Considerable confusion exists on what is the 'size' of a proton event. With the greatly improved sensitivity of detectors on satellites, it appears that almost every solar 'hiccup' is being reported as a solar proton event with the descriptive adjective 'large' being applied to events that are very much smaller in particle flux and average energy than some of the classic solar particle events of the 19th solar cycle. In view of this confusion, it appears that some type of classification system for discrete solar proton events would be beneficial to scientists utilizing solar proton information for various geophysical analyses.
The system described here is the result of a preliminary proton event classification system proposed during the Inter-Union Commission on Solar-Terrestrial Physics (IUCSTP) Plenary Meeting held in London in January, 1969. A revised three-digit system was presented at the Third ESLAB/ESRIN Symposium, Noordwijk, The Netherlands, in September 1969 , and then finalized for presentation at the IUCSTP and COSPAR meetings held in Leningrad in May 1970. At that time both the IUCSTP Bureau and COSPAR officially approved this classification system for international adoption.
This system, described in Table , is based on the solar proton flux in the vicinity of the Earth measured in units of cm -2 sec-1 ster-1 at the time of maximum intensity, and is intended to convey the response of three types of sensors to a solar particle event. The commonly available data relating directly to the solar proton intensity are obtained from: (a) direct satellite measurements; (b) riometer absorption measurements in the polar cap; and (c) the counting-rate of a high latitude cosmic-ray neutron monitor.
These data are usually available relatively quickly and are not dependent on sophisticated interpretation of other types of geophysical measurements.
The first digit represents the measurement of the E> 10 MeV proton flux by a satellite within the Earth-Moon system, the second digit represents the 30 MHz absorption measured by a sunlit polar riometer, and the third digit represents the response of a high latitude sea level neutron monitor. The classification system is designed so that positive values of the index indicate an event measurable by groundbased sensors, a zero index indicates no significant geophysical effects (such as a PCA), and a negative index number indicates particle fluxes so low that they are not measurable by ground-based equipment.