Solar flare
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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
Flares also occur on other stars, where the term stellar flare applies.
Description
Solar flares are eruptions of
Flares occur in
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]
Frequency
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]
The
Duration
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
Post-eruption loops and arcades
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]
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 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]
Classification
Soft X-ray
The modern classification system for solar flares uses the letters A, B, C, M, or X, according to the peak
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]
Importance
An earlier classification system, what is sometimes referred to as the flare importance, was based on
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]
Duration
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]
Effects
Terrestrial
X-rays and extreme ultraviolet radiation emitted by solar flares are absorbed by the
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 in soft X-ray radiation. The
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 |
Magnetic crochet
The increased ionization of the D and E layers of the ionosphere caused by large solar flares increases the
In space
For astronauts in low earth orbit an expected radiation dose from the electromagnetic radiation emitted during a solar flare is about 0.05
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 spectral lines. They normally produce bremsstrahlung in X-rays and synchrotron radiation in radio.[citation needed]
History
Optical observations
Solar flares were first observed by
Since flares produce copious amounts of radiation at
Radio observations
During
Space telescopes
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.
Examples of large solar flares
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
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]
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[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]
See also
- Aurora
- Gamma-ray burst
- Hyder flare
- Moreton wave
- Neupert effect
- Sun in culture
- Sun in fiction
- Superflare
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