The growth of our knowledge of the Earth's outer radiation belt

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Abstract
The development of our knowledge of the outer Van Allen radiation zone is surveyed. In general, the approach within each topic is historical. The early discoveries of the gross features of the radiation belt, the difficulties encountered in determining the electron energy spectrum, and the misinterpretation of geomagnetic storm intensity changes due to inadequate knowledge of the electron spectrum are discussed. Also considered are the equatorial pitch angle distributions of electrons, the presence of protons in the outer zone, and the question of a ‘slot’ between the inner and outer zones. The calculations of the neutron albedo contribution of electrons to the outer zone are reviewed; the contribution appears to be inadequate, because the resulting energy spectrum is incorrect, the intensity inadequate, and the required lifetime too long. Observations of electron precipitation into the atmosphere are briefly reviewed, and several possibilities for the connection of such observations to the intensity and lifetimes of trapped electrons are listed.