g a mission [Feyn-96]. J. Feyman and co-workers at JPL have performed a statistical analysis including data on solar particle event proton fluences from the past three solar cycles. Their findings show that the largest events, such as the August-72 and October-89 events, belong in the same statistical distribution as other events. They provide a Monte Carlo based tool for assessing probability of exceeding a given fluence level during a specified mission duration. This model is especially important for geostationary and interplanetary missions, and it has rapidly gained acceptance. The key point is that our understanding of even the gross features of the space radiation environment is not precise. Revisions and enhancements to the existing models are ongoing. IV-15 P6 (> 16 Mev) P7 (> 30 MeV) P8 (> 80 MeV) Solar Maximum (1991) Altitude (km) Figure 6. Recent dosimetry data from TIROS/NOAA satellites provide the basis for a revised model for trapped protons. The above figure and discussion in [Hous-98], by S.L. Houston and K.A. Pfitzer, indicate about twice the flux predicted by AP-8 for the three energy bins listed in the legend. TOTAL IONIZING DOSE AND PROTONS In this section we discuss several issues specific to total ionizing dose deposited by protons. Topics will include typical situations where proton-induced TID may be important to satellite systems, a discussion of the equivalence between proton dose and ionizing dose from other sources in the natural space environment, and finally a discussion of microdosimetry issues specific to protons. The emphasis will be on ionizing dose with the recognition that protons also deposit non-ionizing dose, which causes displacement damage. That is treated in the segment IVB of these notes. 3.1 Proton-Induced Total Ionizing Dose: Mechanisms and Issues As protons traverse a solid, their positive charge presents an electrostatic force to the orbital electrons of the surrounding material. Excited electrons are freed from their bound state thereby creating electron-hole pairs.