Solar wind

• 1. Introduction

  ›

  The solar wind is a stream of charged particles pouring outward from the Sun's outermost atmospheric layer, the corona, at speeds of up to 750 kilometers per second. It touches nearly everything in the Solar System, carving the shape of Earth's magnetosphere, blowing comet tails, stripping planets of their atmospheres, and painting the sky with auroras. And yet for most of human history, nobody knew it existed.

  The questions that frame this story are deceptively simple. Where does the solar wind come from? How does something as vast and hot as the corona fling matter continuously into space? And what happens when that invisible river of particles finally hits something? The answers took centuries to piece together, from a 19th-century astronomer watching the Sun in a moment of brightness, to a spacecraft in 2021 that dove directly into the zone where the solar wind is actually born.

• 2. Early Clues and Contested Ideas

  ›

  Richard C. Carrington, a British astronomer, was the first to hint that particles flow from the Sun toward Earth. In 1859, Carrington and Richard Hodgson independently made the first recorded observations of a solar flare, a sudden localized brightening on the solar disc. The very next day, a powerful geomagnetic storm struck Earth. Carrington suspected a connection, and he was right. That storm is now understood as the arrival of a coronal mass ejection interacting with Earth's magnetosphere.

  Irish academic George FitzGerald later built on this intuition by arguing that matter was being continuously accelerated away from the Sun, taking several days to reach Earth. The idea moved slowly through scientific circles. In 1910, British astrophysicist Arthur Eddington essentially described the solar wind in a footnote to an article about Comet Morehouse, having made a similar suggestion at a Royal Institution address the year before. Even so, his proposition was never fully embraced.

  Norwegian scientist Kristian Birkeland pushed the concept further. His geomagnetic surveys showed that auroral activity was nearly uninterrupted, suggesting a constant source. He proposed in 1916 that solar rays consist of both negative and positive charges: electrons and ions together. Three years after Birkeland's proposal, in 1919, British physicist Frederick Lindemann arrived at the same conclusion independently, suggesting the Sun ejects both protons and electrons. These two voices were ahead of their time, but the observational tools to confirm them didn't yet exist.

• 3. Parker's Four Lines of Algebra

  ›

  Around the 1930s, scientists deduced from the corona's behavior during total solar eclipses that its temperature must reach roughly a million degrees Celsius. Later spectroscopic work confirmed this extraordinary temperature. By the mid-1950s, British mathematician Sydney Chapman had calculated what such a superheated gas would do and concluded that the corona must extend beyond Earth's orbit. German astronomer Ludwig Biermann, also in the 1950s, was studying why comet tails always point away from the Sun regardless of which direction the comet travels. He concluded that a steady stream of solar particles must be pushing those tails outward. German astronomer Paul Ahnert is credited with being the first to connect solar wind specifically to the direction of a comet's tail, based on observations of the comet Whipple-Fedke (1942g).

  In 1956, Biermann visited the University of Chicago, where he discussed his comet findings with astrophysicist Eugene Parker. Parker also spoke with Chapman, who told him the corona is so hot that it should extend all the way to Earth's orbit. Parker concluded that the corona and solar corpuscular radiation must be the same thing. He later recalled that the mathematics involved was just "four lines of algebra."

  Parker named his concept the solar wind deliberately. He explained: "I called it the solar wind because I felt that solar corpuscular radiation gives the wrong idea. With that term, one thinks of individual particles being shot out, which was the original picture we had. But it really is an ordinary flow of gas." His model showed that a million-degree corona cannot remain static: pressure forces must drive a flow that accelerates from subsonic near the Sun to supersonic beyond a critical point. He also predicted that solar rotation would wind the outward-flowing magnetic field lines into a spiral pattern in the ecliptic, now called the Parker spiral.

  When Parker submitted his results to The Astrophysical Journal in 1958, two reviewers recommended rejection. One wrote: "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, Parker's own colleague at the University of Chicago and future Nobel prize-winner Subrahmanyan Chandrasekhar, found no obvious errors and overruled the reviewers, publishing the paper even though he personally disagreed with Parker's theory.

• 4. Confirmation From Space

  ›

  Parker's theoretical predictions were described as "a unique example in astrophysics" because of how quickly observations confirmed them. In January 1959, the Soviet spacecraft Luna 1 became the first to directly detect and measure the solar wind, using hemispherical ion traps. Luna 2, Luna 3, and the more distant Venera 1 all verified the finding. Then, three years after Luna 1, American geophysicist Marcia Neugebauer and collaborators used the Mariner 2 spacecraft to perform a similar measurement. Mariner 2 data revealed something unexpected: the solar wind is not uniform. It exists in two distinct types, a slow component and a fast component.

  The first numerical simulation of the solar wind in the solar corona was performed by Pneuman and Kopp in 1971. In 1990, the Ulysses probe launched specifically to study the solar wind from high solar latitudes, because all prior observations had been confined to the ecliptic plane near the Solar System's equator.

  In the late 1990s, the Ultraviolet Coronal Spectrometer aboard the SOHO spacecraft observed that the fast solar wind from the Sun's poles accelerates far more rapidly than Parker's model predicted. Parker's model placed the transition to supersonic flow at about four solar radii, roughly 3,000,000 kilometers from the photosphere. The actual transition point appears to be much lower, perhaps only one solar radius, about 700,000 kilometers, above the photosphere. This gap pointed toward an additional acceleration mechanism still not fully understood, though work by Hannes Alfven, who won the Nobel Prize in Physics in 1970, provided gravitational and electromagnetic groundwork for explaining it.

