The solar wind is a stream of charged particles that come from the Sun's outer atmosphere, called the corona. This plasma is mostly made up of electrons, protons, and alpha particles that have kinetic energy between 0.5 and 10 keV. The solar wind plasma also contains small amounts of other particles, such as heavy ions and atomic nuclei from elements like carbon, nitrogen, oxygen, neon, magnesium, silicon, sulfur, and iron. It also includes rare traces of other nuclei and isotopes, such as phosphorus, titanium, chromium, and nickel's isotopes Ni, Ni, and Ni. The interplanetary magnetic field is combined with the solar-wind plasma. The solar wind changes in density, temperature, and speed over time and across the Sun's latitude and longitude. Its particles can escape the Sun's gravity because of their high energy, which comes from the high temperature of the corona. This high temperature is caused by the coronal magnetic field. The boundary where the corona ends and the solar wind begins is called the Alfvén surface.

At a distance of more than a few solar radii from the Sun, the solar wind reaches speeds of 250–750 km/s and is supersonic, meaning it moves faster than the speed of fast magnetosonic waves. The solar wind is no longer supersonic at the termination shock. Other related events include auroras (northern and southern lights), comet tails that always point away from the Sun, and geomagnetic storms that can change the direction of magnetic field lines.

History

The idea that particles flow from the Sun to Earth was first proposed by British astronomer Richard C. Carrington in 1859. Along with Richard Hodgson, Carrington observed a sudden bright flash on the Sun, later called a solar flare. This event is now known to often happen with the release of material and magnetic energy from the Sun's atmosphere, called a coronal mass ejection. The next day, a strong geomagnetic storm was recorded, and Carrington thought it might be connected. Scientists now know the storm was caused by the coronal mass ejection reaching Earth and interacting with Earth's magnetic field. Later, Irish scientist George FitzGerald suggested that matter is regularly pushed away from the Sun and reaches Earth after several days.

In 1910, British scientist Arthur Eddington proposed the idea of a solar wind, though he did not name it, in a footnote about Comet Morehouse. His idea was not widely accepted, even though he had made a similar suggestion earlier. At that time, he thought the material might be electrons, but later suggested it could be ions.

Norwegian scientist Kristian Birkeland first proposed that the ejected material includes both ions and electrons. His studies of Earth's magnetic field showed that auroras, or northern and southern lights, are nearly constant. He believed these lights are caused by particles from the Sun hitting Earth and concluded that Earth is constantly hit by "rays of electric particles" from the Sun. In 1916, he wrote that solar rays likely include both negative electrons and positive ions. In 1919, British scientist Frederick Lindemann also suggested the Sun sends out both protons and electrons.

By the 1930s, scientists learned the Sun's corona, or outer atmosphere, must be extremely hot—about a million degrees Celsius—based on observations during solar eclipses. Later studies confirmed this temperature. In the 1950s, British mathematician Sydney Chapman calculated that such a hot corona would expand far into space. Around the same time, German astronomer Ludwig Biermann noticed that comet tails always point away from the Sun, no matter the comet's direction. He proposed this happens because the Sun emits a constant stream of particles that pushes the tails away. German astronomer Paul Ahnert later linked this observation to the solar wind, based on studies of comet Whipple–Fedke.

In 1956, Biermann discussed his findings with astrophysicist Eugene Parker at the University of Chicago. Parker also spoke with Sydney Chapman, who said the corona is so hot it should extend to Earth's orbit. Parker then proposed that the corona and solar particles are the same thing. He called this phenomenon the "solar wind" because he felt the term "solar corpuscular radiation" suggested individual particles, but the flow is more like gas. He said the math to describe the solar wind was simple, requiring only "four lines of algebra."

Parker argued that the corona is so hot it can escape the Sun's gravity despite being far away. As the Sun's gravity weakens, the gas flows outward like a de Laval nozzle, changing from slow to fast movement. When Parker solved equations for this process, he found a solution that matched the solar wind. His theory also predicted the shape of the Sun's magnetic field, which forms a spiral pattern called the Parker spiral.

