TRITONS IONOSPHERIC SOURCE - ELECTRON-PRECIPITATION OR PHOTOIONIZATION

We have performed an evaluation to determine whether or not Neptune's magnetospheric electrons can provide the ionization of Triton's ionosp here as previously suggested or whether photoionization is the dominan t ionization mechanism. Our approach has been to determine the accessi bility of magnetospheric electrons to Triton's ionosphere. Using scali ng relationships based on Venus and Titan observations, we have develo ped estimates of the centrifugal, gradient B and E x B drifts. We have computed trajectories of magnetospheric electrons and studied their a ccessibility to the Triton ionosphere. The following conclusions can b e reached from this study: (1) Centrifugal drift delivers electrons to the ionopause. If centrifugal drift is impaired, then electron precip itation is severely limited. (2) Low-energy electrons (E < 5 keV) are lost through E x B drift around the ionopause. (3) At higher electron energy the probability of precipitation increases. If the electron gyr oradius is small relative to the ionopause thickness, then at pitch an gles similar to 90 degrees grad B drift dominates with trapping of ele ctrons in the ionopause and subsequent exclusion from the ionosphere. At pitch angles 0 degrees and 180 degrees curvature drift dominates, a nd electrons will precipitate on entry into the ionopause. If the elec tron gyroradius is large compared to the ionopause thickness, then ele ctrons will precipitate at any pitch angle. Mass loading is estimated to be unimportant at Triton, and this contributes to the importance of E x B drift and the exclusion of low-energy electrons to Triton's ion osphere. Our calculations have intentionally overestimated the effects of centrifugal drift to present the best case for electron precipitat ion. Although collisions are more important for low-energy electrons ( E < 5 keV), we estimate that cross-field diffusion is small for ionopa use heights greater than 725 km. At higher electron energies where col lisions are less important, the threshold energy above which electrons become untrapped is only dependent upon the ionopause thickness and n ot collisions. Pressure balance arguments show that the ionopause is t hick with Delta(z) > 200 km. A magnetized ionosphere would be equivale nt to the high ram pressure case for the Venus interaction. A thick io nopause would contribute to prevention of precipitation of magnetosphe ric electrons into Triton's ionosphere when E < 50 keV. Although our c alculations at the present level of development cannot rule out the im portance of electron precipitation as the source of Triton's ionospher e, we suggest that photoionization be considered viable for the produc tion of Triton's ionosphere.