Solar Proton Events (or SPEs) are defined as a flux (of protons with energies higher than 10MeV) greater than 10 particles cm-2 s-1 ster-1 (per centimeter-squared per second per steradian) for more than fifteen minutes. During an SPE, protons and other particles are accelerated to very high velocities by the release of extreme energy during a solar flare or a shockwave from a Coronal Mass Ejection (or CME). The protons travel along the extended magnetic field lines of the Sun, some making their way to the Earth.

Once caught in the trap of the Earth's magnetic field, the solar protons travel to the weaker regions of the field lines and enter the ionospheric region of the atmosphere, producing further ionization. This may cause a significant radiation increase, leading to disturbance of electronic systems.

Because of the Earth's atmosphere and extensive magnetosphere, humans on the Earth's surface are adequately protected from the radiation increase from an SPE. However, astronauts in space (for example, on the Moon or orbiting Earth) are extremely vulnerable to this radiation. In order to protect these astronauts and the future of space exploration, it is important to learn as much as possible about SPEs and both their short- and long-term effects.