Solar proton event

A Solar Flare, image taken by the TRACE satellite (photo by NASA)

A solar proton event (or proton storm) occurs when protons emitted by the Sun become accelerated to very high energies either close to the Sun during a solar flare or in interplanetary space by the shocks associated with coronal mass ejections. Besides protons, the events can include other nuclei like helium ions and HZE ions, meaning that the event is sometimes a solar particle event. These high energy protons and ions cause several effects. They can penetrate the Earth's magnetic field and cause ionization in the ionosphere. The effect is similar to auroral events, the difference being that electrons and not protons are involved. Energetic solar protons are also a significant radiation hazard to spacecraft and especially astronauts, who can receive large amounts of absorbed dose from the ionizing radiation.

Solar protons normally have insufficient energy to penetrate through the Earth's magnetic field. However, during unusually strong solar flare events, protons can be produced with sufficient energies to penetrate deeper into the Earth's magnetosphere and ionosphere. Regions where deeper penetration can occur includes the north pole, south pole, and South Atlantic magnetic anomaly.

Above copied from:  http://en.wikipedia.org/wiki/Solar_proton_event

Protons greater than 10
Mev have a chance of crossing threshold on day one (14 Apr) and have a
slight chance of crossing threshold on day two (15 Apr).