Triton's ionospheric source: Electron precipitation or photoionization

Details Triton's ionospheric source: Electron precipitation or photoionization Triton's ionospheric source: Electron precipitation or photoionization

May 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996jgr...10110863s&link_type=abstract

Journal of Geophysical Research, Volume 101, Issue A5, p. 10863-10876

Physics

4

Planetology: Solar System Objects: Neptunian Satellites, Ionosphere: Planetary Ionospheres, Magnetospheric Physics: Magnetosphere/Ionosphere Interactions, Magnetospheric Physics: Magnetosphere Interactions With Satellites And Rings

Scientific paper

We have performed an evaluation to determine whether or not Neptune's magnetospheric electrons can provide the ionization of Triton's ionosphere as previously suggested or whether photoionization is the dominant ionization mechanism. Our approach has been to determine the accessibility of magnetospheric electrons to Triton's ionosphere. Using scaling relationships based on Venus and Titan observations, we have developed estimates of the centrifugal, gradient B and E×B drifts. We have computed trajectories of magnetospheric electrons and studied their accessibility to the Triton ionosphere. The following conclusions can be reached from this study: (1) Centrifugal drift delivers electrons to the ionopause. If centrifugal drift is impaired, then electron precipitation is severely limited. (2) Low-energy electrons (E<5 keV) are lost through E×B drift around the ionopause. (3) At higher electron energy the probability of precipitation increases. If the electron gyroradius is small relative to the ionopause thickness, then at pitch angles ~90° grad B drift dominates with trapping of electrons in the ionopause and subsequent exclusion from the ionosphere. At pitch angles 0° and 180° curvature drift dominates, and electrons will precipitate on entry into the ionopause. If the electron gyroradius is large compared to the ionopause thickness, then electrons will precipitate at any pitch angle. Mass loading is estimated to be unimportant at Triton, and this contributes to the importance of E×B drift and the exclusion of low-energy electrons to Triton's ionosphere. Our calculations have intentionally overestimated the effects of centrifugal drift to present the best case for electron precipitation. Although collisions are more important for low-energy electrons (E<5 keV), we estimate that cross-field diffusion is small for ionopause heights greater than 725 km. At higher electron energies where collisions are less important, the threshold energy above which electrons become untrapped is only dependent upon the ionopause thickness and not collisions. Pressure balance arguments show that the ionopause is thick with δz>200 km. A magnetized ionosphere would be equivalent to the high ram pressure case for the Venus interaction. A thick ionopause would contribute to prevention of precipitation of magnetospheric electrons into Triton's ionosphere when E<50 keV. Although our calculations at the present level of development cannot rule out the importance of electron precipitation as the source of Triton's ionosphere, we suggest that photoionization be considered viable for the production of Triton's ionosphere.

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