I.2.1 Overview

During energetic events on the Sun, large fluxes of energetic protons are produced which can reach the Earth. Solar particle events, because of their unpredictability and large variability in magnitude, duration and spectral characteristics, are treated statistically. Current models make the assumption that large events are confined to a 7year period defined as solar maximum. Although large events are absent during the remaining 4 solar minimum years of the 11year solar cycle (see clause 6) the occasional small event can still occur.

An inherent input to probablistic models is the use of a “confidence level”. As a result:

• the risks of encountering environments can be more severe than the one predicted, and

• margins are implicitly assumed by applying high confidence levels.

Two main approaches are currently described in the literature to specify the mission integrated fluence: the ESP model approach and the JPL model approach.