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An X5.4-class solar flare as seen in 131 Å on 6 March 2012. The flare appears as a bright point in the center of the image.
An X5.4-class solar flare as seen in 131 Å on 6 March 2012. The flare appears as a bright point in the center of the image.

A solar flare is an intense eruption of electromagnetic radiation in the Sun's atmosphere.[1] Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other solar phenomena.

Solar flares occur in a power-law spectrum of magnitudes; an energy release of typically 1020 joules of energy suffices to produce a clearly observable event, while a major event can emit up to 1025 joules.[2] Although originally observed in the visible electromagnetic spectrum, especially in the H-alpha emission line of hydrogen, they can now be detected from radio wave to gamma-ray radiation.

Flares also occur on other stars, where the term stellar flare applies.


Solar flares affect all layers of the solar atmosphere (photosphere, chromosphere, and corona). The plasma medium is heated to tens of millions of kelvins, while electrons, protons, and heavier ions are accelerated to near the speed of light. Flares produce electromagnetic radiation across the electromagnetic spectrum at all wavelengths, from radio waves to gamma rays. Most of the energy is spread over frequencies outside the visual range; the majority of the flares are not visible to the naked eye and must be observed with special instruments. 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 produce coronal mass ejections (CMEs), although the relationship between CMEs and flares is still not well understood.[citation needed]

Associated with solar flares are flare sprays.[3] They involve faster ejections of material than eruptive prominences,[4] and reach velocities of 20 to 2000 kilometers per second.[5]


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.[6]

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.[7] 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.

Post-eruption loops and arcades

A post-eruption arcade present after an X5.7-class solar flare during the Bastille Day solar storm.[8]
A post-eruption arcade present after an X5.7-class solar flare during the Bastille Day solar storm.[8]

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.[9] 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.[10]

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.[9]


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.[11] On the Sun, magnetic reconnection may happen on solar arcades – a series of closely occurring loops following magnetic lines of force. 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.[12] 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's 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. Scientists are unable to forecast flares.[citation needed]


Multi-spacecraft observations of the 20 March 2014 X-class flare.

Soft X-ray classification

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 the GOES spacecraft in geosynchronous orbit 35,786 kilometres (22,236 miles) above the Earth's surface.

Classification Approximate peak flux range at 0.1-0.8 nanometre
(watts/square metre)
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,[13] 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, and only 50% more powerful than an X2.[14] An X2 is four times more powerful than an M5 flare.[15] 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, the largest event registered occurred on 2003 and was measured to be X28 with some uncertainty as at this scale most satellite instrument overload.[16][17]

H-alpha classification

An earlier flare classification 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.[18]


Massive X6.9-class solar flare, August 9, 2011

Solar flares pose no direct danger to humans on the Earth's surface. The harmful electromagnetic radiation emitted by flares, primarily X-rays, are absorbed by the daylight side of Earth's atmosphere and do not reach the Earth's surface. 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 cause increase drag on satellites in low Earth orbit and can lead to orbital decay over time.[19]

The radiation risks posed by solar flares are a major concern in discussions of a human mission to Mars, the Moon, or other planets. Energetic protons can pass through the human body, causing biochemical damage,[20] presenting a hazard to astronauts during interplanetary travel. Some kind of physical or magnetic shielding would be required to protect the astronauts. Most proton storms take at least two hours from the time of visual detection to reach Earth's orbit. A solar flare on January 20, 2005, released the highest concentration of protons ever directly measured, which would have given astronauts on the moon little time to reach shelter.[21][22]

Radio blackouts

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. The NOAA classifies radio blackouts by the peak intensity of the associated flare.

Classification Associated solar flare Description[23]
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


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.


Optical observations

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. It was an extraordinarily intense white light flare, a flare emitting a high amount of light in the visual spectrum.[24]

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.

Radio observations

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.

Space telescopes

Since the beginning of space exploration, telescopes have been sent to space, where it is possible to detect wavelengths shorter than UV, which are completely absorbed by the Earth's atmosphere, and where flares may be very bright. Since the 1970s, the GOES series of satellites observe the Sun in soft X-rays, and their observations became the standard measure of flares, diminishing the importance of the H-alpha classification. Hard X-rays were observed by many different instruments, the most important today being the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Nonetheless, UV observations are today the stars of solar imaging with their incredible fine details that reveal the complexity of the solar corona. Spacecraft may also bring radio detectors at extremely long wavelengths (as long as a few kilometres) that cannot propagate through the ionosphere.

Examples of large solar flares

Short narrated video about Fermi's observations of the highest-energy light ever associated with an eruption on the Sun as of March 2012
Active Region 1515 released an X1.1-class flare from the lower right of the Sun on 6 July 2012, peaking at 7:08 PM EDT. This flare caused a radio blackout, labeled as an R3 on the National Oceanic and Atmospheric Administrations scale that goes from R1 to R5.
Space weather—March 2012.[25]
Space weather—March 2012.[25]

The most powerful flare ever observed was the first one to be observed,[26] on 1 September 1859, and was reported by British astronomer Richard Carrington and independently by an observer named Richard Hodgson. The event is named the Solar storm of 1859, or the "Carrington event". The flare was visible to a naked eye (in white light), and produced stunning auroras down to tropical latitudes such as Cuba or Hawaii, and set telegraph systems on fire.[27] The flare left a trace in Greenland ice in the form of nitrates and beryllium-10, which allow its strength to be measured today.[28] Cliver and Svalgaard[29] reconstructed the effects of this flare and compared with other events of the last 150 years. In their words: "While the 1859 event has close rivals or superiors in each of the above categories of space weather activity, it is the only documented event of the last ∼150 years that appears at or near the top of all of the lists." The intensity of the flare has been estimated to be around X50.[30]

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.[31] Later analysis of the ionospheric effects suggested increasing this estimate to X45.[32] This event produced the first clear evidence of a new spectral component above 100 GHz.[33]

Other large solar flares also occurred on 2 April 2001 (X20+),[34] 28 October 2003 (X17.2+ and 10),[35] 7 September 2005 (X17),[34] 9 August 2011 (X6.9),[36][37] 7 March 2012 (X5.4),[38][39] and 6 September 2017 (X9.3).[40]


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[41] is commonly used as a starting point for flare prediction.[42] 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.[43] 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.[44] A physics-based method that can predict imminent large solar flares was proposed by Institute for Space-Earth Environmental Research (ISEE), Nagoya University.[45]

In popular culture

A solar flare has been the main plot device for science fiction stories:

See also


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External links

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