Solar wind

• 1. Historical Observations And Predictions

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  In 1859, British astronomer Richard C. Carrington watched a sudden flash of brightness on the solar disc that would later be known as a solar flare. The next day, Earth experienced a powerful geomagnetic storm that disrupted telegraph systems and convinced him of a connection between the Sun and our planet. Irish academic George FitzGerald followed this observation by suggesting matter was regularly accelerated away from the Sun to reach Earth after several days. In 1910, British astrophysicist Arthur Eddington wrote a footnote about Comet Morehouse where he essentially suggested the existence of particles flowing outward without naming it. He had also postulated at a Royal Institution address the previous year that ejected material consisted of electrons rather than ions. Norwegian scientist Kristian Birkeland proposed in 1916 that solar rays were neither exclusively negative nor positive but contained both kinds. His geomagnetic surveys showed auroral activity was almost uninterrupted, leading him to conclude Earth was being continually bombarded by rays of electric corpuscles emitted by the Sun. Three years later, British physicist Frederick Lindemann suggested the Sun ejects particles of both polarities including protons as well as electrons. Around the 1930s, scientists concluded the temperature of the solar corona must be a million degrees Celsius because of how it extended into space during total solar eclipses. Later spectroscopic work confirmed this extraordinary temperature to be true. In the mid-1950s, British mathematician Sydney Chapman calculated properties of gas at such high temperatures and determined the corona must extend way out into space beyond Earth's orbit. German astronomer Ludwig Biermann became interested in the fact that comet tails always point away from the Sun regardless of travel direction. He postulated this happens because the Sun emits a steady stream of particles pushing the tail away. Paul Ahnert is credited with first relating solar wind to the direction of a comet's tail based on observations of Comet Whipple, Fedke in 1942.

• 2. Spacecraft Discovery Missions

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  In January 1959, the Soviet spacecraft Luna 1 directly observed the solar wind and measured its strength using hemispherical ion traps. This discovery was verified by subsequent missions including Luna 2, Luna 3, and the more distant measurements of Venera 1. Three years later, American geophysicist Marcia Neugebauer and collaborators used the Mariner 2 spacecraft to perform similar measurements. Mariner 2 data revealed two types of solar wind consisting of low-speed and high-speed components. The Ulysses probe launched in 1990 studied the solar wind from high solar latitudes for the first time since all prior observations had been made near the Solar System's ecliptic plane. From May 10 to the 12th of May 1999, NASA's Advanced Composition Explorer and 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 which caused a highly unusual polar rain event where visible aurora appeared over the North Pole. During this period, Earth's magnetosphere increased to between 5 and 6 times its normal size. The STEREO mission launched in 2006 to study coronal mass ejections and the solar corona using stereoscopy from two widely separated imaging systems. Each STEREO spacecraft carried two heliospheric imagers capable of imaging the solar wind itself via Thomson scattering of sunlight off free electrons. On the 13th of December 2010, Voyager 1 determined that the velocity of the solar wind at its location had slowed to zero. Voyager project scientist Edward Stone explained that the wind was no longer moving outward but only moving sideways to end up going down the tail of the heliosphere. In 2018, NASA launched the Parker Solar Probe named in honor of American astrophysicist Eugene Parker on a seven-year mission to make twenty-four orbits of the Sun. The probe will pass within 0.04 astronomical units of the Sun's surface during its final perihelion passes.

• 3. Acceleration Mechanisms And Physics

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  While early models relied primarily on thermal energy to accelerate material, by the 1960s it became clear thermal acceleration alone cannot account for high speeds. An additional unknown mechanism likely relates to magnetic fields in the solar atmosphere. The Sun's corona is heated to over a megakelvin resulting in particles with speeds described by a Maxwellian distribution. The mean velocity of these particles is about 50 kilometers per second which is well below the solar escape velocity of 618 kilometers per second. However, a few particles achieve energies sufficient to reach terminal velocity allowing 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 3 times 10^36 per second. Thus the total mass loss each year is about 2 times 10^-14 solar masses or approximately 1.3 to 1.9 million tonnes per second. This is equivalent to losing a mass equal to Earth every 150 million years. Since the Sun's formation only about 0.01% of its initial mass has been lost through the solar wind. In March 2023 solar extreme ultraviolet observations showed small-scale magnetic reconnection could be a driver of the solar wind as swarms of nanoflares produce short-lived streams of hot plasma and Alfvén waves at the base of the corona.

