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Solar phenomena are natural phenomena which occur within the atmosphere of the Sun. They take many forms, including solar wind, radio wave flux, solar flares, coronal mass ejections, [1] coronal heating and sunspots.
These phenomena are believed to be generated by a helical dynamo, located near the center of the Sun's mass, which generates strong magnetic fields, as well as a chaotic dynamo, located near the surface, which generates smaller magnetic field fluctuations. [2] All solar fluctuations together are referred to as solar variation, producing space weather within the Sun's graviational field.
Solar activity and related events have been recorded since the eighth century BCE. Throughout history, observation technology and methodology advanced, and in the 20th century, interest in astrophysics surged and many solar telescopes were constructed. The 1931 invention of the coronagraph allowed the corona to be studied in full daylight.
The Sun is a star located at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma and magnetic fields. [3] [4] It has a diameter of about 1,392,684 kilometres (865,374 mi), [5] around 109 times that of Earth, and its mass (1.989×1030 kilograms, approximately 330,000 times that of Earth) accounts for some 99.86% of the total mass of the Solar System. [6] Chemically, about three quarters of the Sun's mass consists of hydrogen, while the rest is mostly helium. The remaining 1.69% (equal to 5,600 times the mass of Earth) consists of heavier elements, including oxygen, carbon, neon and iron. [7]
The Sun formed about 4.567 billion [lower-alpha 1] [8] years ago from the gravitational collapse of a region within a large molecular cloud. Most of the matter gathered in the center, while the rest flattened into an orbiting disk that became the balance of the Solar System. The central mass became increasingly hot and dense, eventually initiating thermonuclear fusion in its core.
The Sun is a G-type main-sequence star (G2V) based on spectral class and it is informally designated as a yellow dwarf because its visible radiation is most intense in the yellow-green portion of the spectrum. It is actually white, but from the Earth's surface it appears yellow because of atmospheric scattering of blue light. [9] In the spectral class label, G2 indicates its surface temperature, of approximately 5770 K ( the UAI will accept in 2014 5772 K ) and V indicates that the Sun, like most stars, is a main-sequence star, and thus generates its energy via fusing hydrogen into helium. In its core, the Sun fuses about 620 million metric tons of hydrogen each second. [10] [11]
The Earth's mean distance from the Sun is approximately 1 astronomical unit (about 150,000,000 km; 93,000,000 mi), though the distance varies as the Earth moves from perihelion in January to aphelion in July. [12] At this average distance, light travels from the Sun to Earth in about 8 minutes, 19 seconds. The energy of this sunlight supports almost all life [lower-alpha 2] on Earth by photosynthesis, [13] and drives Earth's climate and weather. [14] As recently as the 19th century scientists had little knowledge of the Sun's physical composition and source of energy. This understanding is still developing; a number of present-day anomalies in the Sun's behavior remain unexplained.
Many solar phenomena change periodically over an average interval of about 11 years. This solar cycle affects solar irradiation and influences space weather, terrestrial weather and climate.
The solar cycle also modulates the flux of short-wavelength solar radiation, from ultraviolet to X-ray and influences the frequency of solar flares, coronal mass ejections and other solar eruptive phenomena.
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A coronal mass ejection (CME) is a massive burst of solar wind and magnetic fields rising above the solar corona. [15] Near solar maxima, the Sun produces about three CMEs every day, whereas solar minima feature about one every five days. [16] CMEs, along with solar flares of other origin, can disrupt radio transmissions and damage satellites and electrical transmission line facilities, resulting in potentially massive and long-lasting power outages. [17] [18]
Coronal mass ejections often appear with other forms of solar activity, most notably solar flares, but no causal relationship has been established. Most weak flares do not have CMEs; most powerful ones do. Most ejections originate from active regions on the Sun's surface, such as sunspot groupings associated with frequent flares. Other forms of solar activity frequently associated with coronal mass ejections are eruptive prominences, coronal dimming, coronal waves and Moreton waves, also called solar tsunami.
Magnetic reconnection is responsible for CME and solar flares. Magnetic reconnection is the name given to the rearrangement of magnetic field lines when two oppositely directed magnetic fields are brought together. This rearrangement is accompanied with a sudden release of energy stored in the original oppositely directed fields. [19] [20]
When a CME impacts the Earth's magnetosphere, it temporarily deforms the Earth's magnetic field, changing the direction of compass needles and inducing large electrical ground currents in Earth itself; this is called a geomagnetic storm and it is a global phenomenon. CME impacts can induce magnetic reconnection in Earth's magnetotail (the midnight side of the magnetosphere); this launches protons and electrons downward toward Earth's atmosphere, where they form the aurora.
