Diurnal and seasonal variability of Uranus’ magnetosphere under different IMF
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The magnetosphere of Uranus is far from well known since there was only one fly-by measurement in history. In order to study the magnetosphere and its coupling mechanism with the solar wind, we used our multifluid magnetohydrodynamics (MHD) model [Cao and Paty, 2017] to successfully simulate the variation of the global magnetosphere of Uranus and have predicted potential favorable reconnection locations. We investigated the existence of a “switch-like” magnetosphere at Uranus for both equinox and solstice seasons, where the planetary rotation drives the interchange between an open magnetosphere and a closed magnetosphere each Uranus day. This periodic reconnection is predicted to occur upstream of the magnetopause, with a frequency that corresponds to the planetary rotation (once per 17.24 hours). The locations of the bow shock and magnetopause in our model are validated against measurements made by Voyager 2. In examining the evolution of the magnetic field configuration along with that of high plasma beta regions, which in combination indicate where the system is favorable for reconnection, we found that the occurrence of reconnection is highly dependent on the rotation of the planetary magnetic field in both equinox and solstice seasons. These periodic reconnection events in our simulation support the hypothesis of a periodic “switch-like” magnetosphere at Uranus during different seasons. We then investigated the diurnal and seasonal variations of the magnetopause boundary under different Interplanetary Magnetic Field (IMF) orientations, combined with Voyager 2’s measurement. We quantitatively analyzed the characteristics and variability of Uranus’ magnetopause and cusp in terms of the subsolar’s standoff distance, the flaring parameter and the cusp indentation, which give us an initial intuition of the asymmetric structure of the magnetopause. Our results show that the asymmetry of the magnetopause is highly dependent on the rotation of Uranus under specific IMF orientations. The shape of the magnetopause is also affected by the off-centered dipole moment. Our model can be applied to other planets with different magnetic geometries, such as the exoplanets and the Neptune-Triton system.