include the
aurora An aurora (plural: auroras or aurorae), sometimes referred to as polar lights (aurora polaris), northern lights (aurora borealis), or southern lights (aurora australis), is a natural light display in the Earth's sky, predominantly seen in high-l ...
( and ), the
plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood plasma ...
tails of
comet A comet is an icy, small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astro ...

s that always point away from the Sun, and
geomagnetic storm . Sizes are not to scale. A geomagnetic storm (commonly referred to as a solar storm) is a temporary disturbance of the Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth' ...
s that can change the direction of magnetic field lines.

# History

## Observations from Earth

The existence of particles flowing outward from the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

to the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wit ...

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 A solar flare is a sudden flash of increased brightness on the Sun, usually observed near its surface and in close proximity to a sunspot group. Powerful flares are often, but not always, accompanied by a coronal mass ejection. Even the most ...

. 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 . Sizes are not to scale. A geomagnetic storm (commonly referred to as a solar storm) is a temporary disturbance of the Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth' ...
is now attributed to the arrival of the coronal mass ejection in near-Earth space and its subsequent interaction with the Earth's
magnetosphere In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mat ...

. Irish academic later suggested that matter was being regularly accelerated away from the Sun, reaching the Earth after several days. In 1910, British astrophysicist
Arthur Eddington Sir Arthur Stanley Eddington (28 December 1882 – 22 November 1944) was an English astronomer, physicist, and mathematician. He was also a philosopher of science and a populariser of science. The Eddington limit, the natural limit to the lumi ...

essentially suggested the existence of the solar wind, without naming it, in a footnote to an article on . Eddington's proposition was never fully embraced, even though he had also made a similar suggestion at a
Royal Institution The Royal Institution of Great Britain (often the Royal Institution, abbreviated 'Ri' or 'RI') is an organisation for scientific education and research, based in the City of Westminster City of Westminster is an Inner London, inner London Ci ...

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
ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ...
s. The idea that the ejected material consisted of both ions and electrons was first suggested by Norwegian scientist
Kristian Birkeland Kristian Olaf Bernhard Birkeland (13 December 1867 – 15 June 1917) was a Norwegian Norwegian, Norwayan, or Norsk may refer to: *Something of, from, or related to Norway, a country in northwestern Europe *Norwegians, both a nation and an ethnic ...

. 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 Frederick Alexander Lindemann, 1st Viscount Cherwell, ( ; 5 April 18863 July 1957) was a British physicist who was prime scientific adviser to Winston Churchill in World War II. Lindemann was a brilliant but arrogant intellectual, who quarrell ...
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 A corona (, in turn derived from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the f ...
must be a million degrees
Celsius The degree Celsius is a unit of temperature on the Celsius scale, a temperature scale Scale of temperature is a methodology of calibrating the physical quantity temperature in metrology. Empirical scales measure temperature in relation to conv ...

because of the way it extended into space (as seen during a total
solar eclipse A solar eclipse occurs when a portion of the Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The re ...

). Later
spectroscopic Spectroscopy is the study of the interaction between matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately compo ...

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 Ludwig Franz Benedikt Biermann (March 13, 1907 in Hamm Hamm (, Latin: ''Hammona'') is a city in North Rhine-Westphalia, Germany. It is located in the northeastern part of the Ruhr area. As of 2016 its population was 179,397. The city is situ ...
became interested in the fact that the tail of a
comet A comet is an icy, small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astro ...