A solar flare is a sudden flash of brightness observed over the Sun's surface or the solar limb, which is interpreted as an energy release of up to 6 × 1025 joules (about a sixth of the total Sun's energy output each second or 160 billion megatons of TNT equivalent, over 25,000 times more energy than released from the impact of Comet Shoemaker–Levy 9 with Jupiter). It may be followed by a coronal mass ejection. [21] The flare ejects clouds of electrons, ions and atoms through the corona into space. These clouds typically reach Earth a day or two after the event. [22] Similar phenomena in other stars are known as stellar flares.
Solar flares strongly influence space weather near the Earth. They can produce streams of highly energetic particles in the solar wind, known as a solar proton event. These particles can impact the Earth's magnetosphere in the form of a geomagnetic storm and present radiation hazards to spacecraft and astronauts.
A solar proton event (SPE), or "proton storm", occurs when particles (mostly protons) emitted by the Sun become accelerated either close to the Sun during a flare or in interplanetary space by CME shocks. The events can include other nuclei such as helium ions and HZE ions. These particles cause multiple effects. They can penetrate the Earth's magnetic field and cause ionization in the ionosphere. The effect is similar to auroral events, except that protons rather than electrons are involved. Energetic protons are a significant radiation hazard to spacecraft and astronauts. [23] Energetic protons can reach Earth within 30 minutes of a major flare's peak.
A prominence is a large, bright, gaseous feature extending outward from the Sun's surface, often in the shape of a loop. Prominences are anchored to the Sun's surface in the photosphere and extend outwards into the corona. While the corona consists of high temperature plasma, which does not emit much visible light, prominences contain much cooler plasma, similar in composition to that of the chromosphere.
Prominence plasma is typically a hundred times cooler and denser than coronal plasma. A prominence forms over timescales of about an earthly day and may persist for weeks or months. Some prominences break apart and form CMEs.
A typical prominence extends over many thousands of kilometers; the largest on record was estimated at over 800,000 kilometres (500,000 mi) long [24] – roughly the solar radius.
When a prominence is viewed against the Sun instead of space, it appears darker than the background. This formation is called a solar filament. [24] It is possible for a projection to be both a filament and a prominence. Some prominences are so powerful that they eject matter at speeds ranging from 600 km/s to more than 1000 km/s. Other prominences form huge loops or arching columns of glowing gases over sunspots that can reach heights of hundreds of thousands of kilometers. [25]
Sunspots are relatively dark areas on the Sun's radiating 'surface' (photosphere) where intense magnetic activity inhibits convection and cools the Photosphere. Faculae are slightly brighter areas that form around sunspot groups as the flow of energy to the photosphere is re-established and both the normal flow and the sunspot-blocked energy elevate the radiating 'surface' temperature. Scientists began speculating on possible relationships between sunspots and solar luminosity in the 17th century. [26] [27] Luminosity decreases caused by sunspots (generally < - 0.3%) are correlated with increases (generally < + 0.05%) caused both by faculae that are associated with active regions as well as the magnetically active 'bright network'. [28]
The net effect during periods of enhanced solar magnetic activity is increased radiant solar output because faculae are larger and persist longer than sunspots. Conversely, periods of lower solar magnetic activity and fewer sunspots (such as the Maunder Minimum) may correlate with times of lower irradiance. [29]
Sunspot activity has been measured using the Wolf number for about 300 years. This index (also known as the Zürich number) uses both the number of sunspots and the number of sunspot groups to compensate for measurement variations. A 2003 study found that sunspots had been more frequent since the 1940s than in the previous 1150 years. [30]
Sunspots usually appear as pairs with opposite magnetic polarity. [31] Detailed observations reveal patterns, in yearly minima and maxima and in relative location. As each cycle proceeds, the latitude of spots declines, from 30 to 45° to around 7° after the solar maximum. This latitudinal change follows Spörer's law.
For a sunspot to be visible to the human eye it must be about 50,000 km in diameter, covering 2,000,000,000 square kilometres (770,000,000 sq mi) or 700 millionths of the visible area. Over recent cycles, approximately 100 sunspots or compact sunspot groups are visible from Earth. [lower-alpha 3] [32]
Sunspots expand and contract as they move about and can travel at a few hundred meters per second when they first appear.
The solar wind is a stream of plasma released from the Sun's upper atmosphere. It consists of mostly electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in density, temperature and speed over time and over solar longitude. These particles can escape the Sun's gravity because of their high energy.
