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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.
Solar flares are thought to occur when stored magnetic energy in the Sun's atmosphere accelerates charged particles in the surrounding plasma. This results in the emission of electromagnetic radiation across the electromagnetic spectrum.
High-energy electromagnetic radiation from solar flares is absorbed by the daylight side of Earth's upper atmosphere, in particular the ionosphere, and does not reach the surface. This absorption can temporarily increase the ionization of the ionosphere which may interfere with short-wave radio communication. The prediction of solar flares is an active area of research.
Flares also occur on other stars, where the term stellar flare applies.
Solar flares are eruptions of electromagnetic radiation originating in the Sun's atmosphere. [1] They affect all layers of the solar atmosphere (photosphere, chromosphere, and corona). [2] The plasma medium is heated to >107 kelvin, while electrons, protons, and heavier ions are accelerated to near the speed of light.[ citation needed ] Flares emit electromagnetic radiation across the electromagnetic spectrum at all wavelengths, from radio waves to gamma rays. [2]
Flares occur in active regions, often around sunspots, where intense magnetic fields penetrate the photosphere to link the corona to the solar interior. Flares are powered by the sudden (timescales of minutes to tens of minutes) release of magnetic energy stored in the corona. The same energy releases may also produce coronal mass ejections (CMEs), although the relationship between CMEs and flares is still not well understood. [3]
Associated with solar flares are flare sprays. [4] They involve faster ejections of material than eruptive prominences, [5] and reach velocities of 20 to 2000 kilometers per second. [6]
The frequency of occurrence of solar flares varies with the 11-year solar cycle. It can range from several per day during solar maximum to less than one every week during solar minimum. Additionally, more powerful flares are less frequent than weaker ones. For example, X10-class (severe) flares occur on average about eight times per cycle, whereas M1-class (minor) flares occur on average about 2000 times per cycle. [7]
Erich Rieger discovered with coworkers in 1984 an approximately 154 day period in the occurrence of gamma-ray emitting solar flares at least since the solar cycle 19. [8] The period has since been confirmed in most heliophysics data and the interplanetary magnetic field and is commonly known as the Rieger period. The period's resonance harmonics also have been reported from most data types in the heliosphere.
The frequency distributions of various flare phenomena can be characterized by power-law distributions. For example, the peak fluxes of radio, extreme ultraviolet, and hard and soft X-ray emissions; total energies; and flare durations (see § Duration) have been found to follow power-law distributions. [9] [10] [11] [12] : 23–28
The duration of a solar flare depends heavily on the wavelength of the electromagnetic radiation used in its calculation. This is due to different wavelengths being emitted through different processes and at different heights in the Sun's atmosphere.
A common measure of flare duration is the full width at half maximum (FWHM) time of soft X-ray flux within the wavelength bands 0.05 to 0.4 and 0.1 to 0.8 nanometres (0.5 to 4 and 1 to 8 ångströms ) measured by the GOES spacecraft in geosynchronous orbit. The FWHM time spans from when a flare's flux first reaches halfway between its maximum flux and the background flux and when it again reaches this value as the flare decays. Using this measure, the duration of a flare ranges from approximately tens of seconds to several hours with a median duration of approximately 6 and 11 minutes in the 0.05 to 0.4 and 0.1 to 0.8 nanometre bands, respectively. [13] [14]
After the eruption of a solar flare, post-eruption loops made up of hot plasma begin to form across the neutral line separating regions of opposite magnetic polarity near the flare's source. These loops extend from the photosphere up into the corona and form along the neutral line at increasingly greater distances from the source as time progresses. [16] The existence of these hot loops is thought to be continued by prolonged heating present after the eruption and during the flare's decay stage. [17]
In sufficiently powerful flares, typically of C-class or higher, the loops may combine to form an elongated arch-like structure known as a post-eruption arcade. These structures may last anywhere from multiple hours to multiple days after the initial flare. [16] In some cases, dark sunward-traveling plasma voids known as supra-arcade downflows may form above these arcades. [18]
Flares occur when accelerated charged particles, mainly electrons, interact with the plasma medium. Evidence suggests that the phenomenon of magnetic reconnection leads to this extreme acceleration of charged particles. [19] On the Sun, magnetic reconnection may happen on solar arcades – a type of prominence consisting of a series of closely occurring loops following magnetic lines of force. [20] These lines of force quickly reconnect into a lower arcade of loops leaving a helix of magnetic field unconnected to the rest of the arcade. The sudden release of energy in this reconnection is the origin of the particle acceleration. The unconnected magnetic helical field and the material that it contains may violently expand outwards forming a coronal mass ejection. [21] This also explains why solar flares typically erupt from active regions on the Sun where magnetic fields are much stronger.
