The very large solar proton events, those having an omni-directional solar proton fluence greater than 109 cm−2 at energies \textgreater30 MeV, are the events that impose operational constraints on manned space missions and equipment. A systematic survey of these very large omni-directional fluence solar proton events shows an association with two solar source group locations. One group is the classical situation, where the spacecraft is well connected to the solar active region by the interplanetary magnetic field. The other and most often-occurring group is the interplanetary shock dominated event resulting from a fast coronal mass ejection (CME) shock near the location of the spacecraft-Sun line. This result is consistent with observations of solar particle data from Earth-orbiting spacecraft for the last 4 solar cycles and also with Earth-based observations of very large solar particle events for the last 7 solar cycles. We now have an ∼450-year record of these very large solar proton events from the analysis of the impulsive nitrate deposition events found in polar ice. The frequency distribution of these large events is consistent with the frequency distribution derived from the analysis of radionuclides found in moon rocks. These frequency distributions show that solar proton events with \textgreater 30 MeV omni-directional fluence exceeding 6 × 109 cm−2 are very rare.
