Solar wind

The Sun emits streams of protons, electrons, and some heavier particles in all directions. Known as the solar wind, the outward flow of material in these streams comes from the outermost region of the Sun’s atmosphere, the corona.

Overview

The Sun’s (its outer atmosphere) does not end abruptly but gradually decreases in density as it extends billions of kilometers into space. The outward movement and expansion of the corona are functions of distance from the Sun. Expansion close to the Sun is very slow since the pull of gravity is dominant, but as the distance from the Sun increases, the outward flow increases. This flow of gas is the solar wind, the term devised by Eugene N. Parker in 1958 in his classic paper on the dynamics of interplanetary gas. The is a stream of ionized gas constantly blown away from the Sun at high speed in all directions. It is composed primarily of electrons, protons, alpha particles (helium nuclei), and some heavier ions.

The rate of mass loss as a result of the outflow of material via the solar wind is only on the order of 10^-14 or 10^-15 of the per year (or about 10^-9 of the Earth’s mass per year). Since the Sun is losing mass, however, the total of the Sun is decreasing. As a result, the angular speed of rotation of the Sun also decreases with time, at least on stellar evolutionary timescales. A study of surface rotation rates of young solar-type stars in the Pleiades and Hyades star clusters shows that they have rotation rates some ten times faster than that of the Sun.

At the distance of Earth’s orbit from the Sun (150 million kilometers), the average number density of ions under “quiet Sun” conditions (periods when the Sun is not exhibiting high activity, as during solar maxima) is five particles per cubic centimeter. Varying solar activity can cause this number to vary widely from the average value; measurements from space probes yield a range of 0.4 to eighty particles per cubic centimeter. The temperature of the particles in the solar wind at the Sun is about one million kelvins. By the time they reach the Earth, their temperature has dropped to 200,000 kelvins on average, but because their density is so low, no appreciable heat transfer to Earth occurs. Again, there is considerable variation, from a minimum of 5,000 kelvins to a maximum of one million kelvins.

The use of the word “wind” is appropriate, considering the speeds involved. At the Earth’s orbit, the solar wind whips by at approximately 400 to 500 kilometers per second on average, though there are large fluctuations ranging between 200 kilometers per second minimum and 1,000 kilometers per second maximum. The solar wind is composed mostly of electrons and protons, with a helium abundance averaging 5 percent but ranging from 0 to 25 percent.

Historically, it was known that there is a correlation between solar activity and geomagnetic storms. A large summary of historical data correlating geomagnetic activity and solar activity was published in 1940 by Chapman and J. Bartels. In 1931, nine years prior to this comprehensive summary, the initial model to provide an explanation for the connection was proposed by Sydney Chapman and V. C. A. Ferraro. The model involved streams of ionized (electrically charged) gas ejected from the Sun at the time of solar flares. The interaction of these ionized gas streams, trapped in the magnetic polar regions of the Earth, with the Earth’s atmosphere triggered the northern and southern lights (also known as the aurora borealis and australis). This initial model was updated in 1951 by Ludwig Biermann, who suggested that, rather than occasional streams, there was a continuous outward flux of charged particles from the Sun into interplanetary space. This revised model was designed to explain the antisolar spikes observed in some comets. The classic paper by Parker in 1958 predicted that interplanetary space is filled with a solar wind. The content and speed of this predicted wind would be functions of the temperature of the corona. Parker predicted that the solar wind speed in the vicinity of the Earth would range from 400 to 800 kilometers per second.

Parker’s paper came out just at the dawn of the space age. Since then, numerous space missions have gathered massive amounts of data on the solar wind that could not have been obtained any other way. Between 1959 and 1961, the Soviet interplanetary probes Luna 2 and 3, Venus 1, and Mars 1, using rudimentary (by today’s standards) particle detectors, confirmed Parker’s prediction. Also in 1961, the initial findings of the Soviet space probes were confirmed by the United States’ Explorer 10 satellite using a Faraday-cup probe. The first long-term monitoring of the solar wind was conducted by the US Mariner 2 spacecraft on its three-month mission to Venus in 1962. Mariner 2 again confirmed Parker’s prediction by measuring a continuous solar wind with speeds ranging from 319 to 771 kilometers per second. However, the Mariner results disclosed something not predicted by Parker: gusts in the solar wind up to 1,000 kilometers per second that were phase-correlated with the rotational period of the Sun.

The reason for the correlation between the Sun’s rotation, solar wind gusts, and geomagnetic perturbations with a recurrent twenty-seven-day period (which matched the Sun’s synodic rotation period) was not determined until rocket and X-ray data of the Sun revealed the existence of nonmagnetic, long-lived holes in the corona. The coronal holes with the rest of the Sun, and since they are nonmagnetic, they permit ions an easy exit from what normally are magnetically confined regions of the Sun. The streams of ions passing through the coronal holes have higher-than-normal speeds because they are able to escape without having to overcome the additional magnetic force effects usually present at the Sun’s surface. Data from Skylab firmly established that coronal holes are the source of the high-speed streams (gusts) of the solar wind. This discovery is among the most well-established solar-terrestrial connections and is useful in making daily forecasts of geomagnetic disturbances.