Although there is a general agreement on the source of a flare's energy, the mechanisms involved are still not well understood. It is not clear how the magnetic energy is transformed into the kinetic energy of the particles, nor is it known how some particles can be accelerated to the GeV range (109 electron volt) and beyond. There are also some inconsistencies regarding the total number of accelerated particles, which sometimes seems to be greater than the total number in the coronal loop.[ citation needed ]
The modern classification system for solar flares uses the letters A, B, C, M, or X, according to the peak flux in watts per square metre (W/m2) of soft X-rays with wavelengths 0.1 to 0.8 nanometres (1 to 8 ångströms ), as measured by GOES satellites in geosynchronous orbit.
Classification | Peak flux range (W/m2) |
---|---|
A | < 10−7 |
B | 10−7 – 10−6 |
C | 10−6 – 10−5 |
M | 10−5 – 10−4 |
X | > 10−4 |
The strength of an event within a class is noted by a numerical suffix ranging from 1 up to, but excluding, 10, which is also the factor for that event within the class. Hence, an X2 flare is twice the strength of an X1 flare, an X3 flare is three times as powerful as an X1. M-class flares are a tenth the size of X-class flares with the same numeric suffix. [22] An X2 is four times more powerful than an M5 flare. [23] X-class flares with a peak flux that exceeds 10−3 W/m2 may be noted with a numerical suffix equal to or greater than 10.
This system was originally devised in 1970 and included only the letters C, M, and X. These letters were chosen to avoid confusion with other optical classification systems. The A and B classes would later be added in the 1990s as instruments became more sensitive to weaker flares. Around the same time, the backronym moderate for M-class flares and extreme for X-class flares began to be used. [24]
An earlier classification system, what is sometimes referred to as the flare importance, was based on H-alpha spectral observations. The scheme uses both the intensity and emitting surface. The classification in intensity is qualitative, referring to the flares as: faint (f), normal (n), or brilliant (b). The emitting surface is measured in terms of millionths of the hemisphere and is described below. (The total hemisphere area AH = 15.5 × 1012 km2.)
Classification | Corrected area (millionths of hemisphere) |
---|---|
S | < 100 |
1 | 100–250 |
2 | 250–600 |
3 | 600–1200 |
4 | > 1200 |
A flare then is classified taking S or a number that represents its size and a letter that represents its peak intensity, v.g.: Sn is a normal sunflare. [25]
Solar flares can also be classified based on their duration as either impulsive or long duration events (LDE). The time threshold separating the two is not well defined. The SWPC regards events requiring 30 minutes or more to decay to half maximum as LDEs, whereas Belgium's Solar-Terrestrial Centre of Excellence regards events with duration greater than 60 minutes as LDEs. [26] [27]
X-rays and extreme ultraviolet radiation emitted by solar flares are absorbed by the daylight side of Earth's atmosphere and do not reach the Earth's surface. Therefore, solar flares pose no direct danger to humans on Earth. However, this absorption of high-energy electromagnetic radiation can temporarily increase the ionization of the upper atmosphere, which can interfere with short-wave radio communication, and can temporarily heat and expand the Earth's outer atmosphere. This expansion can increase drag on satellites in low Earth orbit, which can lead to orbital decay over time. [28]
The temporary increase in ionization of the daylight side of Earth's atmosphere, in particular the D layer of the ionosphere, can interfere with short-wave radio communications that rely on its level of ionization for skywave propagation. Skywave, or skip, refers to the propagation of radio waves reflected or refracted off of the ionized ionosphere. When ionization is higher than normal, radio waves get degraded or completely absorbed by losing energy from the more frequent collisions with free electrons. [1]
The level of ionization of the atmosphere correlates with the strength of the associated solar flare in soft X-ray radiation. The NOAA classifies radio blackouts by the peak soft X-ray intensity of the associated flare.
