A solar flare is a sudden, intense burst of radiation from the Sun's surface. It happens when magnetic field lines in the corona become twisted and snap, releasing stored energy. The energy heats plasma to millions of degrees and accelerates particles to near the speed of light. The flare can last minutes to hours and can be seen across the electromagnetic spectrum.

Flares are classified by their X-ray brightness. A-class flares are tiny. X-class flares are the most powerful. An X-class flare can cause radio blackouts on the sunlit side of Earth, disrupt GPS signals, and endanger astronauts. The radiation reaches Earth in about eight minutes, traveling at the speed of light.

Flares often accompany coronal mass ejections, but not always. A flare is a burst of radiation. A CME is a burst of matter. When they occur together, the effects can be severe. The Carrington Event of 1859 was probably caused by a powerful flare and CME. It set telegraph wires on fire and produced auroras visible near the equator.

Scientists monitor the Sun continuously with satellites like SDO and SOHO. They can predict when a flare is likely, but not exactly when it will happen. Space weather forecasting is improving, but it is still an inexact science.