  From May 10 to the 12th of May 1999, NASA's Advanced Composition Explorer and WIND spacecraft observed a 98% decrease in solar wind density. This allowed energetic electrons to travel to Earth in narrow beams called strahl, which triggered a highly unusual polar rain event: a visible aurora appeared over the North Pole. During the same period, Earth's magnetosphere expanded to between five and six times its normal size.

  On the 13th of December 2010, Voyager 1 determined that the solar wind at its location, roughly 10.8 billion miles from Earth, had slowed to zero. Voyager project scientist Edward Stone described it: "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."

• 5. Slow Wind, Fast Wind

  ›

  Near Earth, the solar wind is not one thing but two. The slow solar wind moves at roughly 300 kilometers per second and carries a temperature of about 100,000 kelvin. Its composition closely matches the corona. The fast solar wind moves at a typical 750 kilometers per second with a temperature around 800,000 kelvin, and its composition matches the Sun's photosphere instead. The slow wind is twice as dense and more variable than its faster counterpart.

  The slow wind appears to originate from a band around the Sun's equatorial region called the streamer belt, where magnetic flux opens outward over closed magnetic loops. Observations between 1996 and 2001 showed that during solar minimum, slow wind emission occurred at latitudes up to 30-35 degrees. As the solar cycle approached maximum, emission expanded toward the poles, and at solar maximum the poles themselves emitted slow solar wind.

  The fast wind comes from coronal holes, funnel-like regions of open magnetic field lines concentrated especially around the Sun's magnetic poles. Plasma is generated by small magnetic fields from convection cells in the solar atmosphere, which carry material into the narrow necks of coronal funnels located only 20,000 kilometers above the photosphere. When those field lines reconnect, the plasma is released into the funnel and hurled outward.

  Both slow and fast winds can be interrupted by coronal mass ejections, or CMEs, which are large, fast-moving bursts of plasma driven by the release of magnetic energy at the Sun. CMEs cause shock waves in the heliosphere, launching electromagnetic waves and accelerating particles that form showers of ionizing radiation. When a CME strikes Earth's magnetosphere, it temporarily deforms Earth's magnetic field, changing compass needle directions and inducing large electrical ground currents. CME impacts can also trigger magnetic reconnection in Earth's magnetotail, launching protons and electrons downward into the atmosphere to form auroras.

• 6. What the Wind Does to Worlds

  ›

  Earth is largely shielded from the solar wind by its magnetic field. Most charged particles are deflected, though some are trapped in the Van Allen radiation belt and a smaller number travel along electromagnetic pathways into the upper atmosphere in the auroral zones. The levels of ionizing radiation and radio interference around Earth can vary by factors of hundreds to thousands depending on solar wind conditions. The shape and location of the magnetopause can shift by several Earth radii, at times exposing geosynchronous satellites directly to the solar wind.

  Studies from the European Space Agency's Cluster mission found evidence that the solar wind can penetrate Earth's magnetosphere more easily than previously assumed. Scientists directly observed unexpected wave patterns, specifically Kelvin-Helmholtz instability waves at the magnetopause, forming under solar wind conditions that were formerly thought to be unfavorable for their generation. These findings indicate the magnetosphere acts more as a filter than a solid barrier.

  Venus, with an atmosphere 100 times denser than Earth's but virtually no magnetic field, has developed a comet-like tail that extends all the way to Earth's orbit under the sculpting pressure of the solar wind. Mars, though four times farther from the Sun than Mercury, is thought to have lost up to a third of its original atmosphere to solar wind stripping. The NASA MAVEN mission measured the rate of atmospheric loss from Mars at about 100 grams per second.

  Mercury, the closest planet to the Sun, bears the solar wind's full force. It has an intrinsic magnetic field that normally protects its surface, but during coronal mass ejections the magnetopause can be pressed all the way to the planet's surface, allowing direct contact. The Moon has no atmosphere or magnetic field at all. Project Apollo missions deployed passive aluminum collectors to sample the solar wind directly, and the lunar soil they returned confirmed that the lunar regolith is enriched in atomic nuclei deposited over time by the solar wind. Those embedded elements may prove useful as resources for future lunar expeditions.

• 7. The Edge of the Bubble

  ›

  The solar wind does not travel forever unchecked. It pushes outward into the interstellar medium, which is the rarefied hydrogen and helium gas filling the galaxy, and inflates a vast bubble called the heliosphere. The point where the solar wind's pressure can no longer push back the interstellar medium is called the heliopause, which is generally considered the outer boundary of the Solar System. Voyager 1 detected the heliopause at roughly 120 astronomical units from the Sun.

  Before reaching the heliopause, the solar wind passes through the termination shock, a boundary where its supersonic flow abruptly slows. Voyager 2 crossed the termination shock more than five times between the 30th of August and the 10th of December 2007. It crossed that boundary roughly a billion kilometers closer to the Sun than the distance where Voyager 1 had encountered the same shock.

  On the 28th of April 2021, NASA's Parker Solar Probe made the journey in the opposite direction, traveling inward. During its eighth flyby of the Sun, at 18.8 solar radii from the surface, the probe encountered the specific magnetic and particle conditions that mark the Alfvén surface, the boundary where the corona ends and the solar wind formally begins. It was the first spacecraft to cross into that region. Parker Solar Probe was launched in 2018, named in honor of Eugene Parker himself, who at age 91 was present to watch the launch. The probe's mission spans seven years and twenty-four orbits, each bringing it closer to the Sun, ultimately to within 0.04 astronomical units of the solar surface.