Aurora on Triton?

If the planetary magnetic field of Neptune lies within the range of values that have been predicted for it (or even if the field is considerably smaller), Voyager 2 will find Triton immersed within the plasmasphere of a rotationally dominated magnetosphere. We expect the ionosphere of Triton to be highly conducting, which, coupled with Triton's 40 km/sec velocity relative to the magnetospheric plasma, leads to a limiting current that produces an induced magnetosphere, as occurs with solar‐wind flow past Venus or Mars. The induced magnetosphere, analogous to that observed near Saturn's satellite Titan, serves to deviate the flowing plasma and reduce the potential across Triton. The limiting current in this case can probably be carried by the local plasma with drift speeds below the thermal speed. For these conditions, we do not expect current‐driven instabilities to occur, and any auroral emissions would probably not be detectable against the background airglow of Triton's atmosphere. However, if Triton were to have a weak intrinsic magnetic field (for example, application of Blackett's law to Triton indicates a surface field of 400 nT, although a 20–80 nT surface field on Triton would suffice), the current would be funneled into an auroral zone. This would produce a concentration of current that may trigger specific auroral acceleration processes, such as double‐layer formation, and produce auroral optical and radio emissions detectable by Voyager instruments. Thus the presence of an aurora, particularly one that is localized, will argue for the presence of an intrinsic field on Triton.