Solar flare

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Heliophysics Phenomena
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Two successive photos of a solar flare phenomenon. The solar disc was blocked in these photos for better visualization of the flare prominence.
Solar flare and its prominence recorded on June 7, 2011 by SDO in extreme ultraviolet.
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Heliophysicist Alex Young from NASA Goddard Space Flight Center's predictions for solar activity in 2012.
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Evolution of magnetism on the Sun.

A solar flare is a sudden brightening observed over the Sun's surface or the solar limb, which is interpreted as a large energy release of up to 6 × 1025 joules of energy (about a sixth of the total energy output of the Sun each second or 160,000,000,000 megatons of TNT equivalent, over 25,000 times more energy than released from the impact of Comet Shoemaker–Levy 9 with Jupiter). They are mainly followed by a colossal coronal mass ejection also known as a CME.[1] The flare ejects clouds of electrons, ions, and atoms through the corona of the sun into space. These clouds typically reach Earth a day or two after the event.[2] The term is also used to refer to similar phenomena in other stars, where the term stellar flare applies.

Solar flares affect all layers of the solar atmosphere (photosphere, chromosphere, and corona), when the medium plasma is heated to tens of millions of kelvins and electrons, protons, and heavier ions are accelerated to near the speed of light. They produce radiation across the electromagnetic spectrum at all wavelengths, from radio waves to gamma rays, although most of the energy goes to frequencies outside the visual range and for this reason the majority of the flares are not visible to the naked eye and must be observed with special instruments. Flares occur in active regions 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 (CME), although the relation between CMEs and flares is still not well established.

X-rays and UV radiation emitted by solar flares can affect Earth's ionosphere and disrupt long-range radio communications. Direct radio emission at decimetric wavelengths may disturb operation of radars and other devices operating at these frequencies.

Solar flares were first observed on the Sun by Richard Christopher Carrington and independently by Richard Hodgson in 1859[3] as localized visible brightenings of small areas within a sunspot group. Stellar flares have also been observed on a variety of other stars.

The frequency of occurrence of solar flares varies, from several per day when the Sun is particularly "active" to less than one every week when the Sun is "quiet", following the 11-year cycle (the solar cycle). Large flares are less frequent than smaller ones.

Heliophysics Phenomena



Stellar flare hits HD 189733b (artist's impression).[4]

Flares occur when accelerated charged particles, mainly electrons, interact with the plasma medium. Scientific research has shown that the phenomenon of magnetic reconnection is responsible for the acceleration of the charged particles. On the Sun, magnetic reconnection may happen on solar arcades – a series of closely occurring loops of magnetic lines of force. These lines of force quickly reconnect into a low 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 in 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.[5] This also explains why solar flares typically erupt from what are known as the active regions on the Sun where magnetic fields are much stronger on average.

Although there is a general agreement on the flares' causes, the details are still not well known. It is not clear how the magnetic energy is transformed into the particle kinetic energy, nor is it known how the particles are accelerated to energies as high as 10 MeV (mega 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. We are unable to forecast flares, even to this day.


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Powerful X-class flares create radiation storms that produce auroras and can give airline passengers flying over the poles small radiation doses.
On August 1, 2010, the Sun shows a C3-class solar flare (white area on upper left), a solar tsunami (wave-like structure, upper right) and multiple filaments of magnetism lifting off the stellar surface.[6]

Solar flares are classified as A, B, C, M or X according to the peak flux (in watts per square metre, W/m2) of 100 to 800 picometre X-rays near Earth, as measured on the GOES spacecraft.

ClassificationPeak Flux Range at 100-800 picometre
(Watts/square metre)
A< 10−7
B10−7 - 10−6
C10−6 - 10−5
M10−5 - 10−4
X> 10−4


Within a class there is a linear scale from 1 to 9.n (apart from X), so an X2 flare is twice as powerful as an X1 flare, and is four times more powerful than an M5 flare. X class flares up to at least X28 have been recorded.(see below)

H-alpha classification

An earlier flare classification is based on spectral observations. The scheme uses both the intensity and emitting surface. The classification in intensity is qualitative, referring to the flares as: (f)aint, (n)ormal or (b)rilliant. The emitting surface is measured in terms of millionths of the hemisphere and is described below. (The total hemisphere area AH = 6.2 × 1012 km2.)

ClassificationCorrected Area
[millionths of hemisphere]
S< 100
1100 - 250
2250 - 600
3600 - 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 subflare.[8]


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Massive X6.9 class solar flare, August 9, 2011.
While this flare produced a coronal mass ejection (CME), this CME is not traveling towards the Earth, and no local effects are expected.[9]

Solar flares strongly influence the local space weather in the vicinity of the Earth. They can produce streams of highly energetic particles in the solar wind, known as a solar proton event, or "coronal mass ejection" (CME). These particles can impact the Earth's magnetosphere (see main article at geomagnetic storm), and present radiation hazards to spacecraft, astronauts, and cosmonauts.

