A study of Uranian magnetospheric convection
Hill, Thomas W.
Doctor of Philosophy
In order to understand and explain the low-energy plasma structures observed by the PLS experiment on Voyager 2 in the Uranian inner magnetosphere, an analytic and self-consistent model of a time-dependent solar-wind driven convection system at Uranus has been developed in the corotating coordinate system. Many important results of this model agree with the observations very well. Because of the unusual orientation of the planetary rotation and magnetic dipole axes, magnetic merging on the dayside magnetopause varies as a function of planetary spin, in response to the changing orientation of the planetary magnetic field relative to the upstream interplanetary magnetic field, which is assumed to have a fixed direction for many planetary rotations. Therefore the magnitude of the solar-wind driven convection electric field varies sinusoidally in time with the 17.2 hr planetary spin period, even though the field direction is fixed in the corotating frame in a direction analogous to the dawn-to-dusk direction in the Earth's magnetosphere. By assuming conservation of the first adiabatic invariant we find that the "hot" (few keV) protons observed by the PLS experiment in the inner magnetosphere may be convected Sunward from a pick-up source provided by electron impact ionization of the neutral torus of the outermost satellite Oberon. Under the time-dependent convection field this hot plasma forms a ring-current shielding layer in the region L = 5 $\sim$ 7, similar to an Alfven layer because the hot plasma convection timescale ($\sim$20 days) is much larger than the 17.2 hr period of variation of the convection field. Inside of the shielding layer the time-averaged electric field is much smaller than the time average of the imposed field. The sinusoidal oscillation of the imposed electric field, however, is not significantly shielded by the shielding layer because the shielding timescale ($\sim$30 hr) is longer than the 17.2 hr oscillation period. A fraction of the hot plasma is therefore able to penetrate the shielding layer to form a trapped ring-current population. This trapped ring-current population is sufficiently long-lived to undergo charge-exchange and inelastic collisions with the widely distributed neutral hydrogen corona, resulting in the energy degradation of the "hot" component and the simultaneous appearance of the "intermediate" (few 100 eV) and "warm" (few 10 eV) components evident in the PLS results in the region between L = 5 and L = 7. The region 2 Birkeland current system, in our model, is concentrated near the region of the ring-current shielding layer. By analogy with the Earth's magnetosphere, the lower boundary of the Uranian aurora is predicted by mapping the location of the shielding layer in the magnetic equatorial plane along the magnetospheric magnetic field lines onto the Uranian ionosphere.
Geophysics; Astronomy; Astrophysics