Peak Trapped Proton Fluxes

The TRP module estimates the peak omnidirectional trapped proton flux which will be encountered along a specified spacecraft trajectory. The peak flux is needed for estimating maximum single-event-effect (SEE) rates, which are often a more important design concern than orbit-averaged rates.

The peak flux can be evaluated:

• over the whole trajectory (if the whole-orbit option has been selected); or
• over a restricted segment of the trajectory (if the orbital segments option has been selected).

Peak fluxes are written to output files with names of the form something_peak.tr*, where "something" is the rootname you specify. Peak and average fluxes are calculated simultaneously, so that the TRP module only has to be run once for a specified orbit.

Since low-energy protons do not cause SEEs, TRP evaluates the peak flux only for energies above 10 MeV at the skin of the spacecraft.

One problem in defining the "peak" rate is that the highest proton flux is found at different locations for different energies. We have therefore taken a conservative approach: we estimate the peak flux by superimposing into a single spectrum the highest fluxes found at each energy, even though those highest fluxes may not all be encountered at the same location.

• For most nearly-circular orbits, this procedure will not lead to gross overestimates of peak SEE-rates: for the most important parts of the spectrum, the peak proton fluxes are nearly co-located, spread out over a restricted region of space which the satellite traverses in just a few minutes.
• For more elliptical orbits, in which the peak fluxes may not be so well co-located, you can use the orbital segments option to subdivide your trajectory into smaller segments and to search for a more accurate estimate of the peak flux.
• Locations of the peak >10, >15, >30, >50, and >100 MeV integral fluxes are recorded in the output file's header information. These locations give some indication of the geographical spread of the flux peaks. However, these locations should not be taken too seriously: because of secular changes in Earth's magnetic field, the AP8 models no longer produce accurate maps.

Peak differential proton fluxes are actually evaluated by first stepping along the spacecraft trajectory and recording the largest integral proton flux (that is protons/m²-s-sr above a specific energy) for a number of threshold energies. At the end of the trajectory sampling, the resulting integral spectrum is numerically differentiated, to obtain a differential spectrum (that is, protons/m²-s-sr-MeV vs. kinetic energy in MeV). This procedure will tend to slightly bias the peak spectrum toward higher energies.

TRP also produces a orbit-averaged trapped proton spectrum.