• 4. Fast And Slow Wind States

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  The solar wind exists in two fundamental states termed slow solar wind and fast solar wind with differences extending well beyond speeds alone. In near-Earth space the slow solar wind has a velocity of 400 kilometers per second and a temperature of roughly 100,000 kelvin with composition matching the corona closely. By contrast the fast solar wind has typical velocity of 750 kilometers per second and temperature of 80,000 kelvin while nearly matching the photosphere composition. The slow solar wind is twice as dense and more variable than the fast solar wind. Observations between 1996 and 2001 showed emission of slow solar wind occurred at latitudes up to 30 to 35 degrees during solar minimum then expanded toward poles as cycle approached maximum. At solar maximum the poles were also emitting slow solar wind. The fast solar wind originates from coronal holes which are funnel-like regions of open field lines particularly prevalent around the Sun's magnetic poles. Plasma source consists of small magnetic fields created by convection cells in the solar atmosphere that confine plasma and transport it into narrow necks of coronal funnels located only 20,000 kilometers above the photosphere. The plasma releases when these magnetic field lines reconnect allowing material to escape into interplanetary space.

• 5. Magnetospheric Interactions And Space Weather

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  Where solar wind intersects with planets having well-developed magnetic fields like Earth Jupiter or Saturn particles deflect via Lorentz force creating magnetosphere regions. This region shapes roughly like a hemisphere facing the Sun then draws out in long wake opposite side. Boundary called magnetopause allows some particles to penetrate through partial reconnection of magnetic field lines. Fluctuations in speed density direction and entrained magnetic field strongly affect local space environment causing ionizing radiation levels to vary by factors of hundreds to thousands. Shape and location of magnetopause and bow shock wave can change by several Earth radii exposing geosynchronous satellites to direct solar wind. These phenomena collectively called space weather include geomagnetic storms resulting when pressure of plasmas inside magnetosphere inflates and distorts geomagnetic field. When CME impacts Earth's magnetosphere it temporarily deforms magnetic field changing compass needle directions and inducing large electrical ground currents globally. CME impacts launch protons and electrons downward toward atmosphere forming aurora through magnetic reconnection in Earth's magnetotail. Different patches on Sun give rise to slightly different speeds and densities depending on local conditions forming spiral patterns affecting Earth's magnetosphere similarly but more gently than CMEs. Fast-moving streams tend to overtake slower streams originating westward forming turbulent co-rotating interaction regions giving rise to wave motions and accelerated particles.

• 6. Atmospheric Stripping On Planets

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  Planets with weak or non-existent magnetospheres face atmospheric stripping by the solar wind as demonstrated by Venus Mars and Mercury. Venus has 100 times denser atmosphere than Earth yet little or no geo-magnetic field allowing space probes to discover comet-like tail extending to Earth's orbit. Earth largely protected from solar wind by magnetic field deflecting most charged particles though some trapped in Van Allen radiation belt. Smaller number travel to upper atmosphere and ionosphere in auroral zones observable only during strong phenomena like aurora and geomagnetic storms. Although Mars larger than Mercury and four times farther from Sun thought to have lost up to third of original atmosphere leaving layer one-hundredth as dense as Earth's. Mechanism involves gas caught in bubbles of magnetic field ripped off by solar wind generating electric field accelerating ions into space. In 2015 NASA Mars Atmosphere and Volatile Evolution mission measured rate at about 100 grams per second. Mercury bears full brunt of solar wind since its atmosphere vestigial and transient surface bathed in radiation. During coronal mass ejections magnetopause may press into planet surface allowing solar wind to interact freely with planetary surface. Earth's Moon lacks atmosphere or intrinsic magnetic field consequently surface bombarded with full solar wind. Project Apollo missions deployed passive aluminum collectors attempting to sample solar wind confirming lunar regolith enriched in atomic nuclei deposited from solar wind.

• 7. Heliopause And System Boundaries

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  The Alfvén surface marks boundary separating corona from solar wind defined where coronal plasma's Alfvén speed equals large-scale solar wind speed. Researchers unsure exactly where Alfvén critical surface lay until the 28th of April 2021 when Parker Solar Probe encountered specific conditions at 18.8 solar radii indicating penetration. The solar wind blows bubble in interstellar medium rarefied hydrogen and helium gas permeating galaxy. Point where strength no longer great enough to push back interstellar medium known as heliopause often considered outer border of Solar System. Distance not precisely known probably depends on current velocity of solar wind and local density of interstellar medium but far outside Pluto orbit. Maximum extent of Sun's gravitational influence estimated between 50,000 and 100,000 astronomical units compared to heliopause detected about 120 AU by Voyager 1 spacecraft. Voyager 2 crossed termination shock more than five times between August 30 and the 10th of December 2007 moving outward through heliosheath toward interstellar medium. Scientists hope gain perspective on heliopause from data acquired through Interstellar Boundary Explorer mission launched October 2008.