Radial diffusion of geomagnetically trapped protons observed by the Galileo Energetic Particle Detector

The Galileo spacecraft encountered the Earth once on 8 December 1990 (Earth‐I) and again on 8 December 1992 (Earth‐II). These flybys provided excellent opportunities to evaluate the performance of the Energetic Particle Detector (EPD) and establish analysis procedures in a relatively well‐known environment. Further, because Galileo's Earth flyby trajectories were very rapid and nearly radial, the radiation belt measurements provided an excellent “snapshot” of trapped radiation. Because of the rapid flyby and the 20‐s spin period of Galileo, great care had to be taken to remove time aliasing from the pitch angle distributions. Large anisotropies were also present due to intrinsic density gradients. Spherical harmonics were fitted to the pitch and phase distributions in order to obtain fluxes from which phase space densities could be computed. The phase space density (PSD) was calculated from the fitted count rate for the particles (protons) that conserve the first and second adiabatic invariants. The values of 10.0, 15.0, 20.0, 25.0 30.0, 35.0, 40.0, 45.0, and 50.0 MeV/G were used for the first adiabatic invariant, and the values of 0.10, 0.15, 0.20, 0.25, 0.30, and 0.50 G 0.5 R E were used for the second adiabatic invariant to determine the PSD from Earth‐I and Earth‐II observations. The extracted PSDs were examined for radial diffusion. Results show there is no unique global dependency of the diffusion coefficient to L, except for a limited region of the first and second adiabatic invariants.