Pioneer 11 observations of trapped particle absorption by Amalthea

During its flyby of Jupiter in 1974, Pioneer 11 passed through a region in which low‐energy (∼1 MeV) trapped protons had been exposed to absorption by Amalthea (L ≃ 2.5) only ∼5 hours earlier. In passing through this region, a brief (∼10 s duration) decrease was observed in the counting rate of ∼1‐MeV protons measured by the University of Chicago charged particle instrument. We interpret this decrease as a transient signature of the recent absorption of the proton flux by Amalthea. No such clear effects were observed in fluxes of electrons (E ≥ 3.4 MeV) or high‐energy nuclei (Z > 5, E ⪞ 70 MeV/nucleon) at any crossing of Amalthea's orbit. These observations place an upper limit on the radial diffusion coefficient, κ, for ∼1‐MeV protons at the orbit of Amalthea of 2×10−10 RJ²/s. By comparison with diffusion coefficients derived from absorption effects by Io (L ≃ 6), this limit requires that if the radial diffusion coefficient between L ≃ 2.5 and L ≃ 6 varies with L as Ln, then n ≥ 5, whereas diffusion driven by atmospheric neutral winds would predict κ ∝ L³ for low‐energy protons. Our results are consistent with diffusion driven by fluctuating electric (κ ∝ L7–10) or magnetic (κ ∝ L10) fields.