A solar proton event (SPE), or "proton storm", occurs when particles (mostly
protons) emitted by the Sun become accelerated either close to the Sun during a flare or in interplanetary space by CME shocks. The events can include other nuclei such as helium ions and
HZE ions. These particles cause multiple effects. They can penetrate the Earth's magnetic field and cause
ionization in the
ionosphere. The effect is similar to auroral events, except that protons rather than electrons are involved. Energetic protons are a significant radiation hazard to spacecraft and astronauts.