The near-Earth natural radiation environment can be divided into two categories, the particles trapped in the Van Allen belts and the transient environment. The particles trapped in the near-Earth environment are composed of energetic protons, electrons, and heavy ions. The transient radiation consists of galactic cosmic ray particles and particles from solar events (coronal mass ejections and flares). The cosmic rays have low-level fluxes with energies up to TeV and include all ions in the periodic table. The solar eruptions produce energetic protons, alpha particles, heavy ions, and electrons. To the first order, all of these particle populations are omnidirectional and isotropic.
Space also contains a low energy plasma of electrons and protons with fluxes up to 1012 cm2/sec. In the trapped particle regions, the plasma is the low energy (< 0.1 MeV) component of the charged particles. In the outer regions of the magnetosphere and in interplanetary space, the plasma is associated with the solar wind. Because of its low energy, the plasma is easily stopped by thin layers of material so it is not a hazard to most spacecraft electronics. However, it is damaging to surface materials and can contribute to spacecraft surface charging and discharging problems.
Trapped Protons and Electrons
The trapped particles pose a significant radiation threat to electronic systems and humans. There are large variations in the level of hazard depending on the orbit of the spacecraft, solar activity, and magnetospheric conditions. Both the protons and electrons contribute to total ionizing dose damage. For some electronic parts, single event effects induced by protons are also a hazard. Protons also contribute to degradation due to non-ionizing energy loss. Protons are especially problematic because of their high energies and penetrating power. As mentioned above, low energy electrons are the cause of electrostatic discharging which can be a serious problem for spacecraft in higher altitude orbits (e.g., geostationary) where they are exposed to more intense electron populations. Higher energy electrons can penetrate into the spacecraft, collect in insulator materials, and discharge causing damage to electronics.
Galactic Cosmic Ray Heavy Ions
The flux levels of the galactic cosmic rays (GCRs) are low compared to the trapped particles, but they are hazardous to spacecraft electronics because their high energies make them extremely penetrating. Also, they have a high rate of energy deposition as measured by their linear energy transfer (LET) rate. A particle’s LET is primarily dependent on the density of the target material and, to a lesser degree, the density and thickness of the shielding material. It is their high LET that makes cosmic rays an important contributor to single event effects problems for spacecraft, especially in orbits where the magnetosphere offers little protection.
The total dose deposition in silicon is only 10 rads/year when the GCR environment is at its peak. However, when the GCR dose is converted to dose equivalent in units of rem for biological systems, it can reach dangerous levels for humans. This can be true even for low earth orbits where the effect of the magnetospheric attenuation on the fluence levels of cosmic ray particles is significant.
The particles from solar events are a concern for spacecraft designers. In fact, for spacecraft in orbits exposed to these particles, they are often the driver for setting single event effects requirements. At this time there is no method for predicting when these events will occur. Warnings have short lead times and are not dependable. Experimenters have measured single event upsets on several satellites during solar events and quiet times. The solar proton component of the solar particle events must also be evaluated for the level of degradation damage for both ionizing and non-ionizing effects.
For systems that must operate during a solar particle event, the effect that both the solar protons and the solar heavy-ions has on single effects rates must be evaluated. The heavier ions make only a very small contribution to the dose levels. However, single event effects induced by solar heavy ions pose a serious problem for spacecraft systems that must operate during a solar event, because the particle levels are orders of magnitude higher than the background galactic cosmic rays. For the systems that must operate during a solar particle event, the effect that both the solar protons and the heavy-ions has on single effects rates needs to be evaluated. It is especially important to take the peak flux levels into consideration. When setting part requirements and operational guidelines, one must remember that peak solar particle conditions exist for only a small part of the total mission time.
Protons from solar particle events also contribute to total dose and solar cell damage especially for interplanetary missions and those at geostationary and in geostationary transfer orbits.