Massive solar flares are sometimes associated with CMEs which can trigger geomagnetic storms that have been known to knock out electric power for extended periods of time.

The soft X-ray flux of X class flares increases the ionization of the upper atmosphere, which can interfere with short-wave radio communication and can heat the outer atmosphere and thus increase the drag on low orbiting satellites, leading to orbital decay. Energetic particles in the magnetosphere contribute to the aurora borealis and aurora australis. Energy in the form of hard x-rays can be damaging to spacecraft electronics and are generally the result of large plasma ejection in the upper chromosphere.

The radiation risks posed by coronal mass ejections are a major concern in discussions of a manned mission to Mars, the moon, or other planets. Energetic protons can pass through the human body, causing biochemical damage,[10] 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,[11] taking only 15 minutes after observation to reach Earth, indicating a velocity of approximately one-third light speed, giving astronauts as little as 15 minutes to reach shelter.

An M7.9 class Solar Flare


Flares produce radiation across the electromagnetic spectrum, although with different intensity. They are not very intense at white light, but they can be very bright at particular atomic lines. They normally produce bremsstrahlung in X-rays and synchrotron radiation in radio.


Optical Observations. Richard Carrington observed a flare for the first time on 1 September 1859 projecting the image produced by an optical telescope, without filters. It was an extraordinarily intense white light flare. Since flares produce copious amounts of radiation at , 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 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 25 and 26 February 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 ~100 MHz up to 400 GHz.

Space Telescopes. Since the beginning of Space exploration, telescopes have been sent to space, where they work at wavelengths below UV, which are completely absorbed by the 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 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 very very long wavelengths (as long as a few kilometers) that cannot propagate through the ionosphere.

Optical telescopes

Radio telescopes

Space telescopes

The following spacecraft missions have flares as their main observation target.

Examples of large solar flares

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Short narrated video about Fermi's observations of the highest-energy light ever associated with an eruption on the sun as of June 2012
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Active Region 1515 released an X1.1 class flare from the lower right of the sun on July 6, 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.[13]

The most powerful flare ever observed was the first one to be observed,[14] on September 1, 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.[15] The flare left a trace in Greenland ice in the form of nitrates and beryllium-10, which allow its strength to be measured today (New Scientist, 2005). Cliver & Svalgaard (2004) 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.

In modern times, the largest solar flare measured with instruments occurred on November 4, 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.[16] Later analysis of the ionospheric effects suggested increasing this estimate to X45.[17] This event produced the first clear evidence of a new spectral component above 100 GHz.[18]

Other large solar flares also occurred on April 2, 2001 (X20),[19] October 28, 2003 (X17.2 and 10),[20] September 7, 2005 (X17),[19] February 17, 2011 (X2),[21][22][23] August 9, 2011 (X6.9),[9][24] March 7, 2012 (X5.4),[25][26] July 6, 2012 (X1.1).[27] July 6, 2012- The solar storm hit just after 12 midnight UK time,[28] when an X1.1 solar flare fired out of the AR1515 sunspot. Another X1.4 solar flare from AR 1520 region of the Sun,[29] second in the week, reached the earth on July 15, 2012[30] with a geomagnetic storm of G1–G2 level.[31][32] A X1.8-class flare was recorded on October 24, 2012.[33]

Flare spray

Flare sprays are a type of eruption associated with solar flares.[34] They involve faster ejections of material than eruptive prominences,[35] and reach velocities of 500 to 1200 kilometers per second.[34]


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 most large flares. A simple scheme of sunspot classification due to McIntosh is commonly used as a starting point for flare prediction.[36] Predictions are usually stated in terms of probabilities for occurrence of flares above M or X GOES class within 24 or 48 hours. The U.S. National Oceanic and Atmospheric Administration (NOAA) issues forecasts of this kind.[37]

