Interplanetary proton fluence model: JPL 1991

Abstract

We describe an updated predictive engineering model for the interplanetary fluence of protons with energies >1, >4, >10, >30, and >60 MeV. This has been the first opportunity to derive a model from a data set that has been collected in space over a long enough period of time to produce a valid sample of solar proton events. The model provides a quantitative basis for estimating the exposures to solar protons of spacecraft during missions of varying length and of surfaces and atmospheres of solar system objects. It is derived from the set of data collected 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 data sets contain several major proton events (>10‐MeV fluences exceeding 3 or 4 × 10 9 protons/cm 2 ) comparable to the 1972 event. The method of statistical analysis used in producing the model of the proton environment is the same as that used for earlier models. For the cases of 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 models 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 of the length of the exposure. A method of extending this estimate to other heliocentric distances is described.