Plasma‐dominated magnetic field configurations for the magnetosphere of Uranus

Abstract

We present hydromagnetic equilibrium configurations for the nonrotating “pole‐on” magnetosphere of Uranus. According to a plasma supply mechanism proposed by Cheng (1984) we assume that charged particle sputtering of the water‐ice covered moons of Uranus provides a continuous internal plasma source to the Uranian magnetosphere. We assume further that (1) dynamical changes of the magnetosphere are quasi‐static, (2) the internal plasma source fills the Uranian magnetosphere in a homologous manner, and (3) the plasma residence time within the magnetospheric cavity is long enough to allow the Uranian magnetosphere to evolve from a vacuum configuration toward a plasma‐dominated equilibrium. With these assumptions, we calculated configurational changes of Uranus's magnetosphere, given the amount of the thermal plasma pressure as a free parameter. In order to include Uranus's dipole in the equilibrium calculations the homogeneous Grad‐Shafranov equation for cylindrically symmetric magnetospheres has been complemented by an inhomogeneous source term. We derived linear analytic solutions to the inhomogeneous problem. We found that a plasma ring forms near the equatorial plane of the planet when the thermal pressure reaches about 75% of the maximum amount that would lead to a Harris sheet magnetotail configuration. The appearance of a plasma ring is due to the particular pole‐on orientation of Uranus's dipole; such a ring would not exist in an earth‐type equilibrium magnetosphere.