INTERPLANETARY PROTON FLUENCE MODEL - JPL 1991

We describe an updated predictive engineering model for the interplane tary fluence of protons with energies >1, >4, >10, >30, and >60 MeV. T his has been the first opportunity to derive a model from a data set t hat has been collected in space over a long enough period of time to p roduce a valid sample of solar proton events. The model provides a qua ntitative basis for estimating the exposures to solar protons of space craft during missions of varying length and of surfaces and atmosphere s of solar system objects. It is derived from the set of data collecte d by the IMP and OGO spacecraft between 1963 and 1991. The >10 and >30 MeV data sets cover the period from 1963 to day 126 of 1991. The >1, >4, and >60 MeV data sets were collected between 1973 and 1991. Both d ata sets contain several major proton events (>10-MeV fluences exceedi ng 3 or 4 x 10(9) protons/cm2) comparable to the 1972 event. The metho d of statistical analysis used in producing the model of the proton en vironment is the same as that used for earlier models. For the cases o f the >10 and >30 MeV particles, the fluences are somewhat lower than in our earlier model (JPL 85). No >1, >4, and >60 MeV proton fluence m odels have been published in the literature previously. We present our results in a convenient graphical form which may be used to calculate the 1 AU fluence expected at a given confidence level as a function o f the length of the exposure. A method of extending this estimate to o ther heliocentric distances is described.