Parker's ideas were not immediately accepted. When he submitted his paper in 1958, two reviewers rejected it, calling it "nonsense." However, the editor, Subrahmanyan Chandrasekhar, published it after finding no errors. Later, in 1960, Joseph Chamberlain proposed a slower version of the solar wind, called the "solar breeze," but this was shown to be unstable.

Parker's theory was confirmed by satellite observations, making it a rare example of a scientific prediction quickly verified. In 1959, the Soviet spacecraft Luna 1 first measured the solar wind using ion traps. Later missions, including Luna 2, Luna 3, and Venera 1, confirmed these findings. In 1962, American scientist Marcia Neugebauer used the Mariner 2 spacecraft to discover two types of solar wind: fast and slow.

In 1971, scientists Pneuman and Kopp created the first computer model of the solar wind, including both closed and open magnetic field lines. In 1990, the Ulysses probe studied the solar wind from high above the Sun's poles, unlike earlier missions near the solar plane. In the late 1990s, the SOHO spacecraft's UVCS instrument observed how the fast solar wind accelerates.

Acceleration mechanism

Early models of the solar wind used thermal energy to explain how material is accelerated. However, by the 1960s, scientists realized that thermal energy alone could not explain the high speeds of the solar wind. Another unknown force, likely connected to magnetic fields in the Sun's atmosphere, must be responsible.

The Sun's corona, its outer layer, is a region of hot plasma with temperatures exceeding a million degrees. Particles in the corona move at a range of speeds, following a pattern called a Maxwellian distribution. On average, these particles move at about 145 km/s, which is much slower than the Sun's escape velocity of 618 km/s. However, some particles gain enough energy to reach a speed of 400 km/s, allowing them to escape and join the solar wind. Electrons, because they are much lighter than ions, reach escape velocity more easily. Their movement creates an electric field that helps push ions away from the Sun.

Each second, the solar wind carries away about 1.3 × 10 particles. This means the Sun loses about (2–3) × 10 solar masses each year, or roughly 1.3–1.9 million tonnes per second. Over time, this loss would equal the mass of Earth every 150 million years. However, since the Sun formed, it has only lost about 0.01% of its original mass through the solar wind. Other stars have stronger stellar winds and lose mass at much faster rates.

In March 2023, extreme ultraviolet observations showed that small-scale magnetic reconnection might drive the solar wind. This process involves tiny bursts of energy, called nanoflares, that create jet-like activity known as jetlets. These events produce short bursts of hot plasma and Alfvén waves near the base of the solar corona. This activity may also be linked to the magnetic switchback phenomenon observed in the solar wind.

Properties and structure

The solar wind exists in two main forms: the slow solar wind and the fast solar wind. Their differences are not only in speed but also in other properties. Near Earth, the slow solar wind moves at 300–500 km/s, has a temperature of about 100 kilokelvin, and has a composition similar to the Sun's corona. The fast solar wind moves faster, at about 750 km/s, has a temperature of 800 kilokelvin, and has a composition close to the Sun's photosphere. The slow solar wind is twice as dense and more variable than the fast solar wind.

The slow solar wind comes from a region near the Sun's equator called the "streamer belt." This area forms coronal streamers, which are created by magnetic fields that are open to the heliosphere and wrap around closed magnetic loops. Scientists are still studying exactly how the slow solar wind forms and how material is released. Observations from 1996 to 2001 showed that the slow solar wind was emitted at latitudes up to 30–35° during the solar minimum (a time of low solar activity). As the solar cycle reached its peak, the slow solar wind expanded toward the poles. At solar maximum, the poles also released slow solar wind.

The fast solar wind comes from coronal holes, which are funnel-shaped areas with open magnetic field lines in the Sun's magnetic field. These open lines are common near the Sun's magnetic poles. The plasma source is small magnetic fields created by convection cells in the solar atmosphere. These fields trap plasma and move it into narrow parts of the coronal funnels, which are about 20,000 km above the photosphere. Plasma is released into the funnel when magnetic field lines reconnect.

