Low‐energy plasma observations in the magnetosphere of Uranus

Abstract

The plasma science experiment on Voyager 2 detected a large magnetosphere at Uranus containing a low‐density plasma. All available evidence suggests the plasma is comprised principally of protons and electrons. The protons are subsonic and their motion is consistent with corotation with the planet. In addition, on a longer time scale, they are apparently transported from the nightside of the planet to the dayside by a convective electric field generated by the solar wind. This convective motion is not directly observable in the plasma data; however, there are conspicuous day/night asymmetries in the particle densities which are probably produced by the convection pattern. Features in the spatial distribution are probably explained by a convection boundary in the vicinity of L = 5 produced by this convective electric field. Inside of the boundary plasma moving sunward is excluded. Scaling from models which predict the location of such a boundary in the earth's magnetosphere, we obtain a value of 0.4 mho for the height‐integrated, ionospheric conductivity, consistent with pre‐encounter modeling of the Uranian ionosphere. The time for particles to convect through the Uranian magnetosphere is estimated to be about 1 week. Peak ion densities reach a few protons cm−3. The proton distribution functions are characterized by a “warm” (i.e., subsonic) core and a non‐Maxwellian tail which varies significantly along the spacecraft trajectory. The core temperature is ∼5 to 10 eV. The non‐Maxwellian tail carries most of the plasma energy density in the observed energy range (<6 keV), which is small compared to the energy density of the planetary magnetic field. The spacecraft potential was negative with respect to the ambient plasma for 2 hours while Voyager was outbound from the planet. Analysis of the data gathered in this charging region shows that the predominant proton population is subsonic, and there is no significant “hidden” population of cold, supersonic ions in the inner magnetosphere of Uranus. The core population is probably produced by a local source of protons, while the nonthermal component may be produced by injection of protons from the magnetotail or the auroral region of the ionosphere.