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). American astrophysicist
Eugene Parker Eugene Newman Parker (born June 10, 1927) is an American solar astrophysicist who—in the mid-1950s—developed the theory of the supersonic solar wind and predicted the Parker spiral shape of the solar magnetic field in the outer Solar System ...
realised that heat flowing from the Sun in Chapman's model, and the comet tail blowing away from the Sun in Biermann's hypothesis, had to be the result of the same phenomenon which he termed the "solar wind". In 1957, Parker showed 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 outer coronal atmosphere is able to escape supersonically into interstellar space. Parker was also the first person to notice that the weakening influence of the Sun's gravity has the same effect on
hydrodynamic In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through ...
flow as a
de Laval nozzle A de Laval nozzle (or convergent-divergent nozzle, CD nozzle or con-di nozzle) is a tube which is pinched in the middle, making a carefully balanced, asymmetric shape. It is used to accelerate a compressible fluid to supersonic speeds in the ax ...
, inciting a transition from
subsonic Subsonic may refer to: Motion through a medium * Any speed lower than the speed of sound within a sound-propagating medium * Subsonic aircraft, a flying machine that flies at air speeds lower than the speed of sound * Subsonic ammunition, a type of ...
to supersonic flow. There was strong opposition to Parker's hypothesis on the solar wind; the paper he submitted to ''
The Astrophysical Journal ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a peer-reviewed Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work ( p ...
'' in 1958 was rejected by two reviewers, before being accepted by the editor
Subrahmanyan Chandrasekhar Subrahmanyan Chandrasekhar () (19 October 1910 – 21 August 1995) was an Indian-American astrophysicist who spent his professional life in the United States The United States of America (USA), commonly known as the United States (U ...

.

## Observations from space

In January 1959, the
Soviet The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a socialist state A socialist state, socialist republic, or socialist country, sometimes referred to as a workers' state or workers' republic, is a sovere ...
spacecraft ''
Luna 1 ''Luna 1'', also known as ''Mechta'' (russian: Мечта , '' lit.'': ''Dream''), ''E-1 No.4'' and ''First Lunar Rover'', was the first spacecraft File:Space Shuttle Columbia launching.jpg, 275px, The US Space Shuttle flew 135 times from 1 ...

'' first directly observed the solar wind and measured its strength, using hemispherical ion traps. The discovery, made by Konstantin Gringauz, was verified by '' Luna 2'', '''', 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 '''' spacecraft. The first numerical simulation of the solar wind in the solar corona, including , was performed by Pneuman and Kopp in 1971. The
magnetohydrodynamics Magnetohydrodynamics (MHD; also magneto-fluid dynamics or hydro­magnetics) is the study of the magnetic properties and behaviour of electrically conducting fluid In physics, a fluid is a substance that continually Deformation (mechanics ...
equations in
steady state In systems theory Systems theory is the interdisciplinary study of system A system is a group of Interaction, interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influen ...

were solved iteratively starting with an initial 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 The ecliptic is the plane (geometry), plane of Earth's orbit around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic against the ...

plane. In the late 1990s, the Ultraviolet Coronal Spectrometer (UVCS) instrument on board the
SOHO Soho is an area of the City of Westminster The City of Westminster is a City status in the United Kingdom, city and London boroughs, borough in Inner London which forms a core part of Central London. It is the site of the United Kingdom's ...

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 Solar radius is a unit of distance used to express the size of stars in astronomy relative to the Sun. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the Optical_depth_(astrophysics), optical depth eq ...
(approx. 3,000,000 km) from the
photosphere The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient wor ...
(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. The
STEREO File:Carsoundstereoshift.png, Time difference in a stereophonic recording of a car going past, 250px Stereophonic sound or, more commonly, stereo, is a method of sound reproduction that recreates a multi-directional, 3-dimensional audible per ...

mission was launched in 2006 to study coronal mass ejections and the solar corona, using
stereoscopy consists of a multi-station viewing apparatus and sets of stereo slides. Patented by A. Fuhrmann around 1890. File:Company of ladies watching stereoscopic photographs by Jacob Spoel 1820-1868.jpg, A company of ladies looking at stereoscopic vi ...
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 Thomson scattering is the elastic scattering of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural sc ...
of sunlight off of free electrons. Movies from STEREO revealed the solar wind near the ecliptic, as a large-scale turbulent flow. The ''
Voyager 1 ''Voyager 1'' is a space probe A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel t ...
'' probe reached the end of the solar-wind "bubble" in 2012, at which time the detection of solar wind dropped off precipitously. A similar observation was made six years later by ''
Voyager 2 ''Voyager 2'' is a space probe A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel throu ...
''. In 2018, NASA launched the ''
Parker Solar Probe The Parker Solar Probe (abbreviated PSP; previously Solar Probe, Solar Probe Plus or Solar Probe+) is a NASA The National Aeronautics and Space Administration (NASA; ) is an independent agencies of the United States government, independe ...

'', 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 upright=1.15, The two-body system of interacting primary body (yellow); both are in elliptic orbits around their center of mass">common center of mass (or barycenter), (red +). ∗Periapsis and apoapsis as distances: The smallest and largest ...

