The Neptunian magnetospehre 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.

We 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 &bgr;=2&mgr;0P/B2 values in the two models, we found to our surprise that the free energy F in the tail region of the two-dimensional model becomes independent of the dipole tilt angle. We conjecture the Neptunian magnetotail might assume quasi-static equilibrium states that make the free energy of the system independent of its daily rotation. ¿The American Geophysical Union 1991