At Earth's orbit (1 astronomical unit or AU), the solar wind moves at speeds between 250 and 750 km/s. Its density ranges from 3 to 10 particles per cubic centimeter, and its temperature ranges from 10 to 100 kelvin. On average, the plasma density decreases with the square of the distance from the Sun, while the speed decreases and becomes steady at 1 AU.

Voyager 1 and Voyager 2 measured plasma density between 0.001 and 0.005 particles per cubic centimeter at distances of 80 to 120 AU. Beyond 120 AU, near the heliopause, the density increased rapidly to between 0.05 and 0.2 particles per cubic centimeter. At 1 AU, the solar wind exerts a pressure between 1 and 6 nPa (1–6 × 10⁻⁹ N/m²), though this can vary.

Ram pressure depends on the speed and density of the solar wind. The formula for ram pressure is:

$$ P = frac{1}{2} m_p n V^2 $$

where $ m_p $ is the proton mass, $ P $ is pressure in pascals, $ n $ is density in particles per cubic centimeter, and $ V $ is speed in km/s.

Both the fast and slow solar wind can be interrupted by large bursts of plasma called coronal mass ejections (CMEs). CMEs are caused by the release of magnetic energy on the Sun. They are sometimes called "solar storms" or "space storms" in media reports. CMEs are not always linked to solar flares, which are another form of magnetic energy release. CMEs create shock waves in the heliosphere, producing electromagnetic waves and accelerating particles like protons and electrons, which create ionizing radiation before the CME arrives.

When a CME hits Earth's magnetosphere, it temporarily changes Earth's magnetic field, alters compass directions, and creates strong electrical currents in Earth's crust. This is called a geomagnetic storm and affects the entire planet. CMEs can also cause magnetic reconnection in Earth's magnetotail, sending protons and electrons toward Earth's atmosphere, where they form auroras.

CMEs are not the only cause of space weather. Different areas on the Sun produce solar wind with varying speeds and densities. These differences cause the wind to form spirals with slightly different angles, with fast streams moving more directly and slow streams wrapping around the Sun. Fast streams can overtake slower ones, creating turbulent regions called co-rotating interaction regions. These regions affect Earth's magnetosphere similarly to CMEs but less intensely.

CMEs have a complex structure, with a turbulent, hot, and compressed plasma region (called the sheath) followed by a colder, magnetized plasma region (called a magnetic cloud or ejecta). The sheath and ejecta have different effects on Earth's magnetosphere and space weather, such as the behavior of the Van Allen radiation belts.

Magnetic switchbacks are sudden changes in the solar wind's magnetic field, causing it to bend back on itself. These were first observed by the Ulysses mission, the first spacecraft to fly over the Sun's poles. The Parker Solar Probe later detected switchbacks in 2018.

Solar System effects

Over the Sun's lifetime, the movement of its outer layers and the solar wind escaping into space has slowed the Sun's surface rotation. The solar wind, along with the Sun's radiation, is responsible for the tails of comets. The solar wind also affects radio waves from space that reach Earth through a process called interplanetary scintillation.

When the solar wind reaches a planet with a strong magnetic field, such as Earth, Jupiter, or Saturn, charged particles in the wind are pushed away by a force called the Lorentz force. This area, called the magnetosphere, guides the particles around the planet instead of letting them hit the atmosphere or surface. The magnetosphere is shaped like a hemisphere facing the Sun and stretches into a long tail on the opposite side. The edge of this region is called the magnetopause, and some particles can enter the magnetosphere through a process called magnetic reconnection.

The solar wind shapes Earth's magnetosphere. Changes in the wind's speed, density, direction, and magnetic field affect Earth's space environment. For example, levels of radiation and radio interference can change greatly, and the magnetopause and bow shock wave can shift by several Earth radii, exposing satellites to the solar wind. These effects are called space weather.