, ultimately passing within 0.04
astronomical units The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and equal to about or ~8 light minutes. The actual distance varies by about 3% as Earth orbits the Sun, from a maximum (aphelion ...
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

While early models of the solar wind relied primarily on
thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concepts, such as the internal energy of a system; heat or sensible heat, which are defined as types of energy transfer (as is ...
to accelerate the material, by the 1960s it was clear that thermal acceleration alone cannot account for the high speed of solar wind. An additional unknown acceleration mechanism is required and likely relates to
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

s in the solar atmosphere. The Sun's
corona Corona (from the Latin for 'crown') most commonly refers to: * Stellar corona, the outer atmosphere of the Sun or another star * Coronavirus, a group of RNA viruses ** Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a coronavirus r ...
, or extended outer layer, is a region of plasma that is heated to over a . As a result of thermal collisions, the particles within the inner corona have a range and distribution of speeds described by a
Maxwellian distribution* Maxwell–Boltzmann distribution * The Maxwellians (1991 book) See also

*List of things named after James Clerk Maxwell {{disambig ...
. The mean velocity of these particles is about , which is well below the solar
escape velocity#REDIRECT Escape velocity In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), ...
of . However, a few of the particles achieve energies sufficient to reach the terminal velocity of , which allows them to feed the solar wind. At the same temperature, electrons, due to their much smaller mass, reach escape velocity and build up an electric field that further accelerates ions away from the Sun. The total number of particles carried away from the Sun by the solar wind is about per second. Thus, the total mass loss each year is about
solar mass The solar mass () is a standard unit of mass in astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial obje ...
es, or about 1.3–1.9 Million tonnes per second. This is equivalent to losing a mass equal to the Earth every 150 million years. However, since the Sun's formation, only about 0.01% of its initial mass has been lost through the solar wind. Other stars have much stronger
stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown d ...
s that result in significantly higher mass-loss rates.

# Properties and structure

## Fast and slow solar wind

The solar wind is observed to exist in two fundamental states, termed the slow solar wind and the fast solar wind, though their differences extend well beyond their speeds. In near-Earth space, the slow solar wind is observed to have a velocity of , a temperature of ~100 MK and a composition that is a close match to the
corona Corona (from the Latin for 'crown') most commonly refers to: * Stellar corona, the outer atmosphere of the Sun or another star * Coronavirus, a group of RNA viruses ** Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a coronavirus r ...
. By contrast, the fast solar wind has a typical velocity of , a temperature of 800 MK and it nearly matches the composition of the Sun's
photosphere The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient wor ...
. The slow solar wind is twice as dense and more variable in nature than the fast solar wind. The slow solar wind appears to originate from a region around the Sun's equatorial belt that is known as the "streamer belt", where coronal streamers are produced by magnetic flux open to the heliosphere draping over closed magnetic loops. The exact coronal structures involved in slow solar wind formation and the method by which the material is released is still under debate. Observations of the Sun between 1996 and 2001 showed that emission of the slow solar wind occurred at latitudes up to 30–35° during the
solar minimum Solar minimum is the period of least solar activity in the 11-year solar cycle of the Sun. During this time, sunspot and solar flare activity diminishes, and often does not occur for days at a time. The date of the minimum is described by a smo ...
(the period of lowest solar activity), then expanded toward the poles as the solar cycle approached maximum. At
solar maximum Solar maximum or solar max is a regular period of greatest Sun activity during the 11-year solar cycle. During solar maximum, large numbers of sunspots appear, and the solar irradiance output grows by about 0.07%. At solar maximum, the Sun's ...
, the poles were also emitting a slow solar wind. The fast solar wind originates from coronal holes, which are funnel-like regions of open field lines in the Sun's
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

. Such open lines are particularly prevalent around the Sun's magnetic poles. The plasma source is small magnetic fields created by
convection cell Image:Altocumulus15.jpg, Altocumulus cloud as seen from the Space Shuttle. Altocumulus clouds are formed by convective activity. In the field of fluid dynamics, a convection cell is the phenomenon that occurs when density differences exist within ...
s in the solar atmosphere. These fields confine the plasma and transport it into the narrow necks of the coronal funnels, which are located only 20,000 km above the photosphere. The plasma is released into the funnel when these magnetic field lines reconnect.

