radiation belt dosage. Fortunately, because of their practical importance and because of large scale modification experiments in the past, e.g., the Starfish nuclear event, the physics of radiation belt processes is relatively better understood (Schulz and Lanzerotti, 1973); and we are able to make relatively more definitive assessments in this area. As is described in the morphological features above, magnetospheric particles are not entirely isolated from interactions with their neighbors in the ionospheric and atmospheric regions below. Indeed, these interactions control the lifetime and energy evolution of magnetospheric particles. For this reason, it would be convenient to introduce some basic ideas concerning particle motion in the magnetosphere. The magnetosphere, including the denser plasmasphere, is sufficiently tenuous that energetic particles move under the influences of the dipole-like magnetic field and electric fields without consideration of collisions. A collision, most probably a charge-exchange interaction with the more abundant neutral species existing in the magnetosphere and in the upper layers of the atmosphere, usually means loss of the charged energetic particle from the magnetosphere (precipitation). Thus, the lifetime [[spi:math]] of a charged particle is characterized by the inverse rate of charge-exchange interaction, where Nn is the neutral density involved (requiring various sorts of averaging over the volume of interaction), a is the charge exchange cross-section and v is the characteristic charged particle speed involved. At the lower regions of the magnetosphere (foot-of-field-line), Nn rapidly e-folds over a typical neutral scale height H ~ 20 km; hence, the altitude zce, determined by the functional relation A-3 [[spi:math]] (A1)
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