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.

Solar flares are eruptions of electromagnetic radiation originating in the Sun's atmosphere. They affect all layers of the solar atmosphere (photosphere, chromosphere, and corona). 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 (timescales of minutes to tens of minutes) release of magnetic energy stored in the corona. The same energy releases may also produce coronal mass ejections (CMEs), 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.

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. ^{[citation needed]}