The Sun is the most powerful natural particle accelerator in our solar system, able to accelerate ions to energies of many gigaelectron volts and electrons to hundreds of megaelectron volts. This acceleration occurs as a consequence of transient releases of energy in solar flares and/or coronal mass ejections (CMEs). Solar flares are explosions occurring near sunspots, regions of strong, ∼103 Gauss, magnetic fields.

Traditionally, flares have been detected by ground-based optical observatories as brightenings in the emission of the hydrogenalpha line. In the largest flares, as much as ∼1032 ergs is released in ∼103s. Energy is thought to be stored in the magnetic field and released through some type of instability. Because the number of sunspots waxes and wanes over an ∼11-year cycle, the frequency of occurrence of flares also varies with this solar activity cycle; the last solar maximum occurred from 1989 to 1990.