The solar wind is divided into the slow solar wind and the fast solar wind. The slow solar wind has a velocity of about 400 kilometres per second (250 mi/s), a temperature of 2×105 K and a composition that is a close match to the corona. The fast solar wind has a typical velocity of 750 km/s, a temperature of 8×105 K and nearly matches the photosphere's. [33] [34] The slow solar wind is twice as dense and more variable in intensity than the fast solar wind. The slow wind has a more complex structure, with turbulent regions and large-scale organization. [35] [36]
Both the fast and slow solar wind can be interrupted by large, fast-moving bursts of plasma called interplanetary CMEs, or ICMEs. They cause shock waves in the thin plasma of the heliosphere, generating electromagnetic waves and accelerating particles (mostly protons and electrons) to form showers of ionizing radiation that precede the CME.
Space weather is the environmental condition within the Solar System, including the solar wind. It is studied especially surrounding the Earth, including conditions from the magnetosphere to the ionosphere and thermosphere. Space weather is distinct from terrestrial weather of the troposphere and stratosphere. The term was not used until the 1990s. Prior to that time, such phenomena were considered to be part of physics or aeronomy.
Solar storms are caused by disturbances on the Sun, most often coronal clouds associated with solar flare CMEs emanating from active sunspot regions, or less often from coronal holes. The Sun can produce intense geomagnetic and proton storms capable of causing power outages, disruption or communications blackouts (including GPS systems) and temporary/permanent disabling of satellites and other spaceborne technology. Solar storms may be hazardous to high-latitude, high-altitude aviation and to human spaceflight. [37] Geomagnetic storms cause aurorae. [38]
The most significant known solar storm occurred in September 1859 and is known as the Carrington event. [39] [40]
An aurora is a natural light display in the sky, especially in the high latitude (Arctic and Antarctic) regions, in the form of a large circle around the pole. It is caused by the collision of solar wind and charged magnetospheric particles with the high altitude atmosphere (thermosphere).
Most auroras occur in a band known as the auroral zone, [41] [42] which is typically 3° to 6° wide in latitude and observed at 10° to 20° from the geomagnetic poles at all longitudes, but often most vividly around the spring and autumn equinoxes. The charged particles and solar wind are directed into the atmosphere by the Earth's magnetosphere. A geomagnetic storm expands the auroral zone to lower latitudes.
Auroras are associated with the solar wind. The Earth's magnetic field traps its particles, many of which travel toward the poles where they are accelerated toward Earth. Collisions between these ions and the atmosphere release energy in the form of auroras appearing in large circles around the poles. Auroras are more frequent and brighter during the solar cycle's intense phase when CMEs increase the intensity of the solar wind. [43]
A geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field that interacts with the Earth's magnetic field. The increase in solar wind pressure compresses the magnetosphere and the solar wind's magnetic field interacts with the Earth's magnetic field to transfer increased energy into the magnetosphere. Both interactions increase plasma movement through the magnetosphere (driven by increased electric fields) and increase the electric current in the magnetosphere and ionosphere. [44]
The disturbance in the interplanetary medium that drives a storm may be due to a CME or a high speed stream (co-rotating interaction region or CIR) [45] of the solar wind originating from a region of weak magnetic field on the solar surface. The frequency of geomagnetic storms increases and decreases with the sunspot cycle. CME driven storms are more common during the solar maximum of the solar cycle, while CIR-driven storms are more common during the solar minimum.
Several space weather phenomena are associated with geomagnetic storms. These include Solar Energetic Particle (SEP) events, geomagnetically induced currents (GIC), ionospheric disturbances that cause radio and radar scintillation, disruption of compass navigation and auroral displays at much lower latitudes than normal. A 1989 geomagnetic storm energized ground induced currents that disrupted electric power distribution throughout most of the province of Quebec [46] and caused aurorae as far south as Texas. [47]
A sudden ionospheric disturbance (SID) is an abnormally high ionization/plasma density in the D region of the ionosphere caused by a solar flare. The SID results in a sudden increase in radio-wave absorption that is most severe in the upper medium frequency (MF) and lower high frequency (HF) ranges, and as a result often interrupts or interferes with telecommunications systems. [48]
Geomagnetically induced currents are a manifestation at ground level of space weather, which affect the normal operation of long electrical conductor systems. During space weather events, electric currents in the magnetosphere and ionosphere experience large variations, which manifest also in the Earth's magnetic field. These variations induce currents (GIC) in earthly conductors. Electric transmission grids and buried pipelines are common examples of such conductor systems. GIC can cause problems such as increased corrosion of pipeline steel and damaged high-voltage power transformers.
