NEPTUNE INNER MAGNETOSPHERE AND AURORA - ENERGETIC PARTICLE CONSTRAINTS

A dramatic and peculiar dropout of >500-keV ions (but not electrons) w as observed within Neptune's inner magnetosphere near 2 R(N) as the Vo yager 2 spacecraft approached the planet. Unlike a number of other ene rgetic particle features this feature could not be accounted for by kn own material bodies in the context of the most utilized magnetic field models (neither the offset tilted dipole models nor the spherical har monic model ''O8''). However, the configuration of Neptune's inner mag netosphere is highly uncertain. By applying a novel technique, utilizi ng energetic particle measurements, to constrain the magnetic field co nfiguration of the inner regions, we show that appeals to unobserved m aterials within Neptune's system are unnecessary, and that the ion dro pout feature was, in all likelihood, the result of ion interactions wi th the maximum L excursions of the ring 1989N1R. The constraints also favor the use of the M2 magnetic field model (Selesnick, 1992) over pr evious models. An electron feature was probably absent because the ele ctron interactions with the ring occurred substantially before the ion interactions (about 2 hours for the electrons versus a few minutes fo r the ions). Pitch-angle scattering apparently eliminated the electron signature. Minimum scattering rates determined based on this premise yield enough electron precipitation power to explain the brightest com ponent of Neptune's aurora. We propose that this bright component is a nalogous to the Earth's diffuse aurora.