Even though the solar wind is emitted in all directions from the Sun, the resultant plasma is not uniform. Observations indicate that the speed of the solar wind tends to be higher and more variable at high solar latitudes. The polar regions of the Sun are nearly always covered by coronal holes, and thus, one would expect gusts from the higher latitudes at higher-than-normal speeds.

The Pioneer series of spacecraft carried instrumentation to detect and measure the solar wind. In August 1972, Pioneer 9 (at a distance from the Sun close to Earth’s) recorded solar wind speeds of 1,000 kilometers per second, while Pioneer 10 (which was 214 million kilometers from the Sun, nearing the orbit of Mars) recorded the solar wind at about half that speed. In 1983, Pioneer 10 detected the presence of the solar wind as far out as 4.5 billion kilometers, at the orbit of Neptune. All the Pioneer data show that the average speed of the solar wind changes comparatively little out to Jupiter’s orbit, but the range of fluctuations in speed is remarkably diminished at Jupiter’s orbit compared with the range at Earth’s orbit.

The study of solar wind continued into the twenty-first century and was advanced by images sent back to Earth from orbiters such as the Parker Solar Probe (PSP). In 2023, the PSP traced solar wind back to where it has been generated, allowing for the study of solar wind before it exits the Sun’s corona and enters Earth. The PSP indicated that solar wind is inextricably linked to coronal holes. On December 24, 2024, the PSP flew just 3.8 million miles (6.1 million kilometers) above the solar surface at a speed of 430,000 miles (692,000 kilometers) per hour—the closest flight ever recorded. This proximity allows for unprecedented data collection on solar phenomena, including the origins and acceleration of the solar wind.

Knowledge Gained

The bulk of the solar wind is composed of electrons, protons (hydrogen nuclei), and alpha particles (helium nuclei), but there are also traces of heavier elements, the most abundant of which are measurable at the distance of Earth’s orbit. Solar wind abundances have been derived from Vela (3, 5, and 6) and Apollo (11, 12, 14, 15, 16, and 17) mission data for the elements that are most common in the entire and the Sun’s corona. The agreement of relative abundance values is quite good, considering the varying accuracies of the individual determinations (roughly a factor of two).

The Apollo 17 mission provided data to compute the abundance of iron in the solar wind. Other Apollo missions produced data that resulted in abundance ratios for the light noble gases neon and argon and their isotopes. A major finding of the Apollo program was the detection of the heavier noble gases krypton and xenon in the solar wind and, by implication, the presence of these two elements in the Sun. The anomalous presence of noble gases in meteoritic and lunar surface material is explained by the exposure of these materials to the solar wind.

The ancient solar wind is preserved in approximately the outermost micron of the surface of solid objects since ions with kinetic energies greater than one kilo-electron volt (keV) are trapped on impact with solid surfaces. This record is an easy target for alteration or destruction by a whole host of events. Even so, analyses of lunar material and meteorites indicate that some sort of solar wind has been present at the distance of Earth’s orbit for the past three to five billion years. A study of Apollo 15 deep-drill cores (as deep as three meters) by Donald D. Bogard and L. E. Nyquist concluded that the solar wind at the distance of Earth’s orbit shows little variation over the last 400 million years.

Magnetic fields associated with the planets in the solar system are distorted by interaction with the solar wind. The magnetopause (outer boundary of a planet’s magnetosphere) is where a balance exists between the solar wind and the planet’s magnetic field. The boundary appears to be stable for planets with large magnetic fields. An average value for the size of Earth’s magnetosphere (a term in which the root “sphere” does not necessarily mean spherical in shape but rather refers to the sphere of influence of Earth’s magnetic field) is about thirteen Earth radii. This is a sizable obstruction in the path of the solar wind. The result of this obstacle to the supersonic flow of the solar wind is a standing shock wave. To have a smooth flow about the periphery of the magnetosphere, the solar wind must transition from supersonic to subsonic speeds. In addition, when the plasma is abruptly slowed, its is increased (about tenfold). The point of closest approach by the solar wind to the center of a planet is called the stagnation point. The for Earth is at a distance of about ten Earth radii from the center of the planet (well above the atmosphere).

On December 27, 1984, the Active Magnetospheric Particle Tracer Explorers (AMPTE), a three-satellite cooperative venture by West Germany, Britain, and the United States, generated an artificial cloud of ionized barium. The project was designed to track ionized elements to determine how many of the solar wind’s ionized particles actually enter the Earth’s and to understand the formation and motions of the high-energy particles in the Van Allen trapped-radiation belts. Results from an earlier test in September 1984 (with as the tracer element) had shown that approximately 1 percent of solar-wind particles are transported into the magnetosphere surrounding the Earth.