The production of carbon-14 (radiocarbon: 14C) is related to solar activity. Carbon-14 is produced in the upper atmosphere when cosmic ray bombardment of atmospheric nitrogen (14N) induces the nitrogen to undergo β+ decay, thus transforming into an unusual isotope of carbon with an atomic weight of 14 rather than the more common 12. Because galactic cosmic rays are partially excluded from the Solar System by the outward sweep of magnetic fields in the solar wind, increased solar activity reduces 14C production. [49]
Atmospheric 14C concentration is lower during solar maxima and higher during solar minima. By measuring the captured 14C in wood and counting tree rings, production of radiocarbon relative to recent wood can be measured and dated. A reconstruction of the past 10,000 years shows that the 14C production was much higher during the mid-Holocene 7,000 years ago and decreased until 1,000 years ago. In addition to variations in solar activity, long-term trends in carbon-14 production are influenced by changes in the Earth's geomagnetic field and by changes in carbon cycling within the biosphere (particularly those associated with changes in the extent of vegetation between ice ages).[ citation needed ]
While solar activity has been a main driver of climate change over geologic time, its role in the warming that began in the twentieth century does not appear to have been significant. [50]
Solar activity and related events have been regularly recorded since the time of the Babylonians. Early records described solar eclipses, the corona and sunspots.
Soon after the invention of telescopes, in the early 1600s, astronomers began observing the Sun. Thomas Harriot was the first to observe sunspots, in 1610. Observers confirmed the less-frequent sunspots and aurorae during the Maunder minimum. [51]
Solar spectrometry began in 1817. [52] Rudolf Wolf gathered sunspot observations as far back as the 1755–1766 cycle. He established a relative sunspot number formulation (the Wolf or Zürich sunspot number) that became the standard measure. Around 1852, Sabine, Wolf, Gautier and von Lamont independently found a link between the solar cycle and geomagnetic activity. [52]
On 2 April 1845, Fizeau and Foucault first photographed the Sun. Photography assisted in the study of solar prominences, granulation, spectroscopy and solar eclipses. [52]
On 1 September 1859, Richard C. Carrington and separately R. Hodgson first observed a solar flare. [52] Carrington and Gustav Spörer discovered that the Sun exhibits differential rotation, and that the outer layer must be fluid. [52]
In 1907–08, George Ellery Hale uncovered the Sun's magnetic cycle and the magnetic nature of sunspots. Hale and his colleagues later deduced Hale's polarity laws that described its magnetic field. [52]
Bernard Lyot's 1931 invention of the coronagraph allowed the corona to be studied in full daylight. [52]
The Sun was, until the 1990s, the only star whose surface had been resolved. [53] Other major achievements included understanding of: [54]
In the later twentieth century, satellites began observing the Sun, providing many insights. For example, modulation of solar luminosity by magnetically active regions was confirmed by satellite measurements of total solar irradiance (TSI) by the ACRIM1 experiment on the Solar Maximum Mission (launched in 1980). [28]
A corona is the outermost layer of a star's atmosphere. It is a hot but relatively dim region of plasma populated by intermittent coronal structures known as solar prominences or filaments.
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between 0.5 and 10 keV. The composition of the solar wind plasma also includes a mixture of materials found in the solar plasma: trace amounts of heavy ions and atomic nuclei of elements such as carbon, nitrogen, oxygen, neon, magnesium, silicon, sulfur, and iron. There are also rarer traces of some other nuclei and isotopes such as phosphorus, titanium, chromium, and nickel's isotopes 58Ni, 60Ni, and 62Ni. Superimposed with the solar-wind plasma is the interplanetary magnetic field. The solar wind varies in density, temperature and speed over time and over solar latitude and longitude. Its particles can escape the Sun's gravity because of their high energy resulting from the high temperature of the corona, which in turn is a result of the coronal magnetic field. The boundary separating the corona from the solar wind is called the Alfvén surface.
A solar flare is a relatively intense, localized emission of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other eruptive solar phenomena. The occurrence of solar flares varies with the 11-year solar cycle.
Space weather is a branch of space physics and aeronomy, or heliophysics, concerned with the varying conditions within the Solar System and its heliosphere. This includes the effects of the solar wind, especially on the Earth's magnetosphere, ionosphere, thermosphere, and exosphere. Though physically distinct, space weather is analogous to the terrestrial weather of Earth's atmosphere. The term "space weather" was first used in the 1950s and popularized in the 1990s. Later, it prompted research into "space climate", the large-scale and long-term patterns of space weather.
A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave.
A coronal mass ejection (CME) is a significant ejection of magnetic field and accompanying plasma mass from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accepted theoretical understanding of these relationships has not been established.
