Solar proton event
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.
Solar protons normally have insufficient energy to penetrate the Earth's magnetic field. However, during unusually strong flares, protons can be produced with sufficient energies to reach the Earth's magnetosphere and ionosphere around the north pole and south pole.
Protons are charged particles and are therefore influenced by magnetic fields. When the energetic protons leave the Sun, they preferentially follow (or are guided by) the Sun's powerful magnetic field. When solar protons enter the Earth's magnetosphere where the magnetic fields are stronger than solar magnetic fields, they are guided by the Earth's magnetic field into the polar regions where the majority of the Earth's magnetic field lines enter and exit.
Energetic protons that are guided into the polar regions collide with atmospheric constituents and release their energy through the process of ionization. The majority of the energy is extinguished in the extreme lower region of the ionosphere (around 50–80 km in altitude). This area is particularly important to ionospheric radio communications because this is the area where most of the absorption of radio signal energy occurs. The enhanced ionization produced by incoming energetic protons increases the absorption levels in the lower ionosphere and can have the effect of completely blocking all ionospheric radio communications through the polar regions. Such events are known as Polar Cap Absorption events (or PCAs). These events commence and last as long as the energy of incoming protons at approximately greater than 10 MeV (million electron volts) exceeds roughly 10 pfu (particle flux units or particles⁄sr·cm2·s) at geosynchronous satellite altitudes.
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