## Pressure

The wind exerts a pressure at typically in the range of (), although it can readily vary outside that range. The
ram pressure Ram pressure is a pressure exerted on a body moving through a fluid medium, caused by relative bulk motion of the fluid rather than random thermal motion. It causes a drag (physics), drag force to be exerted on the body. Ram pressure is given in t ...
is a
function Function or functionality may refer to: Computing * Function key A function key is a key on a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern comp ...
of wind speed and density. The formula is :$P = m_p \cdot n \cdot V^2 = 1.6726 \times 10^ \cdot n \cdot V^2$ where mp is the
proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

mass, pressure P is in nPa (nanopascals), n is the density in particles/cm3 and V is the speed in km/s of the solar wind.

## Coronal mass ejection

Both the fast and slow solar wind can be interrupted by large, fast-moving bursts of plasma called coronal mass ejections, or CMEs. CMEs are caused by a release of magnetic energy at the Sun. CMEs are often called "solar storms" or "space storms" in the popular media. They are sometimes, but not always, associated with
solar flare A solar flare is a sudden flash of increased brightness on the Sun, usually observed near its surface and in close proximity to a sunspot group. Powerful flares are often, but not always, accompanied by a coronal mass ejection. Even the most ...

s, which are another manifestation of magnetic energy release at the Sun. CMEs cause shock waves in the thin plasma of the heliosphere, launching electromagnetic
wave In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

s and accelerating particles (mostly
proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

s and
electrons The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has m ...
) to form showers of
ionizing radiation Ionizing radiation (or ionising radiation), including nuclear radiation, consists of s or s that have sufficient to s or s by detaching s from them. The particles generally travel at a speed that is greater than 1% of , and the electromagnetic w ...
that precede the CME. When a CME impacts the Earth's magnetosphere, it temporarily deforms the Earth's
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

, changing the direction of
compass A compass is a device that shows the cardinal direction The four cardinal directions, or cardinal points, are the directions north, east, south, and west, commonly denoted by their initials N, E, S, and W. East and west are perpendicular ( ...

needles and inducing large electrical ground currents in Earth itself; this is called a
geomagnetic storm . Sizes are not to scale. A geomagnetic storm (commonly referred to as a solar storm) is a temporary disturbance of the Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth' ...
and it is a global phenomenon. CME impacts can induce
magnetic reconnectionImage:reconnection.gif, 380px, Magnetic reconnection: This view is a cross-section through four magnetic domains undergoing separator reconnection. Two separatrices (see text) divide space into four magnetic domains with a separator at the center of ...

in Earth's
magnetotail In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a star or planet with an active interior Dynamo t ...
(the midnight side of the magnetosphere); this launches protons and electrons downward toward Earth's atmosphere, where they form the
aurora An aurora (plural: auroras or aurorae), sometimes referred to as polar lights (aurora polaris), northern lights (aurora borealis), or southern lights (aurora australis), is a natural light display in the Earth's sky, predominantly seen in high-l ...
. CMEs are not the only cause of space weather. Different patches on the Sun are known to give rise to slightly different speeds and densities of wind depending on local conditions. In isolation, each of these different wind streams would form a spiral with a slightly different angle, with fast-moving streams moving out more directly and slow-moving streams wrapping more around the Sun. Fast-moving streams tend to overtake slower streams that originate westward of them on the Sun, forming turbulent co-rotating interaction regions that give rise to wave motions and accelerated particles, and that affect Earth's magnetosphere in the same way as, but more gently than, CMEs.

# Solar System effects

Over the Sun's lifetime, the interaction of its surface layers with the escaping solar wind has significantly decreased its surface rotation rate. The wind is considered responsible for comets' tails, along with the Sun's radiation. The solar wind contributes to fluctuations in Sky, celestial radio waves observed on the Earth, through an effect called interplanetary scintillation.

## Magnetospheres

Where the solar wind intersects with a planet that has a well-developed
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

(such as Earth, Jupiter or Saturn), the particles are deflected by the Lorentz force. This region, known as the
magnetosphere In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mat ...

