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Trapped Heavy Ions

Energetic trapped heavy ions, with atomic number Z > 2, have been observed in Earth's inner magnetosphere. However, such ions are not believed to constitute a significant single-event-effect hazard. Trapped heavy ions are NOT included in the CREME96 software.

  • All observations to date indicate that highly-ionizing trapped ions (atomic number Z > 6) have energies which are too low to penetrate typical minimum satellite shielding.
  • Trapped He ions (atomic number Z=2) have been reported at high energies, up to nearly ~100 MeV/nuc. However, He ions are generally not sufficiently ionizing to cause single event effects through direct ionization; and trapped protons overwhelmingly dominate the rates of nuclear-interaction induced single-event effects.

There appear to be several sources of energetic trapped heavy ions in Earth's magnetosphere:

  1. Anomalous Cosmic Rays (ACRs) appear to be the main source of trapped ions (primarily N, O, Ne) with energies above 10 MeV/nuc in the inner magnetosphere. In interplanetary space, these ACRs are singly-ionized (at least for energies up to ~30 MeV/nuc). Because of this low charge state, these ions are not effectively deflected by Earth's magnetic field. As a result, they can penetrate deep into Earth's magnetosphere, where collisions with atoms in the upper atmosphere cause them to lose electrons. The loss of electrons collapses the ions' gyroradii, and the ions become trapped in Earth's magnetic field.

Trapped ACRs were discovered as a "third" radiation belt around Earth in 1991. See:

  • N.L. Grigorov et al.,"Evidence for Trapped Anomalous Cosmic Ray Oxygen Ions in the Inner Magnetosphere", Geophysical Research Letters 18, 1959-1962 (1991).

Since 1992, trapped ACRs and have been extensively studied by instruments aboard the SAMPEX. See:

  • R.S. Selesnick et al.,"Geomagnetically Trapped Anomalous Cosmic Rays", Journal of Geophysical Research 100, 9503-9518 (1995).
  • M.D. Looper et al., "Trapped Anomalous Cosmic Rays Near the Geomagnetic Cutoff", Journal of Geophysical Research 101, 24747-24753 (1996).
  • R.A. Mewaldt et al., "Anomalous Cosmic Rays: The Principle Source of High Energy Heavy Ions in the Radiation Belts", Geophysical Monograph 97: Radiation Belts: Models and Standards, eds. J.F.Lemaire et al., (Washington: American Geophysical Union, 1996).

The SAMPEX observations showed that the trapped ACRs have very steep spectra. Thus, they are unlikely to have any impact on spacecraft design except for very lightly shielded components (< 50 mils) and in small portions of certain orbits. For further discussion of the potential radiation hazard posed by trapped ACRs, see:

  • A.J. Tylka et al.,"LET Spectra of Trapped Anomalous Cosmic Rays", Advances in Space Research, 17 (2)47-(2)50, (1996).

It should also be noted that no excess single event effect rates were observed aboard the SAMPEX satellite when it passed through the most intense portions of the low-altitude trapped ACR belt. [K. LaBel, private communication].

  1. Geomagnetically trapped Fe ions, with energies extending up to ~40 MeV/nuc, were observed aboard the LDEF satellite. The origin of these Fe ions is unknown; they may have been a transient trapped population associated with the very large solar particle events of September-October 1989. For further information, see:
    1. Jonathal, R. Beaujean, and W. Enge, "Trapped Iron Measured on LDEF", Proceedings 23rd ICRC (Calgary) 3, 445-448 (1993).
  • A.J. Tylka, et al., "On Purported Observations of Partially Ionized Galactic Cosmic Rays", Astrophysical Journal Letters 438, L83-L86 (1995).
  1. Energetic trapped He ions may arise from several sources. SAMPEX reported trapped He ions at L < 2 with spectra extending to at least ~20 MeV/nuc. These trapped He ions were apparently produced by nuclear interactions between trapped protons and atoms in the upper atmosphere. See:
      1. Selesnick and R.A. Mewaldt, "Atmospheric Production of Radiation Belt Light Isotopes", Journal of Geophysical Research 101, 19745-19757 (1996).

CRRES reported trapped He ions at even higher energies, with spectra extending to ~100 MeV/nuc. The origin of the CRRES trapped He is not clear, but these ions may have been associated with solar particle ("flare") events and/or magnetospheric acceleration processes caused by geomagnetic storms. Comparison between the CRRES fluxes and the AP8MAX model suggests He/proton ratios of ~10-6 - 10-7 above 40 MeV/nuc. See:

    1. Chen et al.,"Energetic Helium Particles Trapped in the Magnetosphere", Geophysical Research Letters 21, 1583-1586 (1994).
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