Classification | Associated solar flare | Description [7] |
---|---|---|
R1 | M1 | Minor radio blackout |
R2 | M5 | Moderate radio blackout |
R3 | X1 | Strong radio blackout |
R4 | X10 | Severe radio blackout |
R5 | X20 | Extreme radio blackout |
The increased ionization of the D and E layers of the ionosphere caused by large solar flares increases the electrical conductivity of these layers allowing for the flow of electric currents. These ionospheric currents induce a magnetic field which can be measured by ground-based magnetometers. This phenomenon is known as a magnetic crochet or solar flare effect (SFE). [29] The former name derives from its appearance on magnetometers resembling a crochet hook.[ citation needed ] These disturbances are on the order of a few nanoteslas, which is relatively minor compared to those induced by geomagnetic storms. [30]
For astronauts in low earth orbit an expected radiation dose from the electromagnetic radiation emitted during a solar flare is about 0.05 gray, which is not immediately lethal on its own. Of much more concern for astronauts is the particle radiation associated with solar particle events. [31] [ better source needed ]
Flares produce radiation across the electromagnetic spectrum, although with different intensity. They are not very intense in visible light, but they can be very bright at particular spectral lines. They normally produce bremsstrahlung in X-rays and synchrotron radiation in radio.[ citation needed ]
Solar flares were first observed by Richard Carrington and Richard Hodgson independently on 1 September 1859 by projecting the image of the solar disk produced by an optical telescope through a broad-band filter. [33] [34] It was an extraordinarily intense white light flare, a flare emitting a high amount of light in the visual spectrum. [32]
Since flares produce copious amounts of radiation at H-alpha,[ citation needed ] adding a narrow (≈1 Å) passband filter centered at this wavelength to the optical telescope allows the observation of not very bright flares with small telescopes. For years Hα was the main, if not the only, source of information about solar flares. Other passband filters are also used.
During World War II, on February 25 and 26, 1942, British radar operators observed radiation that Stanley Hey interpreted as solar emission. Their discovery did not go public until the end of the conflict. The same year Southworth also observed the Sun in radio, but as with Hey, his observations were only known after 1945. In 1943, Grote Reber was the first to report radioastronomical observations of the Sun at 160 MHz. The fast development of radioastronomy revealed new peculiarities of the solar activity like storms and bursts related to the flares. Today ground-based radiotelescopes observe the Sun from c. 15 MHz up to 400 GHz.
Because the Earth's atmosphere absorbs much of the electromagnetic radiation emitted by the Sun with wavelengths shorter than 300 nm, space-based telescopes allowed for the observation of solar flares in previously unobserved high-energy spectral lines. Since the 1970s, the GOES series of satellites have been continuously observing the Sun in soft X-rays, and their observations have become the standard measure of flares, diminishing the importance of the H-alpha classification. Additionally, space-based telescopes allow for the observation of extremely long wavelengths—as long as a few kilometres—which cannot propagate through the ionosphere.
The most powerful flare ever observed is thought to be the flare associated with the 1859 Carrington Event. [36] While no soft X-ray measurements were made at the time, the magnetic crochet associated with the flare was recorded by ground-based magnetometers allowing the flare's strength to be estimated after the event. Using these magnetometer readings, its soft X-ray class has been estimated to be greater than X10. [37] The soft X-ray class of the flare has also been estimated to be around X50. [38] [39]
In modern times, the largest solar flare measured with instruments occurred on 4 November 2003. This event saturated the GOES detectors, and because of this its classification is only approximate. Initially, extrapolating the GOES curve, it was estimated to be X28. [40] Later analysis of the ionospheric effects suggested increasing this estimate to X45. [41] This event produced the first clear evidence of a new spectral component above 100 GHz. [42]
In 2016, Juan José Curto, alongside his colleagues, estimated the Carrington Event and 2003 event to be at least X45, [43] confirming these estimates in a 2020 review. [44]
Other large solar flares also occurred on 2 April 2001 (X20+), [45] 28 October 2003 (X17.2+ and 10), [46] 7 September 2005 (X17), [45] 9 August 2011 (X6.9), [47] 7 March 2012 (X5.4), [48] 6 September 2017 (X9.3), [49] [50] 31 December 2023 (X5), [51] and 22 February 2024 (X6.3). [52]
Current methods of flare prediction are problematic, and there is no certain indication that an active region on the Sun will produce a flare. However, many properties of sunspots and active regions correlate with flaring. For example, magnetically complex regions (based on line-of-sight magnetic field) called delta spots produce the largest flares. A simple scheme of sunspot classification due to McIntosh, or related to fractal complexity [53] is commonly used as a starting point for flare prediction. [54] Predictions are usually stated in terms of probabilities for occurrence of flares above M- or X-class within 24 or 48 hours. The U.S. National Oceanic and Atmospheric Administration (NOAA) issues forecasts of this kind. [55] MAG4 was developed at the University of Alabama in Huntsville with support from the Space Radiation Analysis Group at Johnson Space Flight Center (NASA/SRAG) for forecasting M- and X-class flares, CMEs, fast CME, and Solar Energetic Particle events. [56] A physics-based method that can predict imminent large solar flares was proposed by Institute for Space-Earth Environmental Research (ISEE), Nagoya University. [57]
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 ionosphere is the ionized part of the upper atmosphere of Earth, from about 48 km (30 mi) to 965 km (600 mi) above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an important role in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on Earth. It also affects GPS signals that travel through this layer.
