Injection and loss mechanisms for energetic ions in the inner Jovian magnetosphere

Abstract

The pronounced depletion of energetic ions observed by Voyager outside the orbit of Io can be understood as a steady state balance between particle loss and inward diffusion. For ions above 1 MeV/nuc (nucleon) G sweeping loss by Io itself is unimportant; rather the removal mechanisms involve processes associated with the extensive torus of neutral and ionized gas emanating from the satellite. There is compelling evidence to suggest that losses throughout an extended region in the outer torus, exterior to the orbit of Io, occur at a rate comparable to the upper limit imposed by strong pitch angle diffusion; although the plasma waves responsible for such scattering have yet to be detected. This is by far the dominant process for removing energy from the inner Jovian magnetosphere, and it also provides a viable energy source for exciting intense Jovian auroral emissions. Charge exchange removal of equatorially trapped lower‐energy (≲10 MeV/nuc G) heavy ions (e.g., O+ or S+) can become significant over a limited region near and just inside the orbit of Io, but this process appears to have little overall effect on the energetics of the Jovian system. The trapped particle distribution function in fact exhibits little evidence for effective loss inside the orbit of Io, presumably due to the absence of intense plasma waves. An equilibrium solution for the ion phase space density requires a pronounced increase in the rate of radial diffusion between the orbit of Io and the outer torus (L > 7). This suggests a change in the dominant mechanism for cross‐L transport consistent with the anticipated onset of rapid centrifugally driven interchange diffusion with DLL ≈ 5×10−6 (L/8)4 RJ²/s in the outer torus. In contrast, the diffusion rate near Io is substantially slower, DLL (L ≃ 6) ≈ 2×10−7 RJ²/s. This is comparable to the value deduced earlier from Pioneer 11 data by Thomsen et al., who suggested neutral winds in the Jovian ionosphere as a driving mechanism.