Survey of electrons in the Uranian magnetosphere: Voyager 2 observations

Abstract

We present results of an analysis of the Voyager 2 plasma science experiment (PLS) electron measurements made during the Uranus encounter. The energy coverage is 10 eV ≤ E ≤ 5950 eV. The electron distribution functions are composed of a thermal (cold) and a non‐Maxwellian suprathermal (hot) component. Within the inner magnetosphere the electron density ne ranged between 0.02 cm−3 and 1.0 cm−3, while the electron temperatures Te ranged between ∼10 eV near the terminator and ∼100 eV within the nightside hemisphere. Thermal (suprathermal) electron temperatures Tc (Th) ranged from ∼7 to 10 eV (20–200 eV) near the terminator and from ∼10 to 30 eV (500 eV to 2 keV) within the nightside hemisphere. The observations revealed no significant electron fluxes above the detection limit at all observed energies in the dayside hemisphere, but electron fluxes orders of magnitude larger extended over the full PLS energy range in the nightside hemisphere. The large day‐night asymmetry together with the spin axis alignment with the solar direction and the large tilt ∼60° of the planetary magnetic dipole indicate that solar wind driven time dependent magnetospheric convection will be an important transport mechanism within the Uranian magnetosphere. The energy‐time evolution of the plasma boundary for ions and electrons near the terminator (L ∼ 6.7) at 1648 spacecraft event time (SCET) on January 24, 1986, is remarkably similar to that observed during substorms by spacecraft within the earth's magnetosphere at geosynchronous orbit in the dusk‐midnight quadrant. We show that Miranda cannot be the cause of the plasma sheet inner edge and that present “steady state” shielding models cannot account for the thinness of the “inner edge” (< 500 km) because of expected periodic merging (enhanced convection) every ∼17 hours on the dayside magnetopause. Resonant charge exchange collisions between protons and Uranus' hydrogen cloud are used to estimate an upper limit for the hydrogen corona density NH ∼200 cm−3 at L = 5. Using the local time asymmetry in the E ∼ 6 keV electron fluxes and the observation of E > 20 keV electron fluxes at all local times sampled by the spacecraft (Krimigis et al., 1986), we estimate that the steady state convection time τconv of the plasma is between 1 and 3 days. Finally, the local time asymmetry in the radio emissions from Uranus is consistent with the day‐night asymmetry in the keV electron fluxes observed by PLS which may provide the free energy for the radio emission.