See also


  1. ^ Kopp, G.; Lawrence, G and Rottman, G. (2005). "The Total Irradiance Monitor (TIM): Science Results". Solar Physics 20 (1–2): 129–139. Bibcode 2005SoPh..230..129K. doi:10.1007/s11207-005-7433-9.
  2. ^ Menzel, Whipple, and de Vaucouleurs, "Survey of the Universe", 1970
  3. ^ "Description of a Singular Appearance seen in the Sun on September 1, 1859", Monthly Notices of the Royal Astronomical Society, v20, pp13+, 1859
  4. ^ "Dramatic change spotted on a faraway planet". ESA/Hubble Press Release. Retrieved 28 June 2012.
  5. ^ "The Mysterious Origins of Solar Flares", Scientific American, April 2006
  6. ^ "Great Ball of Fire". NASA. Retrieved May 21, 2012.
  7. ^ Tamrazyan, Gurgen P. (1968). "Principal Regularities in the Distribution of Major Earthquakes Relative to Solar and Lunar Tides and Other Cosmic Forces". ICARUS (Elsevier) 9: 574–592. Bibcode 1968Icar....9..574T. doi:10.1016/0019-1035(68)90050-X
  8. ^ Tandberg-Hanssen, Einar; Emslie, A. Gordon (1988). "The physics of solar flares".
  9. ^ a b "Sun Unleashes X6.9 Class Flare". NASA. Retrieved March 7, 2012.
  10. ^ "New Study Questions the Effects of Cosmic Proton Radiation on Human Cells". Retrieved 2008-10-11.
  11. ^ A New Kind of Solar Storm
  12. ^ "Japan launches Sun 'microscope'". BBC. 2006-09-23. Retrieved 2009=05-19.
  13. ^ "Extreme Space Weather Events". National Geophysical Data Center. Retrieved May 21, 2012.
  14. ^ "A Super Solar Flare". NASA. 6 May 2008. Retrieved 22 December 2012.
  15. ^ Bell, Trudy E.; Phillips, Tony (2008). "A Super Solar Flare". Science@NASA. Retrieved May 21, 2012.
  16. ^ "SOHO Hotshots". Retrieved May 21, 2012.
  17. ^ "Biggest ever solar flare was even bigger than thought | SpaceRef - Your Space Reference". SpaceRef. 2004-03-15. Retrieved May 21, 2012.
  18. ^
  19. ^ a b "BIGGEST SOLAR X-RAY FLARE ON RECORD - X20". NASA. Retrieved May 21, 2012.
  20. ^ "X 17.2 AND 10.0 FLARES!". NASA. Retrieved May 21, 2012.
  21. ^ Hendrix, Susan (2012-03-07). "Valentine's Day Solar Flare" (video included). Nasa Goddard Space Flight Center. Retrieved May 21, 2012.
  22. ^ "Solar flare to jam Earth's communications". ABC. Retrieved May 21, 2012.
  23. ^ Kremer, Ken. "Sun Erupts with Enormous X2 Solar Flare". Universe Today. Retrieved May 21, 2012.
  24. ^ Bergen, Jennifer. "Sun fires powerful X6.9-class solar flare". Retrieved May 21, 2012.
  25. ^ Zalaznick, Matt. "Gimme Some Space: Solar Flare, Solar Storm Strike". The Norwalk Daily Voice. Retrieved July 19, 2012.
  26. ^ "Geomagnetic Storm Strength Increases". NASA. Retrieved July 9, 2012.
  27. ^ Fox, Karen (July 7, 2012). "Sunspot 1515 Release X1.1 Class Solar Flare". Nasa Goddard Space Flight Center. Retrieved July 14, 2012.
  28. ^ "Massive 'X Class' Solar Flare Bursts From Sun, Causing Radio Blackouts (VIDEO)". Huffington Post UK. July 9, 2012. Retrieved July 14, 2012.
  29. ^ "Big Sunspot 1520 Releases X1.4 Class Flare With Earth-Directed CME". NASA. July 12, 2012. Retrieved July 14, 2012.
  30. ^ "Solar storm rising, to hit Earth today". The Times of India. Retrieved July 14, 2012.
  31. ^ "'Minor' solar storm reaches Earth". Retrieved July 15, 2012.
  32. ^ "Space Weather Alerts and Warnings Timeline: July 16, 2012". NOAA. Retrieved July 17, 2012.
  33. ^ "Sun Unleashes Powerful Solar Flare". Sky News. October 24, 2012. Retrieved October 24, 2012.
  34. ^ a b Tarou Morimoto, Hiroki Kurokawa. "Effects of Magnetic and Gravity forces on the Acceleration of Solar Filaments and Coronal Mass Ejections". Retrieved 2009-10-08.
  35. ^ E. Tandberg-Hanssen, Sara F. Martin and Richard T. Hansen (1980). "Dynamics of flare sprays". Solar Physics.
  36. ^ Wheatland, M. S. (2008). "A Bayesian approach to solar flare prediction.". The Astrophysical Journal (IOP Publishing) 609 (2): 1134-1139. doi:10.1086/421261.
  37. ^ "Space Weather Prediction Center". NOAA. Retrieved August 1, 2012.

External links