A solar flare is an intense burst of radiation coming from the release of magnetic energy associated with sunspots. It is a tremendous explosion on the Sun that happens when energy stored in twisted magnetic fields (usually above sunspots) is suddenly released. Flares are our solar systems largest explosive events and are seen as bright areas on the sun. They can last from minutes to hours and heat material to many millions of degrees, producing a burst of radiation across the electromagnetic spectrum, including from radio waves to x-rays and gamma rays. Scientists classify solar flares according to their brightness in the x-ray wavelengths, with X-class flares being the biggest and most powerful.

Solar flares are thought to occur when stored magnetic energy in the Suns atmosphere accelerates charged particles in the surrounding plasma. This results in the emission of electromagnetic radiation across the electromagnetic spectrum. Flares affect all layers of the solar atmosphere (photosphere, chromosphere, and corona), and the plasma medium is heated to >107 kelvin, while electrons, protons, and heavier ions are accelerated to near the speed of light. Flares are also sites where particles (electrons, protons, and heavier particles) are accelerated.

Solar flares are different from coronal mass ejections (CMEs), which are huge bubbles of gas threaded with magnetic field lines that are ejected from the Sun over the course of several hours. Although some CMEs are accompanied by flares, it is now known that most CMEs are not associated with flares. Solar flares directly affect the ionosphere and radio communications at the Earth, and also release energetic particles into space. Therefore, to understand and predict space weather and the effect of solar activity on the Earth, an understanding of both CMEs and flares is required.