A study from the European Space Agency's Cluster mission found that the solar wind may enter Earth's magnetosphere more easily than previously thought. Scientists observed unexpected waves in the solar wind that allow charged particles to pass through the magnetopause. This suggests the magnetosphere acts more like a filter than a solid barrier. The Cluster spacecraft, arranged in a special formation, helped scientists study these effects in three dimensions.

Research identified changes in the interplanetary magnetic field (IMF), influenced by a process called Kelvin–Helmholtz instability, which occurs where two fluids meet. These waves, seen in unexpected places, show how the solar wind can enter Earth's magnetosphere under certain IMF conditions. These findings are important for understanding magnetospheres around other planets.

The solar wind affects cosmic rays interacting with planetary atmospheres. Planets without strong magnetic fields lose their atmospheres over time due to the solar wind.

Venus, the planet closest to Earth, has a much denser atmosphere and no magnetic field. Space missions found a comet-like tail extending to Earth's orbit.

Earth's magnetic field protects it from the solar wind, pushing most charged particles away. Some particles get trapped in the Van Allen radiation belts. A few particles reach Earth's upper atmosphere and ionosphere in the auroral regions. The solar wind is only visible on Earth during strong events like auroras or geomagnetic storms. Bright auroras heat the ionosphere, expanding plasma into the magnetosphere and sending atmospheric material into space. Geomagnetic storms happen when magnetosphere pressure distorts Earth's magnetic field.

Mars, though farther from the Sun than Mercury, has lost much of its atmosphere due to the solar wind. The solar wind strips gas trapped in magnetic field bubbles, sending it into space. The MAVEN mission measured this process, finding that about 100 grams of Mars' atmosphere escape each second.

Mercury, the closest planet to the Sun, experiences the full force of the solar wind. Its weak atmosphere means its surface is exposed to radiation. Mercury has its own magnetic field, so the solar wind usually cannot enter its magnetosphere. However, during solar storms, the magnetosphere may shrink, allowing the solar wind to reach the surface.

The Moon has no atmosphere or magnetic field, so its surface is directly hit by the solar wind. Collectors placed during the Apollo missions captured solar wind particles, which were found in lunar soil. These particles might be useful for future lunar missions.

Limits

The Alfvén surface is the line that separates the corona from the solar wind. It is the place where the speed of the magnetic waves in the corona, called the Alfvén speed, matches the speed of the solar wind moving outward from the Sun.

Scientists were not certain where the Alfvén critical surface of the Sun was located. Using images of the corona taken from far away, they guessed it was between 10 and 20 solar radii from the Sun’s surface. On April 28, 2021, during its eighth close pass by the Sun, NASA’s Parker Solar Probe reached 18.8 solar radii and found the magnetic and particle conditions that showed it had crossed the Alfvén surface.

The solar wind creates a bubble in the interstellar medium, which is the thin gas made of hydrogen and helium that fills the galaxy. The point where the solar wind can no longer push back against this gas is called the heliopause. This is often considered the outer edge of the Solar System. The exact distance to the heliopause is not known and likely depends on the speed of the solar wind and the density of the interstellar gas around the Sun. It is much farther from the Sun than Pluto’s orbit. Scientists hope to learn more about the heliopause using data from the Interstellar Boundary Explorer (IBEX) mission, which began in October 2008.

The heliopause is one of the ways scientists define the edge of the Solar System. Other boundaries include the Kuiper Belt and the distance where the Sun’s gravity is balanced by the gravity of other stars. The farthest reach of the Sun’s gravity is estimated to be between 50,000 and 200,000 astronomical units (about 0.79 to 2 light-years). The heliopause, which marks the outer edge of the heliosphere, has been measured at about 120 astronomical units (18,000 million kilometers) by the Voyager 1 spacecraft.

Between August 30 and December 10, 2007, the Voyager 2 spacecraft crossed the termination shock more than five times. Voyager 2 crossed the shock about 1 billion kilometers closer to the Sun than the 13.5 billion kilometers (90 astronomical units) where Voyager 1 first encountered the termination shock. After crossing the shock, Voyager 2 moved outward through the heliosheath and continued toward the interstellar medium.