The GLE05 Solar Proton Event

What happened

The largest ground-level enhancement (GLE) of solar cosmic rays ever recorded by neutron monitors. A powerful flare on February 23, 1956 accelerated solar protons to energies exceeding 10 GeV, producing a radiation increase of more than 4,500% at some high-latitude stations. Used ever since as the reference upper-bound solar particle event for spacecraft, aviation, and human-spaceflight radiation-shielding standards.