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 C, N, O, Ne, Mg, Si, S, and Fe. There are also rarer traces of some other nuclei and isotopes such as P, Ti, Cr, and 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.
Cosmic noise, also known as galactic radio noise, is not actually sound, but a physical phenomenon derived from outside of the Earth's atmosphere. It can be detected through a radio receiver, which is an electronic device that receives radio waves and converts the information given by them to an audible form. Its characteristics are comparable to those of thermal noise. Cosmic noise occurs at frequencies above about 15 MHz when highly directional antennas are pointed toward the Sun or other regions of the sky, such as the center of the Milky Way Galaxy. Celestial objects like quasars, which are super dense objects far from Earth, emit electromagnetic waves in their full spectrum, including radio waves. The fall of a meteorite can also be heard through a radio receiver; the falling object burns from friction with the Earth's atmosphere, ionizing surrounding gases and producing radio waves. Cosmic microwave background radiation (CMBR) from outer space is also a form of cosmic noise. CMBR is thought to be a relic of the Big Bang, and pervades the space almost homogeneously over the entire celestial sphere. The bandwidth of the CMBR is wide, though the peak is in the microwave range.
X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and satellites. X-ray astronomy uses a type of space telescope that can see x-ray radiation which standard optical telescopes, such as the Mauna Kea Observatories, cannot.
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 temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave.
The solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a nearly periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surface. Over the period of a solar cycle, levels of solar radiation and ejection of solar material, the number and size of sunspots, solar flares, and coronal loops all exhibit a synchronized fluctuation from a period of minimum activity to a period of a maximum activity back to a period of minimum activity.
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 sudden ionospheric disturbance (SID) is any one of several ionospheric perturbations, resulting from abnormally high ionization/plasma density in the D region of the ionosphere and caused by a solar flare and/or solar particle event (SPE). 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.
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).
In solar physics and observation, an active region is a temporary feature in the Sun's atmosphere characterized by a strong and complex magnetic field. They are often associated with sunspots and are commonly the source of violent eruptions such as coronal mass ejections and solar flares. The number and location of active regions on the solar disk at any given time is dependent on the solar cycle.
A nanoflare is a very small episodic heating event which happens in the corona, the external atmosphere of the Sun.
The Bastille Day solar storm was a powerful solar storm on 14-16 July 2000 during the solar maximum of solar cycle 23. The storm began on the national day of France, Bastille Day. It involved a solar flare, a solar particle event, and a coronal mass ejection which caused a severe geomagnetic storm.
In solar physics, a solar particle event (SPE), also known as a solar energetic particle (SEP) 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.
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, coronal heating and sunspots.
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.
Gordon Dean Holman is an emeritus research astrophysicist at the National Aeronautics and Space Administration's (NASA’s) Goddard Space Flight Center in Greenbelt, Maryland. His research mostly focused on obtaining an understanding of high-energy radiation from astronomical objects. This radiation cannot be observed from Earth's surface, but is observed with instruments on satellites launched to orbits above Earth's atmosphere. It is primarily emitted by high-energy electrons interacting with ions. These electrons also emit radiation at radio frequencies which is observed from Earth's surface. Consequently, these observations from space and radio telescopes provide a view of hot gas and energetic particles in the Universe that could not otherwise be obtained. Holman has specialized in the interpretation of these observed emissions to determine the origin and evolution of this hot gas and energetic particles. He has been described as "not just a theorist, he also looks at the data".