A solar flare is a violent explosion in a star's (like the Sun's) atmosphere releasing as much energy as 6 × 1025 Joules. 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. 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 by Richard Christopher Carrington and independently by Richard Hodgson 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.
Two of the largest GOES flares were the X20 events (2 mW/m²) recorded on August 16, 1990 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 . The flare originated in sunspot region 10486, which is shown in the illustration above several days before the flare.
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. 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, 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.
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).
Current methods of flare prediction are probabilistic, 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 most large flares. A simple scheme of sunspot classification due to McIntosh 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 with 24 or 48 hours. The U.S. National Oceanic and Atmospheric Administration (NOAA) issues forecasts of this kind.