Solar flare
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. The typical time profile of these emissions features three identifiable phases: a precursor phase, an impulsive phase when particle acceleration dominates, a gradual phase in which hot plasma injected into the corona by the flare cools by a combination of radiation and conduction of energy back down to the lower atmosphere, and a currently unexplained EUV late phase
that occurs in some flares.
The extreme ultraviolet and X-ray 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.
Physical description
Solar flares are eruptions of electromagnetic radiation originating in the Sun's atmosphere. They affect all layers of the solar atmosphere. The plasma medium is heated to >107 kelvin, while electrons, protons, and heavier ions are accelerated to near the speed of light. Flares emit electromagnetic radiation across the electromagnetic spectrum, from radio waves to gamma rays.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 release of magnetic energy stored in the corona. The same energy releases may also produce coronal mass ejections, although the relationship between CMEs and flares is not well understood.
Associated with solar flares are flare sprays. They involve faster ejections of material than eruptive prominences, and reach velocities of 20 to 2,000 kilometres per second.
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. 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. 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 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 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.
Post-eruption loops and arcades
After the eruption of a solar flare, post-eruption loops made 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. 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.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. In some cases, dark sunward-traveling plasma voids known as supra-arcade downflows may form above these arcades.
Frequency
The frequency of occurrence of solar flares varies with the 11-year solar cycle. It can typically range from several per day during solar maxima to less than one every week during solar minima. Additionally, more powerful flares are less frequent than weaker ones. For example, [|X10-class] flares occur on average about eight times per cycle, whereas M1-class flares occur on average about 2,000 times per cycle.In 1984 Erich Rieger and coworkers discovered an approximately 154-day period in the occurrence of gamma-ray emitting solar flares at least since the solar cycle 19. 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 have been found to follow power-law distributions.
Classification
Soft X-ray
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 of soft X-rays with wavelengths, as measured by GOES satellites in geosynchronous orbit.| Classification | Peak flux range |
| 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. An X2 is four times more powerful than an M5 flare. 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 were 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.
Importance
An earlier classification system, 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, normal, or brilliant. The emitting surface is measured in terms of millionths of the hemisphere and is described below.| Classification | Corrected area |
| S | < 100 |
| 1 | 100–250 |
| 2 | 250–600 |
| 3 | 600–1200 |
| 4 | > 1200 |
A flare is then classified taking S or a number that represents its size and a letter that represents its peak intensity, e.g., Sn is a normal sunflare.
Duration
A common measure of flare duration is the full width at half maximum time of flux in the soft X-ray bands measured by GOES. 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 bands, respectively.Flares can also be classified based on their duration as either impulsive or long duration events. 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.
Effects
The electromagnetic radiation emitted during a solar flare propagates away from the Sun at the speed of light with intensity inversely proportional to the square of the distance from its source region. The excess ionizing radiation, namely X-ray and extreme ultraviolet radiation, is known to affect planetary atmospheres and is of relevance to human space exploration and the search for extraterrestrial life.Solar flares also affect other objects in the Solar System. Research into these effects has primarily focused on the atmosphere of Mars and, to a lesser extent, that of Venus. The impacts on other planets in the Solar System are little studied in comparison. As of 2024, research on their effects on Mercury have been limited to modeling of the response of ions in the planet's magnetosphere, and their impact on Jupiter and Saturn have only been studied in the context of X-ray radiation back scattering off of the planets' upper atmospheres.