Solar Energetic Particle Environment

Solar energetic particle (SEP) events generally constitute the most severe environments in which space systems must operate. Large SEP events produce enhanced particle fluxes in near-Earth space which typically persist for several days.

These events are often colloquially referred to as "flares". Solar flares often occur coincidentally and thus provide useful proxies to signal the onset of SEP events. However, flares are not the causitive agent of the very large, long-duration, high-energy SEP fluxes which are important for space system design. We now know that these events are produced by interplanetary shocks driven by coronal mass ejections (CMEs).

The SEP models in CREME are based on satellite measurements of particles throughout the range of energies and elements relevant for SEU/SEE studies.

• CREME presents three levels of solar-particle intensity:
  • Worst-Week Model: This model is based on SEP fluxes averaged over the 180 hours (=7.5 days) beginning at 1300 UT on 19 October 1989.
  • This week was the most severe SEP environment observed in the last two solar maxima (roughly 1980.5-83.5 and 1989.0-92.0). It can therefore be used as a "99% worst case" environment for systems designed to operate through solar maximum.
  • Worst-Day Model: This model is based on SEP fluxes averaged over 18 hours beginning at 1300 UT on 20 October 1989. This period was the single largest flux enhancement in October 1989. It was caused by the arrival at Earth of a powerful interplanetary shock, which also produced a large geomagnetic storm.
  • For FLUX calculations inside the magnetosphere, you should always use a geomagnetic transmission function calculated with the stormy option with this worst day model.
  • Peak Flux Model: This model is based on the peak five-minute averaged fluxes observed on GOES in October 1989. (Direct measurements of the high-energy heavy-ion fluxes are not possible on such a short time scale. Peak heavy ion fluxes are therefore scaled from the "worst-day" fluxes, using the energy-dependent peak-to-"worst-day" ratios derived from the GOES protons.)
  • This peak-flux is obviously inappropriate for orbit-averaged fluxes and SEU rates, since the result depends critically on spacecraft location. It may be useful, however, for an extreme upper limit calculation. In this case, you should neglect geomagnetic shielding by selecting the interplanetary/geosynchronous option in FLUX. Neglecting all geomagnetic (and solid-Earth) shielding is conservative, especially for low-altitude, low-inclination orbits. The "peak flux" calculation for these orbits will be improved in a future release of CREME.
• CREME also takes into account SEP charge states, which are important for accurate fluxes calculation in low inclination, low-Earth orbits.
• Finally, it should be noted that the above SEP models do not include the contribution of Galactic cosmic rays, which dominate the spectrum at high energies (~500-1000 MeV/nuc, depending upon element). If you are interested in particle fluxes and SEU rates under very thick shielding (>1000 mils Al equivalent), you should add to the SEP results a calculation using solar maximum fluxes from the solar quiet option in FLUX.