The magnetosphere of Neptune: Its response to daily rotation

The Neptunian magnetosphere periodically changes every eight hours between a pole-on magnetosphere with only one polar cusp and an Earth-type magnetosphere with two polar cusps. In the pole-on configuration, the tail current sheet has an almost circular shape with plasma currents closing entirely within the magnetosphere. Eight hours later the tail current sheet assumes an almost flat shape with plasma currents touching the magnetotail boundary and closing over the tail magnetopause. The authors have calculated magnetic field and tail current sheet configurations in a three-dimensional model, but have investigated the plasma- and thermodynamic conditions in a simplified two-dimensional MHD equilibrium magnetosphere. By matching the asymptotic plasma sheet β = 2μ0P/B2 values in the two models, they found to their surprise that the free energy F in the tail region of the two-dimensional model becomes independent of the dipole tilt angle. They conjecture that the Neptunian magnetotail might assume quasi-static equilibrium states that make the free energy of the system independent of its daily rotation