Solar flare

Solar flares occur in active regions where strong, evolving magnetic fields are present. When magnetic configurations become unstable and reconnect, stored magnetic energy can be converted into radiation and energetic particles. A flare emits across a broad range of the electromagnetic spectrum, including ultraviolet, visible, radio and X-ray wavelengths. The observed flare is therefore a radiative event; it does not by definition mean that a large cloud of solar plasma has been ejected.

NOAA's operational flare classes are based on the peak soft X-ray flux measured in the 0.1–0.8 nanometre band by the GOES X-Ray Sensor. The classes A, B, C, M and X step upward by factors of ten in flux; numerical subdivisions indicate strength within a class. The scale describes X-ray output in that measurement band, not the full energy of every aspect of an eruption. An X-class flare is not necessarily accompanied by a coronal mass ejection.

Because electromagnetic radiation travels at light speed, the increased radiation reaches Earth about eight minutes after emission. On the sunlit side of Earth, enhanced X-ray and extreme-ultraviolet radiation can increase ionization in the lower ionosphere. This can absorb high-frequency radio signals and cause radio blackouts, particularly for communications that rely on propagation through the ionosphere. Earth's atmosphere and magnetic field protect people at the surface from most harmful solar radiation, although strong events can affect spacecraft and operations in space.

A coronal mass ejection (CME) is a large expulsion of magnetized plasma from the solar corona, whereas a flare is a burst of electromagnetic radiation. Flares and CMEs often occur in association, but either can be observed without the other. A CME directed toward Earth may take many hours or days to arrive; if its magnetic field and speed are suitable, it can disturb Earth's magnetosphere and drive a geomagnetic storm. The different travel times and physical processes are why space-weather monitoring tracks flares, energetic particles and CMEs as related but distinct phenomena.