Solar‐wind‐driven convection in the Uranian magnetosphere

Abstract

We present an analytic, self‐consistent model of time‐dependent solar‐wind‐driven convection in the magnetosphere of Uranus. 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‐hour 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. We assume that the “hot” (keV) protons observed by the Voyager 2 plasma science instrument in the inner magnetosphere convect sunward from a source in the near tail and form a ring current shielding layer near L = 5. The shielding process requires a time‐dependent model because the convection timescale (∼20 days) is much larger than the 17‐hour period of variation of the convection field. The time‐averaged part of the imposed electric field is strongly attenuated inside the shielding layer, but the sinusoidally varying part of the imposed field penetrates the layer without significant attenuation because the shielding timescale (∼30 hours) is longer than the 17‐hour oscillation period. A fraction of the hot plasma is thereby “scattered” onto closed drift orbits 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” (∼ 100 eV) and "warm" (∼ 10 eV) components evident in the Voyager 2 plasma science measurements between L = 5 and L = 7. The Birkeland current system is concentrated near the ring current shielding layer, consistent with the relatively low latitude of the auroral emissions observed by the Voyager 2 ultraviolet spectrometer.