A MODEL OF TRITONS ROLE IN NEPTUNE MAGNETOSPHERE

Escape of neutral hydrogen (H) and nitrogen (N) from Triton maintains a large neutral cloud, called the Triton torus, in Neptune's magnetosp here. We have developed the first detailed Monte Carlo simulation mode l of the Triton torus that includes the collisionality, the complex ge ometry, the injection of two neutral species from Triton (H and N), an d the combined effects of photoionization, electron impact ionization, and charge exchange. Ionization in Neptune's plasma sheet was modeled using Voyager plasma observations. Collisions cause both the H and N neutral clouds to become more radially extended, both toward Neptune a nd out beyond the magnetopause, as well as more extended in latitude, when compared with collisionless models. Moreover, collisions of H wit h the much more massive N greatly enhance the collisional ejection of H from the system and into Neptune's atmosphere. This effect decreases the probability of H ionization within the magnetosphere relative to that for N, and furthermore causes model results for two-species injec tion from Triton to differ significantly from those for H injection al one. For a hydrogen escape rate from Triton of 5 x 10(25) s(-1) as giv en by photochemical models of Triton's upper atmosphere, a nitrogen es cape rate of 5 x 10(24) s(-1) gives proton and N+ sources of 5.6 x 10( 24) s(-1) and 3.3 x 10(24) s(-1), respectively, whose ratio is close t o the observed ratio of protons to heavies. A nitrogen escape rate of 2 x 10(25) s(-1) for the same hydrogen escape rate of 5 x 10(25) s(-1) , yields an N+ szource more than twice that of protons, inconsistent w ith the Voyager data.