Solar Flares

A solar flare is a sudden flash of brightness observed near the solar surface (see the figure below). It involves a very broad spectrum of emissions ranging from radio, through optical emission, Ultraviolet (UV), Extreme Ultraviolet (EUV) and X-rays to γ-rays, and an energy release from less than 10₂₀ joules up to 10₂₅ joules (roughly the equivalent of 1 billion megatons of TNT, or over 400 times more energy than that released from the impact of Comet Shoemaker–Levy 9 with the Jupiter). Large flares are often, but not always, accompanied by a coronal mass ejection (CME). .

Every flare is different from the others. Richard Carrington first observed a large flare on 1 September 1859 using an optical telescope with a broad-band filter. The flare appears as localized visible brightening of small areas within a sunspot group. Panel (a) of the figure below shows the great 'Seahorse Flare' of 7 August 1972 as observed in the H-alpha line by the Big Bear Solar Observatory (BBSO), which shows a two-ribbon structure connected by bright H-alpha loops. Flares were first observed in radio during the World War II by Grote Reber. As an example, panel (e) of the following figure shows microwave image of a large flare observed by the Nobeyama Radioheliograph. Since the beginning of space exploration after the World War II, solar flares have been observed in EUV, X-rays which are not accessible on the ground. Panel (b) of the following figure shows EUV Post-flare loops of the famous 'Bastille Day Flare' observed at 195 Angstrom on 14 July 2000 by the Transition Region and Coronal Explorer (TRACE) mission. Panel (c) shows soft X-ray cusp-shape post-flare loop observed by the X-ray Telescope (XRT) onboard the Japanese Hinode satellite on 17 December 2006. Panel (d) shows hard X-ray images of the first gamma-ray flare observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). High-energy electrons, ions, and atoms ejected by solar flares have also been detected. Majority of flares are not visible to the naked eye and must be observed with special instruments.

Solar flares affect all layers of the solar atmosphere, including photosphere, chromosphere, and corona, when the plasma medium is heated to tens of millions Kelvin, while cosmic-ray-like electrons, protons and heavier ions are accelerated to near the speed of light. These high energy particles produce radiation across the electromagnetic spectrum from radio waves to gamma rays and heat the background plasma at the same time. Most emission from solar flares is emitted in the EUV band by hot plasmas, which can also produce emission from radio to X-rays. Short timescale narrow band radio pulses can also be produced by coherent plasma physics processes.
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 that use those frequencies. High energy particles can cause radiation damage to space asset. Charged particles propagate along magnetic field lines and typically reach the Earth orbit a few hours after the flare onset. Observations of electromagnetic emission of solar flares can be used to forecast their arrival.
Solar flares are powered by the sudden (timescales of minutes to tens of minutes) release of magnetic energy stored in the non-potential magnetic field in the corona. It can last from a few minutes to tens of hours. Larger flares usually last longer. The same energy release may produce CMEs and jets depending on the topological structure of reconnecting magnetic fields.
Solar flares are classified as A, B, C, M or X according to the order of the peak flux (in watts per square meter, W/m²) of the 1 to 8 Angstrom soft X-rays near the Earth, as measured by a series of Geostationary Operational Environmental Satellite (GOES) launched first in 1970s. Class X corresponds to the most powerful flares with a peak flux greater than 10⁻⁴ W/m². Within a class there is a linear scale from 1 to 9 (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 and 40 times more powerful than a C5 flare. Based on the size of emission surface in the H-alpha line in terms of millionths of the hemisphere, flares are classified as class S (<100), class 1 (100-250), class 2 (250-600), class 3 (600-1200), class 4 (>1200).
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 solar cycle. Large flares are less frequent than smaller ones and the occurrence frequency distribution of the 1-8 Angstrom soft X-ray peak fluxes follows a power-law with an index close to 2.