Magnetospheric convection in the nondipolar magnetic field of Uranus

Abstract

A method for determining the magnetospheric convection electric field using simple analytic approximations under the assumption of uniform ionospheric conductivity is described, and applied to Uranus. Magnetic field models including quadrupole and octupole moments are used to determine the shape of the polar caps and the mapping of the electric field and parallel currents between ionosphere and magnetosphere. The asymmetry in the magnetic field models between the northern and southern hemispheres leads to the inclusion of currents between the hemispheres in order to satisfy the assumption of equipotential magnetic field lines. The results show that the quadrupole moment of the Uranian magnetic field strongly influences magnetospheric convection, and that a significant octupole moment will further alter the flow pattern. Even with these modifications the basic flow is sunward in the inner magnetosphere as inferred previously. The total current which flows along field lines between the two hemispheres due to the asymmetry of the magnetic field is comparable in magnitude to that of the region 1 current system. Time dependent calculations including a self‐consistent electric field show that ring current shielding of the electric field is important and may have formed the most prominent features in the plasma observations made by Voyager 2. The effectiveness of the shielding can be influenced by the magnetic field model. Other features in the data are characteristic of substorm injection, and the model has been used to show that a combination of plasma injection and electric field shielding may be applicable to the interpretation of the Voyager 2 data.