Solar flare

A solar flare is a sudden flash of increased brightness on the Sun, usually observed near its surface
and in close proximity to a sunspot group. 
Powerful flares are often, but not always, accompanied by a coronal mass ejection. Even the most powerful flares are barely detectable in the total solar irradiance (the "solar constant").
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.
Flares are closely associated with the ejection of plasmas and particles through the Sun's corona into outer space; flares also copiously emit radio waves.
If the ejection is in the direction of the Earth, particles associated with this disturbance can penetrate into the upper atmosphere (the ionosphere) and cause bright auroras, and may even disrupt long range radio communication. 
It usually takes days for the solar plasma ejecta to reach Earth. Flares also occur on other stars, where the term stellar flare applies.
High-energy particles, which may be relativistic, can arrive almost simultaneously with the electromagnetic radiations.
Description.
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 and so 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 (CMEs), although the relationship between CMEs and flares is still not well understood.
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 the operation of radars and other devices that use those frequencies.
Solar flares were first observed on the Sun by Richard Christopher Carrington and independently by Richard Hodgson in 1859 as localized visible brightenings of small areas within a sunspot group. Stellar flares can be inferred by looking at the lightcurves produced from the telescope or satellite data of 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 "dormant", following the 11-year cycle (the solar cycle). Large flares are less frequent than smaller ones.
Cause.
Flares occur when accelerated charged particles, mainly electrons, interact with the plasma medium. Evidence suggests that the phenomenon of magnetic reconnection leads to this copious acceleration of charged particles.
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. 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.
Classification.
The 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 X-rays with wavelengths , as measured at the Earth by the GOES spacecraft.
The strength of an event within a class is noted by a numerical suffix ranging from 0 to 9, 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. An X2 is four times more powerful than an M5 flare.
H-alpha classification.
An earlier flare classification was based on Hα 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.)
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".
Hazards.
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 or stellar wind, known as a solar particle event. These particles can impact the Earth's magnetosphere (see main article at geomagnetic storm), and present radiation hazards to spacecraft and astronauts. Additionally, massive solar flares are sometimes accompanied by coronal mass ejections (CMEs) which can trigger geomagnetic storms that have been known to disable satellites and knock out terrestrial electric power grids 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 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, 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.
Observations.
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 atomic lines. They normally produce bremsstrahlung in X-rays and synchrotron radiation in radio.
History.
Optical observations.
Richard Carrington observed a flare for the first time on 1 September 1859 projecting the image produced by an optical telescope through a broad-band filter. It was an extraordinarily intense "white light flare". Since flares produce copious amounts of radiation at Hα, 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 they work at wavelengths shorter than 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 Hα 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 kilometers) that cannot propagate through the ionosphere.
Space telescopes.
The following spacecraft missions have flares as their main observation target.
In addition to these solar observing facilities, many non-solar astronomical satellites observe flares either intentionally (e.g., NuSTAR), or simply because the penetrating hard radiations coming from a flare can readily penetrate most forms of shielding.
Examples of large solar flares.
The most powerful flare ever observed was the first one to be observed, 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. The flare left a trace in Greenland ice in the form of nitrates and beryllium-10, which allow its strength to be measured today. Cliver and Svalgaard 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.
The ultra-fast coronal mass ejection of August 1972 is suspected of triggering magnetic fuses on naval mines during the Vietnam War, and would have been a life-threatening event to Apollo astronauts if it had occurred during a mission to the Moon.
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. Later analysis of the ionospheric effects suggested increasing this estimate to X45. This event produced the first clear evidence of a new spectral component above 100 GHz.
Other large solar flares also occurred on April 2, 2001 (X20), October 28, 2003 (X17.2 and 10), September 7, 2005 (X17), February 17, 2011 (X2), August 9, 2011 (X6.9), March 7, 2012 (X5.4), July 6, 2012 (X1.1). On July 6, 2012, a solar storm hit just after midnight UK time, when an X1.1 solar flare fired out of the AR1515 sunspot. Another X1.4 solar flare from AR 1520 region of the Sun, second in the week, reached the Earth on July 15, 2012 with a geomagnetic storm of G1–G2 level. A X1.8-class flare was recorded on October 24, 2012. There has been major solar flare activity in early 2013, notably within a 48-hour period starting on May 12, 2013, a total of four X-class solar flares were emitted ranging from an X1.2 and upwards of an X3.2, the latter of which was one of the largest year 2013 flares. Departing sunspot complex AR2035-AR2046 erupted on April 25, 2014 at 0032 UT, producing a strong X1.3-class solar flare and an HF communications blackout on the day-side of Earth. NASA's Solar Dynamics Observatory recorded a flash of extreme ultraviolet radiation from the explosion. The Solar Dynamics Observatory recorded an X9.3-class flare at around 1200 UTC on September 6, 2017.
On July 23, 2012, a massive, potentially damaging, solar storm (solar flare, coronal mass ejection and electromagnetic radiation) barely missed Earth. In 2014, Pete Riley of Predictive Science Inc. published a paper in which he attempted to calculate the odds of a similar solar storm hitting Earth within the next 10 years, by extrapolating records of past solar storms from the 1960s to the present day. He concluded that there may be as much as a 12% chance of such an event occurring.
Flare spray.
Flare sprays are a type of eruption associated with solar flares. They involve faster ejections of material than eruptive prominences, and reach velocities of 20 to 2000 kilometers per second.
Prediction.
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 is commonly used as a starting point for flare prediction. 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.
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.
References.
Sources