Geophysical Research Letters

Convection in Neptune's magnetosphere



We assume that nonthermal escape from Triton's atmosphere produces a co-orbiting torus of un-ionized gas (presumably nitrogen and hydrogen) that subsequently becomes ionized by electron impact to populate a partial Triton torus analogous to the Io plasma torus in Jupiter's magnetosphere. Centrifugal and magnetic-mirror forces confine the ions to a plasma sheet located between the magnetic and centrifugal equators. The ionization rate, and the torus ion concentration, is strongly peaked at the two points (approximately 180° apart in longitude) at which Triton's orbit intersects the plasma equator. During the course of Neptune's rotation these intersection points trace out two arcs roughly 75° in longitudinal extent, which we take to be the configuration of the resulting (partial) plasma torus. The partial ring currents produce a quadrupolar (four-cell) convection system that provides rapid outward transport of from the arcs. Ring-current shielding, however, prevents this convection system from penetrating very far inside the plasma-arc distance. We suggest that this convection/shielding process accounts for the radial confinement of ≳150 keV trapped particles within L ≈ 14.3 as observed by the Voyager LECP instrument.