The data from these tests were not definitive and provided only a measure for the specific conditions existing at the time of the test. They indicated that charged particles captured from the solar wind spill out of the outer Van Allen radiation belt (which is inside Earth’s magnetosphere) and enter Earth’s atmosphere around the north and south magnetic poles. The collisions of these charged particles with atoms of oxygen and nitrogen stimulate them to radiate pale greens and bright reds in the northern and southern skies. This colorful display is known as the borealis or (the northern or southern lights), most often seen in zones between 65° and 70° north and south magnetic latitude.

The heliopause is the boundary about fifteen billion kilometers from the Sun (about three times the size of Neptune’s orbit), where the solar wind merges with the and loses its identity. The interface is the outer limit of the heliosphere, the region influenced by the solar wind. Pioneer 10 data showed that the solar wind oscillates with the eleven-year solar cycle. Increases in solar activity and the solar wind result in decreasing numbers of entering galactic cosmic rays, since the more active acts as a shield. Thus, the solar wind influences both the solar and galactic cosmic-ray fluxes at the Earth through a modulation process at the and through magnetic field-line reconnections at the magnetopause.

Context

The of a comet is a “dirty snowball” of various ices with embedded dust and grit. The outer layer of the nucleus sublimates when it nears the Sun, producing a gaseous cloud (the coma) around the nucleus and one or two tails—Type I and Type II. Type II tails are the dust tails. They are smooth, homogeneous, and point generally away from the Sun along a curve. Dust grains in Type II tails have a high area-to-mass ratio, and thus, the effect of pressure is significant; Type II tails are blown away from the Sun by the transfer of momentum from solar photons to the dust grains. Type I tails are the ionic tails and are primarily composed of CO⁺ along with some other ions, such as N₂⁺, CO₂⁺, CH⁺, and OH⁺. Type I tails are long, straight, patchy, and point radially away from the Sun.

Early theories postulated some sort of interplanetary medium to account for the orientation of Type I ionic tails. An extensive study of Type I comet tail orientations by C. Hoffmeister in 1943, long before Parker’s predictions and the actual discovery of the solar wind, required the existence of an interacting resistive medium expanding outward from the Sun. A detailed analysis by John C. Brandt yielded an average expansion speed of 474 ± 21 kilometers per second, with a minimum near 150 kilometers per second. These values match very well the speeds measured for the solar wind by spacecraft. The interaction of the solar wind with the coma of a comet produces a bow shock, a hundred thousand to a million kilometers from the nucleus. This interaction, in turn, carries charged particles from the coma away from the Sun, forming the Type I tail.

In 1981, Brandt and Malcolm B. Niedner published an extensive photographic summary of Type I tail disassociation events (when the ionic tail disconnects from the comet). An angular sector structure of the interplanetary medium and solar wind was identified that rotates with the Sun, within which the direction of the dominant is constant. The basic premise is that as a comet crosses a sector boundary, the sudden reversal of the dominant magnetic field direction causes the charged tail to break away from the rest of the uncharged comet, producing the patchy appearance of Type I tails. Thus, comets can be used as interplanetary magnetic probes to determine the spatial location of the sectors.

Bibliography

Akasofu, Syun-Ichi, and Y. Kamide, editors. The Solar Wind and the Earth. D. Reidel, 1987.

Brandt, John C. Introduction to the Solar Wind. W. H. Freeman, 1970.

Chaisson, Eric, and Steve McMillan. Astronomy Today. 9th ed., Addison-Wesley, 2017.

Foukal, Peter. Solar Astrophysics. 2nd rev. ed., Wiley-VCH, 2004. Wiley, 1990.

Fraknoi, Andrew, et al. Voyages to the Stars and Galaxies. Brooks/Cole-Thomson Learning, 2006.

Freedman, Roger A., and William J. Kaufmann III. Universe. 11th ed., W. H. Freeman, 2019.

Gosling, J. T., and A. J. Hundhausen. “Waves in the Solar Wind.” Scientific American, vol. 236, Mar. 1977, pp. 36-43.

Interrante, Abbey. "What Is the Solar Wind?" NASA, 21 Nov. 2024, science.nasa.gov/sun/what-is-the-solar-wind. Accessed 6 Feb. 2025.

Lea, Robert. “NASA's Sun-Kissing Parker Solar Probe Finds Source of 'Fast' Solar Wind.” Space.com, 7 June 2023, www.space.com/parker-solar-probe-fast-solar-wind-source. Accessed 6 Feb. 2025.

Meyer-Vernet, Nicole. Basics of the Solar Wind. Cambridge UP, 2007.

Moldwin, Mark. An Introduction to Space Weather. Cambridge UP, 2008.

Pisanko, Yuri V., et al. Solar Wind Acceleration. Cambridge Scholars P, 2022.

Raouafi, Nour E. Solar Physics and Solar Wind. Wiley, 2021.

Schneider, Stephen E., and Thomas T. Arny. Pathways to Astronomy. 6th ed., McGraw-Hill, 2021.