THE PLASMASPHERE OF NEPTUNE

We examine the plausible existence of Neptune's plasmasphere and study the drift of particles inside it. Using the O8 magnetic field model [ Connerney et al., 1991] and assuming a uniform solar wind convection e lectric field, the plasma convection time and refining time are calcul ated in a Euler potential coordinate system [Ho et al., 1997]. The pla sma density and refilling time at the equilibrium state are first calc ulated, and the location of the plasmapause is set to be where the ref illing time and convection time are equal. The refilling time as a fun ction of ion speed is then recalculated along field lines, and the pla sma density and temperature are obtained by directly integrating the l ocal ion distribution function over the range of speeds for which the refilling time is less than the convection time. The density calculate d using this model shows sharp drop-offs at approximately 3.25 to 4.5 R-N on the zero magnetic scalar potential surface, a boundary taken to be the plasmapause. Our calculated density compares fairly with the o bserved density along the Voyager trajectory within about 5 R-N. Ion t emperature is also calculated along the field line with results which indicate that high-speed tails of the distribution function might be n eeded to explain the high observed temperature measured along the Voya ger 2 trajectory. Drift trajectories and speeds of 90 degrees pitch an gle particles inside the plasmapause are calculated. Particles of ener gy above tens of eV are gradient drift dominated, and the drift paths of this class of particles are essentially the minimum B contours that are similar to Acuna et al.'s [1993] calculations. Atmospheric precip itation of the J = 0 particles may provide an explanation for the UV e missions, as an alternative to the ''monoprecipitation'' suggested by Paranicas and Cheng [1994]. Drifts of low-energy particles are strongl y affected by the gravitational and centrifugal forces, and because of the largely tilted dipole and the large higher components of magnetic field, the resultant drift is nonaxisymmetric and quite complicated.