A solar storm is a disturbance on the Sun, which can emanate outward across the heliosphere, affecting the entire Solar System, including Earth and its magnetosphere, and is the cause of space weather in the short-term with long-term patterns comprising space climate.
A coronal hole is a temporary region of relatively cool, less dense plasma in the solar corona where the Sun's magnetic field extends into interplanetary space as an open field. Compared to the corona's usual closed magnetic field that arches between regions of opposite magnetic polarity, the open magnetic field of a coronal hole allows solar wind to escape into space at a much quicker rate. This results in decreased temperature and density of the plasma at the site of a coronal hole, as well as an increased speed in the average solar wind measured in interplanetary space. If streams of high-speed solar wind from coronal holes encounter the Earth, they can cause major displays of aurorae. Near solar minimum, when activity such as coronal mass ejections is less frequent, such streams are the main cause of geomagnetic storms and associated aurorae.
A stellar magnetic field is a magnetic field generated by the motion of conductive plasma inside a star. This motion is created through convection, which is a form of energy transport involving the physical movement of material. A localized magnetic field exerts a force on the plasma, effectively increasing the pressure without a comparable gain in density. As a result, the magnetized region rises relative to the remainder of the plasma, until it reaches the star's photosphere. This creates starspots on the surface, and the related phenomenon of coronal loops.
Solar cycle 24 is the most recently completed solar cycle, the 24th since 1755, when extensive recording of solar sunspot activity began. It began in December 2008 with a minimum smoothed sunspot number of 2.2, and ended in December 2019. Activity was minimal until early 2010. It reached its maximum in April 2014 with a 23 months smoothed sunspot number of 81.8. This maximum value was substantially lower than other recent solar cycles, down to a level which had not been seen since cycles 12 to 15 (1878-1923).
Energetic Neutral Atom (ENA) imaging is a technology used to create global images of otherwise invisible phenomena in the magnetospheres of planets and throughout the heliosphere.
Solar cycle 20 was the twentieth solar cycle since 1755, when extensive recording of solar sunspot activity began. The solar cycle lasted 11.4 years, beginning in October 1964 and ending in March 1976. The maximum smoothed sunspot number observed during the solar cycle was 156.6, and the starting minimum was 14.3. During the minimum transit from solar cycle 20 to 21, there were a total of 272 days with no sunspots.
Solar cycle 23 was the 23rd solar cycle since 1755, when extensive recording of solar sunspot activity began. The solar cycle lasted 12.3 years, beginning in August 1996 and ending in December 2008. The maximum smoothed sunspot number observed during the solar cycle was 180.3, and the starting minimum was 11.2. During the minimum transit from solar cycle 23 to 24, there were a total of 817 days with no sunspots. Compared to the last several solar cycles, it was fairly average in terms of activity.
In solar physics, a solar particle event (SPE), also known as a solar energetic particle event or solar radiation storm, is a solar phenomenon which occurs when particles emitted by the Sun, mostly protons, become accelerated either in the Sun's atmosphere during a solar flare or in interplanetary space by a coronal mass ejection shock. Other nuclei such as helium and HZE ions may also be accelerated during the event. These particles can penetrate the Earth's magnetic field and cause partial ionization of the ionosphere. Energetic protons are a significant radiation hazard to spacecraft and astronauts.
The Heliophysics Science Division of the Goddard Space Flight Center (NASA) conducts research on the Sun, its extended Solar System environment, and interactions of Earth, other planets, small bodies, and interstellar gas with the heliosphere. Division research also encompasses geospace—Earth's uppermost atmosphere, the ionosphere, and the magnetosphere—and the changing environmental conditions throughout the coupled heliosphere.
The solar storm of 2012 was a solar storm involving an unusually large and strong coronal mass ejection that occurred on July 23, 2012. It missed Earth with a margin of approximately nine days, as the equator of the Sun rotates around its own axis with a period of about 25 days.
Solar cycle 25 is the current solar cycle, the 25th since 1755, when extensive recording of solar sunspot activity began. It began in December 2019 with a minimum smoothed sunspot number of 1.8. It is expected to continue until about 2030.
The solar storms of August 1972 were a historically powerful series of solar storms with intense to extreme solar flare, solar particle event, and geomagnetic storm components in early August 1972, during solar cycle 20. The storm caused widespread electric‐ and communication‐grid disturbances through large portions of North America as well as satellite disruptions. On 4 August 1972 the storm caused the accidental detonation of numerous U.S. naval mines near Haiphong, North Vietnam. The coronal mass ejection (CME)'s transit time from the Sun to the Earth is the fastest ever recorded.
Every second, the Sun burns 620 million tonnes of hydrogen...