Proton and oxygen plasmas at Uranus

Abstract

Despite the presence of several large, icy moons within the Uranian magnetosphere, the Voyager 2 spacecraft did not detect any heavy ion plasma. This paper estimates the heavy ion density that would be consistent with a heavy neutral torus formed by photosputtering, charged particle sputtering, and micrometeoroid impact vaporization of icy surfaces on the moons, taking into account the large 60° angle between the satellite orbit plane and the magnetic equator. The expected heavy ion density is unobservably small. The observed proton plasma of the inner magnetosphere can be maintained by ionization of the atomic hydrogen corona and by the ionospheric proton source driven by photoelectron escape, for a plasma residence time of ∼30 days. These two proton sources are comparable near L = 5. The same 30‐day residence time for energetic protons implies an upper limit of 10−5 on the fraction of incident solar wind protons that enter the magnetosphere and penetrate to within L < 6 while conserving their first adiabatic invariants.