, causes the particles to travel around the planet rather than bombarding the atmosphere or surface. The magnetosphere is roughly shaped like a Sphere, hemisphere on the side facing the Sun, then is drawn out in a long wake on the opposite side. The boundary of this region is called the magnetopause, and some of the particles are able to penetrate the magnetosphere through this region by partial reconnection of the magnetic field lines. The solar wind is responsible for the overall shape of Earth's magnetosphere. Fluctuations in its speed, density, direction, and Interplanetary magnetic field, entrained magnetic field strongly affect Earth's local space environment. For example, the levels of ionizing radiation and radio interference can vary by factors of hundreds to thousands; and the shape and location of the magnetopause and bow shock wave upstream of it can change by several Earth radii, exposing Geosynchronous orbit, geosynchronous satellites to the direct solar wind. These phenomena are collectively called space weather. From the European Space Agency's Cluster II (spacecraft), Cluster mission, a new study has taken place that proposes that it is easier for the solar wind to infiltrate the magnetosphere than previously believed. A group of scientists directly observed the existence of certain waves in the solar wind that were not expected. A recent study shows that these waves enable incoming charged particles of solar wind to breach the magnetopause. This suggests that the magnetic bubble forms more as a filter than a continuous barrier. This latest discovery occurred through the distinctive arrangement of the four identical Cluster spacecraft, which fly in a controlled configuration through near-Earth space. As they sweep from the magnetosphere into interplanetary space and back again, the fleet provides exceptional three-dimensional insights on the phenomena that connect the sun to Earth. The research characterised variances in formation of the
interplanetary magnetic field The interplanetary magnetic field (IMF), now more commonly referred to as the heliospheric magnetic field (HMF), is the component of the solar magnetic field that is dragged out from the solar corona by the solar wind flow to fill the Solar Syst ...

(IMF) largely influenced by Kelvin–Helmholtz instability (which occur at the interface of two fluids) as a result of differences in thickness and numerous other characteristics of the boundary layer. Experts believe that this was the first occasion that the appearance of Kelvin–Helmholtz waves at the magnetopause had been displayed at high latitude downward orientation of the IMF. These waves are being seen in unforeseen places under solar wind conditions that were formerly believed to be undesired for their generation. These discoveries show how Earth's magnetosphere can be penetrated by solar particles under specific IMF circumstances. The findings are also relevant to studies of magnetospheric progressions around other planetary bodies. This study suggests that Kelvin–Helmholtz waves can be a somewhat common, and possibly constant, instrument for the entrance of solar wind into terrestrial magnetospheres under various IMF orientations.

## Atmospheres

The solar wind affects other incoming cosmic rays interacting with planetary atmospheres. Moreover, planets with a weak or non-existent magnetosphere are subject to atmospheric stripping by the solar wind. Venus, the nearest and most similar planet to Earth, has 100 times denser atmosphere, with little or no geo-magnetic field. Space probes discovered a comet-like tail that extends to Earth's orbit. Earth itself is largely protected from the solar wind by Earth's magnetic field, its magnetic field, which deflects most of the charged particles; however, some of the charged particles are trapped in the Van Allen radiation belt. A smaller number of particles from the solar wind manage to travel, as though on an electromagnetic energy transmission line, to the Earth's upper atmosphere and ionosphere in the auroral zones. The only time the solar wind is observable on the Earth is when it is strong enough to produce phenomena such as the aurora (astronomy), aurora and
geomagnetic storm . Sizes are not to scale. A geomagnetic storm (commonly referred to as a solar storm) is a temporary disturbance of the Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth' ...
s. Bright auroras strongly heat the ionosphere, causing its plasma to expand into the magnetosphere, increasing the size of the plasma geosphere and injecting atmospheric matter into the solar wind. Geomagnetic storms result when the pressure of plasmas contained inside the magnetosphere is sufficiently large to inflate and thereby distort the geomagnetic field. Although Mars is larger than Mercury and four times farther from the Sun, it is thought that the solar wind has stripped away up to a third of its original atmosphere, leaving a layer 1/100th as dense as the Earth's. It is believed the mechanism for this atmospheric stripping is gas caught in bubbles of the magnetic field, which are ripped off by the solar wind. In 2015 the NASA Mars Atmosphere and Volatile Evolution (MAVEN) mission measured the rate of atmospheric stripping caused by the magnetic field carried by the solar wind as it flows past Mars, which generates an electric field, much as a turbine on Earth can be used to generate electricity. This electric field accelerates electrically charged gas atoms, called ions, in Mars' upper atmosphere and shoots them into space. The MAVEN mission measured the rate of atmospheric stripping at about 100 grams (≈1/4 lb) per second.

