A physical model for the magnetosphere of Uranus at solstice time (Pantellini)

Vol. 643
10. Planets and planetary systems

A physical model for the magnetosphere of Uranus at solstice time

by F. Pantellini 2020, A&A, 643, A144

The magnetosphere of Uranus is subject to strong seasonal variations. The variability is a consequence of the planet's rotation axis being nearly parallel to its orbital plane, a situation unique in the Solar System. A short rotation period of 17.24h and a large angle between the rotation axis and magnetic dipole axis induces a twist of the planetary magnetic field lines, which are forced to expand downstream of the planet under the action of the supersonic solar wind. Numerical simulations suggest the scale of the twist to be of the order of several hundred times the Uranian radius. Specifically, for the solstice case, simulations show that the planetary field lines expanding downstream of the planet become organized into two distinct interlaced helically structured bundles. This paper presents a detailed physical description of this tail structure for a Uranus-like magnetosphere at solstice, when the solar wind and the rotation axis are quasi-aligned. In a symmetrized version of the original problem, the paper describes the time evolution of a field line from its emergence through the planet's surface to its final position. It shows, for example, that in the planet frame, the asymptotic position is a unique one-dimensional static double helix, independently of the field line's emergence point. It also shows that each strand of a field line approaches its asymptotic position along a spiraling path, similarly to the path followed by a fluid parcel in a tornado.