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Orbit calculations are carried out to evaluate the effective atmospheric densities for geomagnetically trapped 125‐Mev protons whose guiding centers mirror between 200‐ and 560‐km altitude in the South Atlantic anomaly. Such orbits are limited to the region of B‐L space L = 1.38 and 0.2043 < Bm < 0.2355 gauss. Calculated are the effective atmospheric densities and scale heights experienced by the particle and its guiding center. Rates of energy loss by ionization are also calculated, taking into account atmospheric composition and the ionization energies for each constituent. Atmospheres used in the computations are the Harris and Priester S = 100 and S = 200 models, each diurnally averaged, and the Johnson model for solar minimum. The geomagnetic field is described by the 48‐term spherical harmonic expansion of Jensen and Cain. The numerical results show that for the trajectories considered, the effective atmospheric density averaged over the particle's motion is about twice the density averaged over the guiding center's trajectory. Also, the effective atmospheric scale‐heights can be three to four times the scale height of the model atmosphere, depending upon altitude and time in the solar cycle.

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