Solar flares are huge explosions on the surface of the Sun. They were first observed by Carrington (1860) and Hodgson (1860) in connection with the great aurora of 1859. Flares are very fast processes, with time scales of only a few minutes, and they are thought to be formed as the sunspot related magnetic field structures get twisted and sheared and release energy via magnetic reconnection (Giovanelli, 1946). That they usually occur along the dividing or neutral line between oppositely directed magnetic fields supports this theory.
Flares are characterized and monitored by their brightness in X-rays, but they radiate energy from radio frequencies up to Gamma-rays. Also energetic particles are released: the solar energetic particles (SEP) form a subset of cosmic rays. As a consequence, phenomena like
- Sudden ionospheric disturbances (SID; due to X-rays)
- Polar cap absorption events (PCA)
- Carrington, R. C., Description of a singular appearance seen on the Sun on September 1, 1859, Mon. Not. R. Astron. Soc., 20, 13, 1860.
- Giovanelli, R. G., A theory of chromospheric flares, Nature, 158, 81-, 1946.
- Hodgson, R., On a curious appearance seen in the Sun, Mon. Not. R. Astron. Soc., 20, 16, 1860.
See also Wikipedia on solar flares.