Intense, violent, and sudden, solar flares are enormous explosions on the surface of the Sun. They are caused by a buildup of magnetic energy in the solar atmosphere, and while it may take this energy several hours or days to build up, it takes only minutes for it to be released.

Solar flares heat plasma and other material to several million degrees, accelerating particles and releasing the equivalent of a billion megatons of magnetic energy. Solar flares extend to the corona (the Sun’s outermost atmosphere) and usually occur around sunspots. These sunspots are located within active regions, areas in which magnetic energy is highly concentrated.

Solar flares occur in 11-year cycles. Most sunspots, and therefore most solar flares, occur at the cycle’s peak, during which there may be several flares per day. During quiet periods, there may be less than one solar flare per week.

Solar flares usually cannot be observed with the naked eye, because they are frequently obscured by the bright light of the Sun’s photosphere. In fact, the Sun should never be observed with the naked eye, because this can cause eye damage. To study solar flares, astronomers use special instruments to locate signs of radiation emitted during a flare, and also use telescopes both on Earth and in space, to observe a flare’s radio, optical, and energy emissions.

A solar flare occurs in three stages, each lasting from a few seconds to an hour. In the precursor stage, the built-up magnetic energy is released, and soft X-ray emissions are detected. In the impulsive stage, protons and electrons accelerate, and radio waves, hard x-rays, and gamma rays are emitted. The buildup and deterioration of soft X-rays leads to the final stage, the decay stage.

Solar flares were first recorded on Sept. 1, 1859, when two scientists, Richard C. Carrington and Richard Hodgson, were independently observing sunspots. Scientists have continued to study flares, because of their effects on the Earth’s atmosphere, as well as their potential to harm astronauts and scientific instruments in space. Solar flares can disrupt long-distance radio signals on Earth, interrupt the orbits of man-made satellites, and damage satellites’ electrical mechanisms. It is also feared the radiation released by flares could endanger astronauts.

Solar flares are divided into three categories, based on the brightness of their X-ray wavelengths. X-class flares are the largest, and major enough to cause worldwide radio blackouts, as well as radiation storms in Earth’s upper atmosphere. M-class flares are medium-sized, causing short-lived radio blackouts, and occasionally minor radiation storms. The smallest, C-class flares, are minor and usually have little or no effect on Earth.