Energetic ion and electron phase space densities in the magnetosphere of Uranus

Voyager 2 low-energy charged particle (LECP) data from the magnetosphere of Uranus have been analyzed to obtain proton and electron phase space density profiles. The Uranus proton profiles show an approximately exponential decline with decreasing radius for L<~9 in a relatively dense thermal plasma region with intense plasma wave activity. An analogy with the magnetospheres of Earth, Jupiter, and Saturn suggests a plasmasphere at Uranus. The ion flux tube content in the Uranian radiation belt is less than that in the other three cases. Proton and electron profiles <~100 MeV/G show evidence of an absorption signature near the minimum L of Ariel. Proton profiles >200 MeV/G show evidence of injection events, suggesting substorm processes at Uranus. Electron profiles >100 MeV/G show clear minima at the minimum L values of Miranda, Ariel, and perhaps Umbriel, separated by broad maxima. These profiles are interpreted as implying local injection of energetic electrons. Distributed loss mechanisms at Uranus include satellite sweeping, wave-particle interactions, and charge exchange of protons with an extended hydrogen corona. If the net loss rates of protons and electrons are estimated by calculated satellite sweeping rates, the radial diffusion coefficient is estimated near L=7.5 to be 10^-7 s^-1 to 10^-6 s^-1. If the diffusion coefficient is <~10^-6 s^-1, the power available from inward radial diffusion of energetic protons and electrons is <~5×10⁸ W and is less than the 4×10¹⁰ W needed to maintain the Uranian ultraviolet aurora. Local injection of electrons above 100 MeV/G between the minimum L values of Miranda and Ariel also requires a power of at least 8×10⁷ W. The magnetosphere of Uranus is the third planetary magnetosphere for which evidence of substorm activity has been adduced, after those of Earth and Mercury.