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A solar flare observed by Hinode in the G-band. It can be seen as two narrow, elongated, bright structures (ribbons) over the southern part of the sunspot.

A solar flare is a violent explosion in the Sun's atmosphere releasing up to a total energy of 6.298 × 10²⁵ Joules.^[1] Solar flares take place in the solar corona and chromosphere, heating plasma to tens of millions of kelvins and accelerating electrons, protons and heavier ions to near the speed of light. They produce electromagnetic radiation across the electromagnetic spectrum at all wavelengths from long-wave radio to the shortest wavelength gamma rays.^[2] Most flares occur in active regions around sunspots, where intense magnetic fields emerge from the Sun's surface into the corona. Flares are powered by the sudden (timescales of minutes to tens of minutes) release of magnetic energy stored in the corona.

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 in 1859 as localized brightenings in 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 each week when the Sun is "quiet". Large flares are less frequent than smaller ones. Solar activity varies with an 11-year cycle (the solar cycle). At the peak of the cycle there are typically more sunspots on the Sun, and hence more solar flares.

Solar flares are classified as A, B, C, M or X according to the peak flux (in watts per square meter, W/m²) of 100 to 800 picometer X-rays near Earth, as measured on the GOES spacecraft. Each class has a peak flux ten times greater than the preceding one, with X class flares having a peak flux of order 10^-4 W/m². Within a class there is a linear scale from 1 to 9, so an X2 flare is twice as powerful as an X1 flare, and is four times more powerful than an M5 flare. The more powerful M and X class flares are often associated with a variety of effects on the near-Earth space environment. Although the GOES classification is commonly used to indicate the size of a flare, it is only one measure.

Soft X-ray light curves showing solar flares of different sizes and durations. The red curve represents the total flux in the band 1 to 8 Angstrom, and the blue curve is the flux in 0.5 to 4 Angstrom. Basically, this means that the curves represent the evolution in time of the X-ray power emitted by the Sun in two energy ranges. Each one of the numerous spikes in the curves represents a temporary increase in the emission due to a solar flare.

Two of the largest GOES flares were the X20 events (2 mW/m²) recorded on August 16, 1989 and April 2, 2001. However, these events were outshone by a flare on November 4, 2003 that was the most powerful X-ray flare ever recorded. This flare was originally classified as X28 (2.8 mW/m²). However, the GOES detectors were saturated at the peak of the flare, and it is now thought that the flare was between X40 (4.0 mW/m²) and X45 (4.5 mW/m²), based on the influence of the event on the earth's atmosphere (see [1]). The flare originated in sunspot region 10486, which is shown in the illustration above several days before the flare.

The most powerful flare of the last 500 years is believed to have occurred in September 1859: it was seen by British astronomer Richard Carrington and 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).

Solar flares and associated Coronal Mass Ejections (CMEs) strongly influence our local space weather. They produce streams of highly energetic particles in the solar wind and the Earth's magnetosphere that can present radiation hazards to spacecraft and astronauts. The soft X-ray flux of X class flares increases the ionisation of the upper atmosphere, which can interfere with short-wave radio communication, and can increase the drag on low orbiting satellites, leading to orbital decay. Energetic particles in the magnetosphere contribute to the aurora borealis and aurora australis.

Solar flares release a cascade of high energy particles known as a proton storm. Protons can pass through the human body, doing biochemical damage^{[citation needed]}. Most proton storms take two or more hours from the time of visual detection to reach Earth. A solar flare on January 20, 2005 released the highest concentration of protons ever directly measured^[3], taking only 15 minutes after observation to reach Earth, indicating a velocity of approximately one-third light speed.

The radiation risk posed by solar flares and CMEs is one of the major concerns in discussions of manned missions to Mars or to the moon. Some kind of physical or magnetic shielding would be required to protect the astronauts. Originally it was thought that astronauts would have two hours time to get into shelter, but based on the January 20, 2005 event, they may have as little as 15 minutes to do so.

Filaments surrounding a solar flare, caused by the interaction of the plasma in the Sun's atmosphere with its magnetic field. ^[4]

The new spacecraft Hinode, originally called Solar B, was launched by the Japan Aerospace Exploration Agency in September of 2006 to observe solar flares in more precise detail. The device is meant to study the powerful magnetic fields that are thought to be the source of solar flares. Hopefully this will shed light on what exactly causes this activity, so that scientists might be able to forecast future flares and help minimize damage to satellite equipment and astronauts. See [2].

• Coronal mass ejection
• Geomagnetic storm
• 1 September 1859
• Aurora (astronomy)

• Solar Cycle 24 and VHF Aurora Website (www.solarcycle24.com)
• Solar Weather Site
• STEREO Spacecraft Site
• BBC report on the November 4, 2003 flare
• NASA SOHO observations of flares
• Stellar Flares - D. Montes, UCM.
• The Sun - D. Montes, UCM.
• ASC / Alliances Center for Astrophysical Thermonuclear Flashes
• 'The Sun Kings', lecture by Dr Stuart Clark on the discovery of solar flares given at Gresham College, 12th September 2007 (available as a video or audio download as well as a text file).
• An X Class Flare Region on the Sun - NASA Astronomy Picture of the Day
• Sun|trek website An educational resource for teachers and students about the Sun and its effect on the Earth

• Superflares could kill unprotected astronauts. NewScientist.com. Retrieved on 17 June, 2005.
• Mewaldt, R.A., et al. 2005. Space weather implications of the 20 January 2005 solar energetic particle event. Joint meeting of the American Geophysical Union and the Solar Physics Division of the American Astronomical Society. May 23-27. New Orleans. Abstract.
• Solar Flares NASA Video from 2003
• Solar Flares Solar & Heliospheric Observatory Video from 2002

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Other	Solar System - Solar Variation - Solar Dynamo - Heliospheric Current Sheet - Solar Radiation - Solar Eclipse - Spectral Class: G2