Solar wind

The solar wind is a stream of charged particles released from the Sun’s outermost atmospheric layer, the corona. This plasma is made up mostly of electrons, protons, and alpha particles and also contains trace amounts of heavy ions and atomic nuclei of various elements such as carbon, nitrogen, oxygen, and iron. These particles have enough energy to escape the Sun’s gravity due to the extremely high temperature of the corona.

As the solar wind moves away from the Sun, it speeds up and can reach velocities between 250 and 750 km/s. Far from the Sun, the solar wind is faster than the speed of certain waves, making it supersonic. However, it slows down when it reaches a special area called the termination shock.

The solar wind plays an important role in creating beautiful natural lights known as the aurora, shaping the tails of comets so they always point away from the Sun, and sometimes causing geomagnetic storms that affect Earth’s magnetic field.

History

Observations from Earth

In 1859, British astronomer Richard C. Carrington and Richard Hodgson first saw bright flashes on the Sun, called solar flares. The next day, strong disturbances in Earth's magnetic field were noticed. This showed that material and energy from the Sun could reach Earth.

Later, scientists realized that the Sun constantly sends out a stream of particles, now called the solar wind. In the 1950s, calculations showed that the Sun’s outer atmosphere, called the corona, must be very hot and stretch far into space. Observations of comets also helped scientists understand this stream.

Theoretical prediction

In 1956, Ludwig Biermann discussed his ideas with astrophysicist Eugene Parker at the University of Chicago. Parker used math to show that the Sun’s hot corona must flow outward like a wind. He explained that even though the Sun’s gravity pulls on this material, the heat pushes it away, creating a steady flow into space.

Parker’s idea was not accepted right away, but later it was proven correct. His work helped scientists understand how the solar wind moves and shapes the magnetic field around the Sun.

Observations from space

In 1959, a Soviet spacecraft called Luna 1 was the first to measure the solar wind directly. Later, American spacecraft like Mariner 2 confirmed Parker’s ideas and showed there are different speeds in the solar wind.

Since then, many spacecraft have studied the solar wind from different positions in space. In 2018, NASA launched the Parker Solar Probe to travel closer to the Sun than any spacecraft before, learning more about how the solar wind is created and how it speeds up.

Acceleration mechanism

Early ideas about how the solar wind moves focused on heat from the Sun. But by the 1960s, scientists realized that heat alone couldn't explain how fast the solar wind travels. They found that magnetic fields in the Sun's atmosphere likely play a big role in pushing the solar wind outward.

The Sun's outer layer, called the corona, is very hot — over a million degrees. Particles there move at different speeds, but most are slower than needed to escape the Sun's pull. However, some particles get enough energy to reach the needed speed and become part of the solar wind. Because electrons are much lighter than other particles, they escape first and create an electric field that helps push heavier particles away from the Sun.

The solar wind carries away about 1.3×10³⁶ particles every second. This means the Sun loses a small amount of mass — roughly the weight of Earth — every 150 million years. Even so, over its entire life, the Sun has only lost about 0.01% of its original mass this way. Other stars can lose mass much faster through stronger stellar winds.

Jetlets

In March 2023, special observations showed that tiny magnetic events called jetlets might help drive the solar wind. These jetlets release short bursts of hot material and waves, which could also be linked to a puzzling feature of the solar wind known as magnetic switchbacks.

Properties and structure

The solar wind is a stream of charged particles from the Sun. It has two main types: the slow solar wind and the fast solar wind. The slow solar wind moves at speeds between 300 and 500 kilometers per second and comes from areas near the Sun's equator. The fast solar wind moves much quicker, at around 750 kilometers per second, and comes from areas called coronal holes near the Sun's poles.

Sometimes, big bursts of particles called coronal mass ejections can interrupt the solar wind. These bursts can cause changes in Earth's magnetic field and create beautiful lights in the sky called auroras.

Solar System effects

Main article: Space weather

The Sun sends out a stream of tiny particles called the solar wind. This stream changes how the Sun spins over time and helps create the tails we see on comets. It can also make radio waves on Earth act strangely.

Magnetospheres

Main article: Magnetosphere

When the solar wind meets a planet with a strong magnetic field, like Earth, Jupiter, or Saturn, the particles are pushed away. This creates a protective bubble called a magnetosphere around the planet. The magnetosphere looks like a big half-ball on the side facing the Sun and stretches out on the other side. Some particles can still get through, especially during strong solar storms.

The solar wind shapes Earth's magnetosphere and can change space conditions around our planet. This affects things like radiation levels and radio signals. Recent studies show that the solar wind can enter Earth's magnetosphere more easily than we thought, acting like a filter rather than a solid wall.

Atmospheres

The solar wind also affects space rays coming to planets and can strip away atmospheres from planets without strong magnetic fields.

Venus, Earth’s neighbor, has a thick atmosphere but no strong magnetic field. Space probes found it has a tail stretching toward Earth.

Earth is mostly protected by its magnetic field, but some solar wind particles get trapped and reach the upper atmosphere, creating beautiful lights in the sky called auroras. Strong solar winds can cause big changes in Earth’s magnetic field.

Mars once had a thicker atmosphere, but the solar wind may have blown much of it away. The NASA MAVEN mission measured how quickly this is still happening today.

Moons and planetary surfaces

Mercury, closest to the Sun, feels the solar wind strongly. Usually, its magnetic field protects it, but during big solar storms, the wind can reach its surface.

The Moon has no atmosphere or magnetic field, so the solar wind hits it directly. Samples brought back by Apollo missions showed that the Moon’s surface has particles from the solar wind, which could be useful for future explorers.

Limits

Alfvén surface

Main article: Alfvén surface

The Alfvén surface is the place where the Sun's outer layer stops and the solar wind begins. It is where the speed of the Sun's particles matches the speed of magnetic waves.

Scientists were not sure exactly where this surface was. But on April 28, 2021, NASA's Parker Solar Probe found the conditions that showed it had reached this surface during a close flyby of the Sun.

Outer limits

Main article: Heliopshere

The solar wind creates a kind of bubble around our Solar System in the vast space between the stars. The place where the solar wind can no longer push back against the space between stars is called the heliopause. This is often thought of as the outer edge of our Solar System. We do not know exactly how far it is, but it is far beyond the orbit of Pluto. Scientists are learning more about this from a special spacecraft called the Interstellar Boundary Explorer.

The heliopause helps us understand how far the Solar System reaches, along with the Kuiper Belt and the point where the Sun's pull is matched by other stars. This influence could stretch from about 50,000 astronomical units to 2 light-years, while the heliopause has been found at about 120 astronomical units by the Voyager 1 spacecraft.

The Voyager 2 spacecraft passed through the termination shock several times in late 2007, a bit closer to the Sun than where Voyager 1 met it. It then moved on through this area toward the space between stars.