The Triton-Neptune plasma interaction

[摘要] The Voyager 2 encounter with Neptune and Triton in August of 1989 provided clues to an intriguing problem. Instruments onboard the spacecraft showed a large ionosphere at Triton. Subsequent studies have tried to explain the production of such high levels of ionization but have ignored the possible plasma dynamics originating from the interaction between Neptune's magnetosphere and Triton. This study applies knowledge gained from studying the solar wind-Venus interaction to this case. In doing so we find that observations made by Voyager 2 can be explained by downward convection of magnetospheric plasma into Triton's atmosphere. Furthermore, we find that the flowing momentum is transferred to the moon just below the exobase, calculated here to be approximately 750 km. From this point down the atmosphere is not in hydrostatic equilibrium, and cannot be until below the ionization peak. Finally we show that when the momentum gets transferred to the moon the flow must shut off. This is accomplished when both the convective velocity and magnetic field go to zero. By showing the magnetic field vanishes at an altitude of roughly 650 km, we conclude the accepted mechanism by which the ionosphere is produced to be invalid. This mechanism was identified early-on to be impact ionization from hot, or superthermal, electrons originating in Neptune's magnetosphere. These precipitating hot electrons are thus shown to operate independently of the magnetic field below the exobase. This is a result not previously discovered, and one which implies that the plasma interaction between Neptune's magnetosphere and Triton cannot be ignored.

[发布机构] Rice University

[效力级别] Astrophysics