## Moons and planetary surfaces

Mercury (planet), Mercury, the nearest planet to the Sun, bears the full brunt of the solar wind, and since its atmosphere is vestigial and transient, its surface is bathed in radiation. Mercury has an intrinsic magnetic field, so under normal solar wind conditions, the solar wind cannot penetrate its magnetosphere and particles only reach the surface in the cusp regions. During coronal mass ejections, however, the magnetopause may get pressed into the surface of the planet, and under these conditions, the solar wind may interact freely with the planetary surface. The Earth's Moon has no atmosphere or intrinsic Magnetosphere, magnetic field, and consequently its surface is bombarded with the full solar wind. The Apollo program, Project Apollo missions deployed passive aluminum collectors in an attempt to sample the solar wind, and lunar soil returned for study confirmed that the lunar regolith is enriched in atomic nuclei deposited from the solar wind. These elements may prove Lunar resources, useful resources for future colonization of the moon, lunar colonies.

# Outer limits

The solar wind "blows a bubble" in the interstellar medium (the rarefied hydrogen and helium gas that permeates the galaxy). The point where the solar wind's strength is no longer great enough to push back the interstellar medium is known as the Heliosphere, heliopause and is often considered to be the outer border of the Solar System. The distance to the heliopause is not precisely known and probably depends on the current velocity of the solar wind and the local density of the interstellar medium, but it is far outside Pluto's orbit. Scientists hope to gain perspective on the heliopause from data acquired through the Interstellar Boundary Explorer (IBEX) mission, launched in October 2008. The end of the heliosphere is noted as one of the ways defining the extent of the Solar System, along with the Kuiper belt, Kuiper Belt, and finally the radius at which of the Sun's gravitational influence is matched by other stars. The maximum extent of that influence has been estimated at between 50,000 AU and 2 light-years, compared to the edge of the heliopause (the outer edge of the heliosphere), which has been detected to end about 120 AU by the ''Voyager 1'' spacecraft. The ''Voyager 2'' spacecraft crossed the shock more than five times between August 30 and December 10, 2007. ''Voyager 2'' crossed the shock about a Orders of magnitude (length), Tm closer to the Sun than the 13.5 Tm distance where ''
Voyager 1 ''Voyager 1'' is a space probe A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel t ...
'' came upon the termination shock. The spacecraft moved outward through the termination shock into the heliosheath and onward toward the interstellar medium.

# Notable events

* From May 10 to May 12, 1999, NASA's Advanced Composition Explorer (ACE) and WIND (spacecraft), 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 (astronomy), strahl", which caused a highly unusual "polar rain" event, in which a visible
aurora An aurora (plural: auroras or aurorae), sometimes referred to as polar lights (aurora polaris), northern lights (aurora borealis), or southern lights (aurora australis), is a natural light display in the Earth's sky, predominantly seen in high-l ...
appeared over the North Pole. In addition, Earth's magnetosphere increased to between 5 and 6 times its normal size. *On December 13, 2010, ''
Voyager 1 ''Voyager 1'' is a space probe A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel t ...
'' 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.

* Active region * Deep Space Climate Observatory * Dyson–Harrop satellite * Electric sail * Heliospheric current sheet * Helium focusing cone * Interplanetary medium * List of plasma physics articles * Magnetic sail *
Parker Solar Probe The Parker Solar Probe (abbreviated PSP; previously Solar Probe, Solar Probe Plus or Solar Probe+) is a NASA The National Aeronautics and Space Administration (NASA; ) is an independent agencies of the United States government, independe ...

* Plasmasphere * Solar cycle * Solar sail * Solar Wind Composition Experiment *
STEREO File:Carsoundstereoshift.png, Time difference in a stereophonic recording of a car going past, 250px Stereophonic sound or, more commonly, stereo, is a method of sound reproduction that recreates a multi-directional, 3-dimensional audible per ...